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Meteorite impacts have proved to be a significant source of seismic signal on the Moon, and have now been recorded on Mars by InSight seismometers. Understanding how impacts produce seismic signal is key to the interpretation of this unique data, and to improve their identification in continuous seismic records. Here, we use the seismic Representation Theorem, and particularly the stress glut theory, to model the seismic motion resulting from impact cratering. The source is described by equivalent forces, some resulting from the impactor momentum transfer, and others from the stress glut, which represents the mechanical effect of plasticity and non linear processes in the source region. We condense these equivalent forces into a point-source with a time-varying single force and nine-component moment tensor. This analytical representation bridges the gap between the complex dynamics of crater formation, and the linear point-source representation classically used in seismology. Using the multiphysics modelling software HOSS, we develop a method to compute the stress glut of an impact, and the associated point-source from hypervelocity impact simulations. For a vertical and an oblique impact at 1000 m s−1, we show that the moment tensor presents a significant deviatoric component. Hence, the source is not an ideal isotropic explosion contrary to previous assumptions, and draws closer to a double couple for the oblique impact. The contribution of the point force to the seismic signal appears negligible. We verify this model by comparing two signals: (1) HOSS is coupled to SPECFEM3D to propagate the near-source signal elastically to remote seismic stations; (2) the point-source model derived from the stress-glut theory is used to generate displacements at the same distance. The comparison shows that the point-source model is accurately simulating the low-frequency impact seismic waveform, and its seismic moment is in trend with Lunar and Martian impact data. High-frequencies discrepancies exist, which are partly related to finite-source effects, but might be further explained by the difference in mathematical framework between classical seismology and HOSS' numerical modelling. [ABSTRACT FROM AUTHOR]

Full-waveform inversion (FWI) of seismic data provides quantitative constraints on subsurface structures. Despite its widespread success, FWI of data around the critical angle is challenging because of the abrupt change in amplitude and phase at the critical angle and the complex waveforms, especially in the presence of a sharp velocity contrast, such as at the Moho transition zone (MTZ). Furthermore, the interference of refracted lower crustal (Pg) and upper mantle (Pn) arrivals with the critically reflected Moho (PmP) arrivals in crustal and mantle studies makes the application of conventional FWI based on linearized model updates difficult. To address such a complex relationship between the model and data, one should use an inversion method based on a Bayesian formulation. Here, we propose to use a Hamiltonian Monte Carlo (HMC) method for FWI of wide-angle seismic data. HMC is a non-linear inversion technique where model updates follow the Hamiltonian mechanics while using the gradient information present in the probability distribution, making it similar to iterative gradient techniques like FWI. It also involves procedures for generating distant models for sampling the posterior distribution, making it a Bayesian method. We test the performance and applicability of HMC based elastic FWI by inverting the non-linear part of the synthetic seismic data from a three-layer and a complex velocity model, followed by the inversion of wide-angle seismic data recorded by two ocean bottom seismometers over a 70 Ma old oceanic crustal segment in the equatorial Atlantic Ocean. The inversion results from both synthetic and real data suggest that HMC based FWI is an appropriate method for inverting the non-linear part of seismic data for crustal studies. [ABSTRACT FROM AUTHOR]

In distributed acoustic sensing (DAS), optical fibre is used as sensors, which enables us to observe strain over tens of kilometres at intervals of several metres. S -wave velocity (V s) structures of shallow sediments of high resolution have been obtained from surface wave dispersion curves by applying seismic interferometry to DAS data both onshore and offshore. However, it is known that there is a disadvantage to DAS seismic interferometry. In addition to Rayleigh waves, Love waves are also included. Consequently, the accuracy of the estimated phase velocities for Rayleigh waves is reduced due to the contamination of Love waves. To address this shortcoming, we suggest a spatial autocorrelation (SPAC) method between DAS and the vertical component of seismometer data. The SPAC method is equivalent to seismic interferometry and is useful for obtaining phase-velocity dispersion curves of surface waves from the cross-correlation functions (CCFs) between the records of two receivers. The CCFs obtained from a combination of DAS and vertical seismometer data should contain only Rayleigh waves because Love waves have no vertical component. CCFs between DAS and vertical seismometer data are therefore expected to give more accurate phase velocities of Rayleigh waves than CCFs with DAS data only. In this study, we first formulated analytical expressions of cross-spectra for DAS and three-component seismometer data because seismic observation is generally carried out using a three-component seismometer. A new SPAC method is presented in the form of analytical expressions. We showed that our formulation only includes Rayleigh and not Love waves in the cross-spectra with DAS and the vertical-component seismometer data. We applied our SPAC method to actual DAS and vertical seismometer data recorded on the seafloor. Then, we compared our new SPAC method for DAS and vertical seismometer data with a conventional SPAC method for only DAS data. The results reveal that our new SPAC method can estimate the phase velocities of Rayleigh waves more accurately than the conventional method. In addition, the analytical formulations of the cross-spectrum between DAS and three-component seismometer data, which we obtained in this study, are expected to be useful for the estimation of accurate 3-D structures in the future, although this is not available at the moment due to the lack of an applicable data set. [ABSTRACT FROM AUTHOR]

Accurate seismic phase detection and onset picking are fundamental to seismological studies. Supervised deep-learning phase pickers have shown promise with excellent performance on land seismic data. Although it may be acceptable to apply them to Ocean Bottom Seismometer (OBS) data that are indispensable for studying ocean regions, they suffer from a significant performance drop. In this study, we develop a generalized transfer-learned OBS phase picker—OBSTransformer, based on automated labelling and transfer learning. First, we compile a comprehensive data set of catalogued earthquakes recorded by 423 OBSs from 11 temporary deployments worldwide. Through automated processes, we label the P and S phases of these earthquakes by analysing the consistency of at least three arrivals from four widely used machine learning pickers (EQTransformer, PhaseNet, Generalized Phase Detection and PickNet), as well as the Akaike Information Criterion (AIC) picker. This results in an inclusive OBS data set containing ∼36 000 earthquake samples. Subsequently, we use this data set for transfer learning and utilize a well-trained land machine learning model—EQTransformer as our base model. Moreover, we extract 25 000 OBS noise samples from the same OBS networks using the Kurtosis method, which are then used for model training alongside the labelled earthquake samples. Using three groups of test data sets at subglobal, regional and local scales, we demonstrate that OBSTransformer outperforms EQTransformer. Particularly, the P and S recall rates at large distances (>200 km) are increased by 68 and 76 per cent, respectively. Our extensive tests and comparisons demonstrate that OBSTransformer is less dependent on the detection/picking thresholds and is more robust to noise levels. [ABSTRACT FROM AUTHOR]

Hyuga-nada, off the Pacific coast of Kyushu along the Nankai Trough in southwest Japan, is one of the most active slow earthquake regions around Japan. We estimated the energies of shallow tremors and moments of shallow very low frequency earthquakes (VLFEs) in Hyuga-nada using data from a permanent onshore broadband network and temporary ocean–bottom seismometer observations. The energies and moments of these slow earthquakes have a similar along-strike variation and are generally higher south of the subducted Kyushu–Palau Ridge than near the top of the ridge. This spatial variation is also related to the characteristics of slow earthquake migration. The along-strike migration speed was faster at initiation in the south, where the moments of slow earthquakes are higher. After migration entered the subducted Kyushu–Palau Ridge, its speed was decelerated with a parabolic pattern and their moments became smaller. Assuming a constant patch size of slow earthquakes, we estimated that the stress drop of VLFEs in the south of the subducted ridge was approximately three times higher than that near the top of the subducted ridge. According to our observations and a physical model, this stress drop difference between adjacent regions may cause parabolic migration. We also estimated the scaled energy of slow earthquakes from the ratio of the seismic energy rates of tremors to the seismic moment rates of accompanying VLFEs. The spatial variation in scaled energy is not identified inside the Hyuga-nada. Since the range of scaled energy is similar between the south and near the top of the subducted ridge, the apparent stress may be similar if the rigidity is the same. The dominant range of scaled energy of slow earthquakes in Hyuga-nada is 10−11.5–10−8.5. In addition to having similar or one order smaller values compared to other slow earthquake regions, the range of scaled energy in Hyuga-nada is broader. This broader range suggests wide range of characteristic time and various spectral features of slow earthquakes in Hyuga-nada. Based on a Brownian slow earthquake model, the wide range of characteristic time in this area suggests width variations of slow earthquake source area. [ABSTRACT FROM AUTHOR]

For a more quantitative discussion of slow earthquake activity, we evaluated the detectable limits of very low frequency earthquakes (VLFEs), which are seismic slow earthquakes observed in very low-frequency (< 0.05 Hz) bands in the Nankai subduction zone. We performed numerical simulations using a local 3-D model and used the observed noise level of permanent broad-band seismometers. First, we investigated the effects of the source-time functions on the maximum amplitudes of the VLFE signals at a certain station. The maximum amplitudes of the VLFE signals were controlled by the VLFE moment rate. The detectable limit of VLFEs at each source location can be defined as the lowest moment rate of detectable VLFEs, which radiate signals larger than the noise levels of any component at ≥ 3 stations. For inland seismometers only, the detectable limits of VLFEs at deep (30–40 km) and shallow (≤ 10 km) depths were 1012–1012.3 and 1012.7 N·m s−1, respectively. Due to the geometrical spreading of VLFE signals and large noise levels in horizontal components, offshore seismometers improved the detectability of shallow VLFEs in regions where seismometers were densely deployed. Based on our detectability and published catalogues, shallow slow earthquakes are less active south-southwest off the Kii Peninsula, where geodetic studies expect mechanical coupling. [ABSTRACT FROM AUTHOR]

Microseismic monitoring has been used in geo-energy related activities, such as shale-gas exploitation, mining, deep geothermal exploitation, geotechnical and structural engineering, for detecting and locating fractures, rock failures and micro-earthquakes. The success of microseismic monitoring depends on reliable detection and location of the recorded microseismicity. Multichannel coherence migration (MCM) is a detection and location waveform migration-based approach which does not require phase picking, identification and association and performs well on noisy data. Its caveat is a high computational cost, which impedes its application of MCM on large data sets or for real-time monitoring. To address this issue, we propose an improved approach, the multichannel coherence migration grid search (MCMgs), by introducing an adaptive grid optimization technique. Based on results from synthetic and real data, we show that MCMgs reduces the computation time up to 64 times. In addition, MCMgs generates multiple maximum coherence values with various grid sizes instead of a single (maximum) coherence value that links to a single gridpoint and size, thus resulting in more accurate locations. Our simulation results on different deployment geometries demonstrate that MCMgs is effective even with a small number of recordings available—a minimum of seven. We conduct a sensitivity analysis to assess how the detectability of events is affected by the spatial arrangement of the deployed monitoring array. If a limited number of seismometers are available for deployment, our analysis favours a patch array deployment geometry. We show that 12 seismometers deployed at a patch array geometry can have similar detection and localization capability as a large rectangular array of more than 100 seismometers but at a much lower computational and deployment cost. [ABSTRACT FROM AUTHOR]

Continuous seismic monitoring of volcanoes is challenging due to harsh environments and associated hazards. However, the investigation of volcanic phenomena is essential for eruption forecasting. In seismo-volcanic applications, distributed acoustic sensing (DAS) offers new possibilities for long-duration surveys. We analyse DAS strain rate signals generated by volcanic explosions and tremor at Stromboli volcano (Italy) recorded along 1 km of dedicated fibre-optic cable. We validate DAS recordings with colocated nodal seismometers. Converting node measurements to strain rate, we observe a perfect match in phase between DAS and node waveforms. However, DAS amplitudes appear to be around 2.7 times smaller than those of node records, which we explain as due to the inefficient ground-to-fibre strain transfer in the loose cable. We invert time delays between strain rate waveforms and confirm that the DAS enables us to retrieve a dominant and persistent seismic source in the proximity of active craters. This stable source location is confirmed by node array analyses. Despite an observed high noise level of strain rate signals outside a range of 2–15 Hz, our results demonstrate the potential of this new technology in monitoring volcanic areas. [ABSTRACT FROM AUTHOR]

In this work, we present both 1-D and 3-D shear wave velocity (Vs) models of the oceanic crust and uppermost mantle below the Deep OCean Test ARray area, located ∼ 70 km north of the central section of the Gloria Fault, in the eastern North-Atlantic Ocean. The velocity models are inferred from the dispersion of surface waves recorded on ocean bottom seismometers. Dispersion measurements are obtained from the analysis of ambient seismic noise at short periods (< 14 s) and teleseismic surface waves at long periods (> 14 s) using the two-station method. The 1-D Vs model is inferred from the joint inversion of Rayleigh wave phase velocities and Love wave group and phase velocities. The 3-D tomographic model is obtained by inversion of 2-D Love wave group velocity maps as a function of depth, further constrained by the average of Love wave phase velocities obtained from ambient noise (4–9 s) and the average Rayleigh and Love wave phase velocities calculated from teleseismic data (14–44 s). The 1-D Vs model shows a sediment layer with a low velocity of 1.05 km s−1, similar to previous studies in the region. Below the sediments, we find an oceanic crust with velocities ranging from 3.3 to 4.5 km s−1. The model reaches an unusually high velocity of 4.9 km s−1 in a 20 km thick layer at depths between 16 and 36 km. We interpret this fast velocity layer as indicative of the presence of harzburgite, a residue of enhanced melting that might have been formed by the proximity between the Mid-Atlantic Ridge and the Azores mantle plume. At greater depths the velocity decreases, forming a low-velocity zone that reaches a minimum at ∼ 70 km depth, which we interpret as the maximum depth for the lithosphere–asthenosphere boundary. The 3-D model shows a structure that is mostly horizontally layered, with Vs isocontours at 3.5–4.5 km s−1 highlighting oscillations of the crustal structure with wavelengths of ∼25–30 km. These oscillations may be due to changes in the rate of mantle upwelling and magma supply rate. [ABSTRACT FROM AUTHOR]

It has long been established that plastic flow in the asthenosphere interacts constantly with the overlying lithosphere and plays a pivotal role in controlling the occurrence of geohazards such as earthquakes and volcanic eruptions. Unfortunately, accurately characterizing the direction and lateral extents of the mantle flow field is notoriously difficult, especially in oceanic areas where deployment of ocean bottom seismometers (OBSs) is expensive and thus rare. In this study, by applying shear wave splitting analyses to a dataset recorded by an OBS array that we deployed between mid-2019 and mid-2020 in the South China Sea (SCS), we show that the dominant mantle flow field has a NNW–SSE orientation, which can be attributed to mantle flow extruded from the Tibetan Plateau by the ongoing Indian–Eurasian collision. In addition, the results suggest that E–W oriented flow fields observed in South China and the Indochina Peninsula do not extend to the central SCS. [ABSTRACT FROM AUTHOR]

The 'big' seismic data not only acquired by seismometers but also acquired by vibrometers installed in buildings and infrastructure and accelerometers installed in smartphones will be certainly utilized for seismic research in the near future. Since it is impractical to utilize all the seismic big data in terms of the computational cost, methods which can select observation sites depending on the purpose are indispensable. We propose an observation site selection method for the accurate reconstruction of the seismic wavefield by process-driven approaches. The proposed method selects observation sites suitable for accurately estimating physical model parameters such as subsurface structures and source information to be input into a numerical simulation of the seismic wavefield. The seismic wavefield is reconstructed by the numerical simulation using the parameters estimated based on the observed signals at only observation sites selected by the proposed method. The observation site selection in the proposed method is based on the sensitivity of each observation site candidate to the physical model parameters; the matrix corresponding to the sensitivity is constructed by approximately calculating the derivatives based on the simulations, and then, observation sites are selected by evaluating the quantity of the sensitivity matrix based on the D-optimality criterion proposed in the optimal design of experiments. In this study, physical knowledge on the sensitivity to the parameters such as seismic velocity, layer thickness, and hypocentre location was obtained by investigating the characteristics of the sensitivity matrix. Furthermore, the effectiveness of the proposed method was shown by verifying the accuracy of seismic wavefield reconstruction using the observation sites selected by the proposed method. [ABSTRACT FROM AUTHOR]

SUMMARY: Seismicity along transform faults provides important constraints for our understanding of the factors that control earthquake ruptures. Oceanic transform faults are particularly informative due to their relatively simple structure in comparison to their continental counterparts. The seismicity of several fast-moving transform faults has been investigated by local networks, but as of today there been few studies of transform faults in slow spreading ridges. Here, we present the first local seismicity catalogue based on event data recorded by a temporary broad-band network of 39 ocean–bottom seismometers located around the slow-moving Chain Transform Fault (CTF) along the Mid-Atlantic Ridge (MAR) from 2016 to 2017 March. We locate 972 events in the area by simultaneously inverting for a 1-D velocity model informed by the event P- and S-arrival times. We refine the depths and focal mechanisms of the larger events using deviatoric moment tensor inversion. Most of the earthquakes are located along the CTF (700) and Romanche transform fault (94) and the MAR (155); a smaller number (23) can be observed on the continuing fracture zones or in intraplate locations. The ridge events are characterized by normal faulting and most of the transform events are characterized by strike-slip faulting, but with several reverse mechanisms that are likely related to transpressional stresses in the region. CTF events range in magnitude from 1.1 to 5.6 with a magnitude of completeness around 2.3. Along the CTF we calculate a b-value of 0.81 ± 0.09. The event depths are mostly shallower than 15 km below sea level (523), but a small number of high-quality earthquakes (16) are located deeper, with some (8) located deeper than the brittle-ductile transition as predicted by the 600 °C-isotherm from a simple thermal model. The deeper events could be explained by the control of sea water infiltration on the brittle failure limit. [ABSTRACT FROM AUTHOR]

In this study, the focal mechanism solutions and source parameters of recent earthquakes that occurred in the northeastern region of India have been determined. The region has very complex tectonics as it is subjected to the compressional forces from all sides, due to the collision of the Indian Plate with the Eurasian, Burma and Tibetan plates. Waveform data from deployed broad-band seismographs (BBS) and strong motion accelerographs (SMA) in the northeastern India are used to determine the focal mechanism solutions and source parameters of moderate earthquakes, respectively. The estimated focal mechanisms are used to understand the existing stress field in the region. It is found that the Shillong-Plateau as well as the Indo-Burma subduction zone is dominated by the compressional tectonic regime, Mikir Hills and Bengal basin are dominated by the trans-tension tectonic regime, and the easternmost Himalayan region is dominated by the strike-slip tectonic regime. The maximum horizontal stress direction Sh max is also determined for above subregions. The direction of Sh max is southeast in the Bengal basin, northeast in Mikir Hills and Indo-Burma subduction zone whereas it is NNE in Shillong Plateau and SSW in the eastern Himalayas. The estimated stress drop value of the earthquakes in the region ranges from 2.11 to 23.89 MPa. The relationship between the source parameters and focal mechanisms is also explored. It is found that the earthquakes with a strike-slip mechanism have the highest average stress drop (7.05 MPa) followed by reverse (6.82 MPa) and normal (5.12 MPa) in the northeastern region of India. According to the examined data set, the stress drop is found to be dependent on the type of focal mechanism, seismic moment and hypocentral depths. The comparison of the results with the Kachchh intraplate region in western India shows earthquakes in Kachchh have larger mean stress drop for all types of mechanisms. In both intraplate and interplate regions of India, the stress drop of earthquakes depends on the type of focal mechanism solution. [ABSTRACT FROM AUTHOR]

Icequakes associated with crevassing provide key information on stress fields and water flow pathways inside glaciers, conditions which are sensitive to temperature and precipitation. In this study, we analyse seismic data from 321 days recorded by five broad-band seismometers on Urumqi Glacier No. 1, Tianshan, China, to investigate the source characteristics of icequakes. We first apply a multidimensional autoregressive maximum-likelihood algorithm to detect 12 template events from 62 days of continuous waveform data. The centroid locations of the 12 template events are located using a grid-search approach. We then obtain a catalogue of 41   895 icequakes by compiling all seismic data through a template-matching method. The local magnitudes of all icequakes range between −3.3 and 0.5, with a completeness magnitude of −2.95 and Gutenberg–Richter b -value of 1.08. The particle motion and frequency contents of the 12 template events show that the dominant phases on the vertical component are Rayleigh waves at 5–40 Hz, suggesting that these icequakes were generated by near-surface crevasses. The average daily icequake number for each month and diurnal icequake variations show clear seasonal features. The icequakes may be related to growth of the surface crevasses due to additional melting water and precipitation in summer and to the thermal contraction of the ice body in winter. [ABSTRACT FROM AUTHOR]

This study presents the shear wave velocity (VS) structures of sedimentary sequences and a section of the upper crustal layer in the Fukushima forearc region of the Japan Trench subduction zone, which were obtained by analysing the horizontal-to-vertical (H/V) spectral ratios of ambient vibration records. The H/V curves were derived using 31 d of continuous seismic data from 3 broad-band and 16 short-period ocean bottom seismometer (OBS) stations. Using the broad-band data, H/V ratios from 0.01 to 10 Hz were derived, but the ratios below 0.1 Hz frequencies were unusually large and temporally unstable. Characterization of seismic noise energy from ∼1 yr of seismic data of three broad-band OBSs revealed variable and elevated energy conditions below 0.1 Hz due to typical long-period oceanic noise; we link these observations with the unstable H/V ratios below this frequency. Therefore, H/V analysis was performed in the frequency range of 0.1–10 Hz for both broad-band and short-period OBSs to obtain subsurface VS profiles. For the forward calculation of the H/V ratios in the inversion process, we used the recently developed 'hvgeneralized' method, which is based on the diffuse field assumption, and accounts for the water layer on top of stratified media. Moreover, available prior geological and geophysical information was utilized during the inversion of the H/V curves. We found that subsurface VS ranged from approximately 30 m s−1 at the seabed to approximately 4900 m s−1 at 7000 m below the sea floor (mbsf). Starting with the best model candidate at each OBS location, the effect of the water layer on the H/V curve in the deep ocean was investigated by comparing synthetic H/V curves with and without the water layer. The synthetic H/V analysis revealed that the water layer had a significant effect on H/V amplitudes at higher frequencies (>1 Hz), whereas comparatively little effect was observed at lower frequencies (<1 Hz). This study provides an empirical basis for H/V analysis using OBS data to determine VS down to several kilometres of sedimentary sequences to the upper crust with high-resolution. [ABSTRACT FROM AUTHOR]

Shear wave velocities provide an important constraint on crustal lithology. Limited crustal shear wave data are available from the ultraslow spreading mid-ocean ridges. We combine observations of both compressional (P) and shear (S) waves in ocean–bottom seismometer data from the Southwest Indian Ridge to determine crustal P -wave velocity (Vp), S- wave velocity (Vs), Vp / Vs and Poisson's ratio variations along the ridge at 49°17′E–50°49′E. Similar layered crustal structures were revealed beneath both the magmatically robust segment centres (Vp / Vs of 1.76–1.94, Poisson's ratio of 0.26–0.32) and the non-transform discontinuity (NTD) between them (Vp / Vs of 1.76–2.03 and Poisson's ratio of 0.26–0.34). Because laboratory measurements show an overlap in Poisson's ratio between mafic igneous and ultramafic rocks, particularly at Vp values typical of oceanic Layer 3, it can be difficult to distinguish crustal composition using this parameter only. However, our observed Vp gradients of 0.1 ± 0.1 s−1 suggest that in this area, oceanic Layer 3 consists primarily of mafic igneous rocks both at segment centres and at the NTD. Oceanic crustal Layers 2A and 2B above are likely also to consist of mafic igneous rocks, with some evidence for increased fracturing at the NTD. [ABSTRACT FROM AUTHOR]

We present a new compilation and analysis of broad-band ocean bottom seismometer noise properties from 15 yr of seismic deployments. We compile a comprehensive data set of representative four-component (seismometer and pressure gauge) noise spectra and cross-spectral properties (coherence, phase and admittance) for 551 unique stations spanning 18 U.S.-led experiments. This is matched with a comprehensive compilation of metadata parameters related to instrumentation and environmental properties for each station. We systematically investigate the similarity of noise spectra by grouping them according to these metadata parameters to determine which factors are the most important in determining noise characteristics. We find evidence for improvements in similarity of noise properties when grouped across parameters, with groupings by seismometer type and deployment water depth yielding the most significant and interpretable results. Instrument design, that is the entire deployed package, also plays an important role, although it strongly covaries with seismometer and water depth. We assess the presence of traditional sources of tilt, compliance, and microseismic noise to characterize their relative role across a variety of commonly used seismic frequency bands. We find that the presence of tilt noise is primarily dependent on the type of seismometer used (covariant with a particular subset of instrument design), that compliance noise follows anticipated relationships with water depth, and that shallow, oceanic shelf environments have systematically different microseism noise properties (which are, in turn, different from instruments deployed in shallow lake environments). These observations have important implications for the viability of commonly used seismic analysis techniques. Finally, we compare spectra and coherences before and after vertical channel tilt and compliance noise removal to evaluate the efficacy and limitations of these now standard processing techniques. These findings may assist in future experiment planning and instrument development, and our newly compiled noise data set serves as a building block for more targeted future investigations by the marine seismology community. [ABSTRACT FROM AUTHOR]

Secondary microseisms are the most energetic noise in continuous seismometer recordings. They are generated by interactions between ocean waves, including between gravity waves incident on and reflected from the coast. Coastal reflections of ocean waves leading to coastal microseismic sources are hard to estimate in various global numerical wave models, and independent quantification of these coastal sources through direct measurements can greatly improve these models. Here, we exploit a 41-km-long submarine optical fibre (OF) cable located offshore Toulon, France, using distributed acoustic sensing (DAS). We record both the amplitude and frequency of seafloor strains induced by ocean surface gravity waves, as well as secondary microseisms caused by the interaction of gravity waves incident and reflected from the coast. By leveraging the spatially distributed nature of DAS measurements, additional fundamental information is recovered such as the velocity and azimuth of the waves. We find that on average 30 per cent of the gravity waves are reflected at the coast generating local sources of secondary microseisms that manifest as Scholte waves. These local sources represent the most energetic contribution to the seismic noise recorded along the OF and by an onshore broad-band station located near the DAS interrogator. Furthermore, we estimate a coastal reflection coefficient of ocean surface gravity waves R 2 of about 0.07, which provides improved constraints for seismic noise generation models. In addition, we show that new local sources of microseisms can be generated when gravity waves characteristics (azimuth and frequency content) change and lead to some delays between the OF cable and buoy recordings. These analyses pave the way for a wide use of DAS data to monitor ocean–solid earth interactions as they provide a wealth of information on the reflection of gravity waves, coastal microseismic sources, and new constraints for numerical models of microseismic noise. [ABSTRACT FROM AUTHOR]

Since the Mesozoic, the South China Block (SCB) has experienced multiple episodes of tectonic magmatism accompanied by metallogenesis. To explore the seismic velocity structure of the crust and its connection with mineralization, we constructed a wide-angle reflection and refraction (WARR) profile by deploying 435 short-period seismometers across the Cathaysia Block in Southeast China. Receiver functions and the H–κ stacking method were applied using a reference 2-D velocity model obtained from the analysis of WARR data. Integration of active- and passive-source seismic data enabled us to obtain an image of the crust and the average Vp/Vs ratio value of the SCB and produce several results: (1) The crustal thickness varies west-to-east from 28 km below the Qinhang metallogenic belt to 32 km below the Wuyishan metallogenic belt, and the average value of the Vp/Vs ratio for the crust is 1.71. Both the thickness of the crust and the average Vp/Vs ratio are lower than the average global values for the continental crust. (2) The relatively low Vp/Vs ratio and thin crust along the profile could be generated by thinning of the lower crust in the SCB as a result of the backarc extension related to westward subduction of the Paleo-Pacific plate below East Asia. (3) The Moho depth estimated from H–κ stacking with the help of the background P -wave velocity model modified by the WARR model fits better than using a global seismic velocity model with its common conversion point image, which supports a potential seismic data analysis technique provided with a joint deployment of both active- and passive-source seismic observation system with a shorter period. [ABSTRACT FROM AUTHOR]

Air-to-ground coupled seismic arrivals, resulting from infrasound interacting with the ground surface, can be identified on seismometers co-located with microbarometers. Dense networks of co-located sensors provide detailed spatial information about the infrasonic wavefield, allowing investigation of infrasonic propagation and an assessment of the variability in air-to-ground coupling mechanisms. Signals generated by an explosion at Camp Minden, Louisiana, United States, on 2012 October 16 were recorded across the eastern United States on the USArray Transportable Array. 233 infrasound signals were identified at distances of up to 1566 km from the source, with 105 of these sites, at distances of up to 1147 km from the source, recording above-noise air-to-ground coupled seismic arrivals in the 2–4 Hz passband (chosen to maximise seismic arrival signal-to-noise ratios). The spatially dense infrasound recordings allowed a unique investigation of the azimuthal variation in infrasound amplitudes, showing that stratospheric arrival amplitudes increased by a factor greater than 4 between the edge and centre of the stratospheric propagation duct. Air-to-ground coupling coefficients, calculated as the ratios of temporally coincident peak-to-trough seismic and infrasound time-domain amplitudes, span over two orders of magnitude ([9.0 × 10−8, 5.0 × 10−5] ms−1 Pa−1). Sites exhibiting high coupling coefficients are predominantly located on alluvial sediments that support the generation of air-to-ground coupled Rayleigh waves, resulting in poor coherence between the seismic and infrasonic waveforms. In contrast, sites exhibiting low coupling coefficients are predominantly located on chemically weathered bedrock and only support direct coupling, leading to high coherence between air-to-ground coupled seismic and infrasonic recordings. The Camp Minden explosion observations, and a consideration of expected seismic noise and air-to-ground coupled arrival amplitudes, suggest that co-located pairs of seismometers and microbarometers are likely to record signals from small (tens of tons) explosions at distances of over 1000 km under favourable propagation conditions. Co-located deployments may therefore provide a cost-effective method for augmenting current state-of-the-art infrasound array networks, by providing a higher spatial density of recordings; this may assist in correctly associating recorded signals with, and estimating the location of, source events. [ABSTRACT FROM AUTHOR]

Understanding the crustal structure of the Anatolian Plate has important implications for its formation and evolution, including the extent to which its high elevation is maintained isostatically. However, the numerous teleseismic receiver function studies from which Anatolian Moho depths have been obtained return results that differ by ≤21 km at some seismograph stations. To address this issue, we determine Moho depth and bulk crustal VP / VS ratio (κ) at 582 broad-band seismograph stations, including ∼100 for which H – κ results have not been reported previously. We use a modified H – κ stacking method in which a final solution is selected from a suite of up to 1000 repeat H – κ measurements, each calculated using randomly selected receiver functions and H – κ input parameters. Ten quality control criteria that variously assess the final numerical result, the receiver function data set, and the extent to which the results are clustered tightly, are used to determine station quality. By refining Moho depth constraints, including identifying 182 stations, analysed previously, where H – κ stacking yields unreliable results (particularly in Eastern Anatolia and the rapidly uplifting Taurides), our new crustal model (ANATOLIA-HK21) provides fresh insight into Anatolian crustal structure and topography. Changes in Moho depth within the Anatolian Plate occur on a shorter length-scale than has sometimes previously been assumed. For example, crustal thickness decreases abruptly from >40 km in the northern Kirsehir block to <32 km beneath the Central Anatolian Volcanic Province and Tuz Golu basin. Moho depth increases from 30–35 km on the Arabian Plate to 35–40 km across the East Anatolian Fault into Anatolia, in support of structural geological observations that Arabia–Anatolia crustal shortening was accommodated primarily on the Anatolian, not Arabian, Plate. However, there are no consistent changes in Moho depth across the North Anatolian Fault, whose development along the Intra-Pontide and İzmir-Ankara-Erzincan suture zones was more likely the result of contrasts in mantle lithospheric, not crustal, structure. While the crust thins from ∼45 km below the uplifted Eastern Anatolian Plateau to ∼25 km below lower-lying western Anatolia, Moho depth is generally correlated poorly with elevation. Residual topography calculations confirm the requirement for a mantle contribution to Anatolian Plateau uplift, with localized asthenospheric upwellings in response to slab break-off and/or lithospheric dripping/delamination example candidate driving mechanisms. [ABSTRACT FROM AUTHOR]

The SEIS (Seismic Experiment for Interior Structure) seismometer deployed at the surface of Mars in the framework of the NASA-InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission has been continuously recording the ground motion at Elysium Planitia for more than one martian year. In this work, we investigate the seasonal variation of the near-surface properties using both background vibrations and a particular class of high-frequency seismic events. We present measurements of relative velocity changes over one martian year and show that they can be modelled by a thermoelastic response of the Martian regolith. Several families of high-frequency seismic multiplets have been observed at various periods of the martian year. These events exhibit complex, repeatable waveforms with an emergent character and a coda that is likely composed of scattered waves. Taking advantage of these properties, we use coda wave interferometry (CWI) to measure relative traveltime changes as a function of the date of occurrence of the quakes. While in some families a stretching of the coda waveform is clearly observed, in other families we observe either no variation or a clear contraction of the waveform. These various behaviors correspond to different conditions of illumination at the InSight landing site, depending on the season. Measurements of velocity changes from the analysis of background vibrations above 5 Hz are consistent with the results from CWI. We identify a frequency band structure in the power spectral density (PSD) that can be tracked over hundreds of days. This band structure is the equivalent in the frequency domain of an autocorrelogram and can be efficiently used to measure relative traveltime changes as a function of frequency. We explain how the PSD analysis allows us to circumvent the contamination of the measurements by the Lander mode excitation which is inevitable in the time domain. The observed velocity changes can be adequately modelled by the thermoelastic response of the regolith to the time-dependent incident solar flux at the seasonal scale. In particular, the model captures the time delay between the surface temperature variations and the velocity changes in the subsurface. Our observations could serve as a basis for a joint inversion of the seismic and thermal properties in the first 20 m below InSight. [ABSTRACT FROM AUTHOR]

While distributed acoustic sensing (DAS) has been demonstrated to have great potential in seismology, DAS data often have much higher levels of stochastic and coherent noise (e.g. instrument noise, traffic vibrations) than data collected by traditional seismometers. The linearly, densely spaced nature of DAS arrays presents a suite of opportunities for more innovative processing techniques that can be used to address this issue. One way to take advantage of DAS's array architecture is through the use of curvelets. Curvelets have a non-uniform scaling property that makes them an excellent tool for representing images with discontinuities along piecewise, twice continuously differentiable curves. This anisotropic scaling property makes curvelets an ideal processing tool for DAS data, for which the measured wavefield can be represented as an image composed of curved features. Here, we use the curvelet frame as a tool for the manipulation of DAS signal and demonstrate how this manipulation can improve our ability to identify important features in DAS data sets. We use the curvelet representation to partition the measured wavefield using DAS data collected near Ridgecrest, CA, following the 2019 M w7.1 Ridgecrest earthquake. Here, we isolate the earthquake-induced wavefield from coherent and stochastic noise using the curvelet frame in an effort to improve the results of template matching of the Ridgecrest aftershock sequence. We show that our wavefield-partitioning technique facilitates the identification of over 30 per cent more aftershocks and greatly reduces the magnitude of diurnal depressions in the aftershock catalogue due to cultural noise. [ABSTRACT FROM AUTHOR]

The brutal onset of seismicity offshore Mayotte island North of the Mozambique Channel, Indian Ocean, that occurred in May 2018 caught the population, authorities and scientific community off guard. Around 20 potentially felt earthquakes were recorded in the first 5 d, up to magnitude M w 5.9. The scientific community had little pre-existing knowledge of the seismic activity in the region due to poor seismic network coverage. During 2018 and 2019, the MAYOBS/REVOSIMA seismology group was progressively built between four French research institutions to improve instrumentation and data sets to monitor what we know now as an on-going exceptional submarine basaltic eruption. After the addition of 3 medium-band stations on Mayotte island and 1 on Grande Glorieuse island in early 2019, the data recovered from the Ocean Bottom Seismometers were regularly processed by the group to improve the location of the earthquakes detected daily by the land network. We first built a new local 1-D velocity model and established specific data processing procedures. The local 1.66 low VP / VS ratio we estimated is compatible with a volcanic island context. We manually picked about 125 000 P and S phases on land and sea bottom stations to locate more than 5000 events between February 2019 and May 2020. The earthquakes outline two separate seismic clusters offshore that we named Proximal and Distal. The Proximal cluster, located 10 km offshore Mayotte eastern coastlines, is 20–50 km deep and has a cylindrical shape. The Distal cluster start 5 km to the east of the Proximal cluster and extends below Mayotte's new volcanic edifice, from 50 to 25 km depth. The two clusters appear seismically separated, however our data set is insufficient to firmly demonstrate this. [ABSTRACT FROM AUTHOR]

Tilting of the ground due to loading by the variable atmosphere is known to corrupt very long period horizontal seismic records (below 10 mHz) even at the quietest stations. At BFO (Black Forest Observatory, SW-Germany), the opportunity arose to study these disturbances on a variety of simultaneously operated state-of-the-art broad-band sensors. A series of time windows with clear atmospherically caused effects was selected and attempts were made to model these 'signals' in a deterministic way. This was done by simultaneously least-squares fitting the locally recorded barometric pressure and its Hilbert transform to the ground accelerations in a bandpass between 100 and 3600 s periods. Variance reductions of up to 97 per cent were obtained. We show our results by combining the 'specific pressure induced accelerations' for the two horizontal components of the same sensor as vectors on a horizontal plane, one for direct pressure and one for its Hilbert transform. It turned out that at BFO the direct pressure effects are large, strongly position dependent and largely independent of atmospheric events for instruments installed on piers, while three post-hole sensors are only slightly affected. The infamous 'cavity effects' are invoked to be responsible for these large effects on the pier sensors. On the other hand, in the majority of cases all sensors showed very similar magnitudes and directions for the vectors obtained for the regression with the Hilbert transform, but highly variable from event to event especially in direction. Therefore, this direction most certainly has to do with the gradient of the pressure field moving over the station which causes a larger scale deformation of the crust. The observations are very consistent with these two fundamental mechanisms of how fluctuations of atmospheric surface pressure causes tilt noise. The results provide a sound basis for further improvements of the models for these mechanisms. The methods used here can already help to reduce atmospherically induced noise in long-period horizontal seismic records. [ABSTRACT FROM AUTHOR]

Information on fault zone structure is essential for our understanding of earthquake mechanics, continental deformation and seismic hazard. We use the scattered seismic wavefield to study the subsurface structure of the North Anatolian Fault Zone (NAFZ) in the region of the 1999 İzmit and Düzce ruptures using data from an 18-month dense deployment of seismometers with a nominal station spacing of 7 km. Using the forward- and back-scattered energy that follows the direct P -wave arrival from teleseismic earthquakes, we apply a scattered wave inversion approach and are able to resolve changes in lithospheric structure on a scale of 10 km or less in an area of about 130 km by 100 km across the NAFZ. We find several crustal interfaces that are laterally incoherent beneath the surface strands of the NAFZ and evidence for contrasting crustal structures either side of the NAFZ, consistent with the presence of juxtaposed crustal blocks and ancient suture zones. Although the two strands of the NAFZ in the study region strike roughly east–west, we detect strong variations in structure both north–south, across boundaries of the major blocks, and east–west, parallel to the strike of the NAFZ. The surface expression of the two strands of the NAFZ is coincident with changes on main interfaces and interface terminations throughout the crust and into the upper mantle in the tomographic sections. We show that a dense passive network of seismometers is able to capture information from the scattered seismic wavefield and, using a tomographic approach, to resolve the fine scale structure of crust and lithospheric mantle even in geologically complex regions. Our results show that major shear zones exist beneath the NAFZ throughout the crust and into the lithospheric mantle, suggesting a strong coupling of strain at these depths. [ABSTRACT FROM AUTHOR]

First-arrival traveltime tomography is one of the most used velocity model building techniques especially in sparse wide-angle acquisitions for deep crustal seismic imaging cases. Relying on the inversion of a picked attribute, the absolute traveltimes, the approach is ill-posed in terms of non-uniqueness of the solution. The latter is remedied by proper regularization or the introduction of prior information. Indeed, since traveltime kernels are vulnerable to the velocity–depth ambiguity, the inversion is stabilized by the introduction of complementary data like reflections and explicit reflectors in the velocity models. Here, we propose to supplement first-arrival traveltimes by their slopes, in other words the horizontal component of the slowness vectors at the sources and/or receivers. Slopes are a crucial attribute in state of the art scattering-based or reflection-based tomographic methods like slope tomography or wavefront tomography where the differential information is needed in order to locate the scattering events position or to parametrize the wavefront. The optional but valuable injection of slopes as an objective measure in first-arrival traveltime tomography stabilizes the problem by constraining the emergence angle or in turn implicitly the turning point depth of the rays. We explain why slopes have a tremendous added value in such a tomographic problem and highlight its remedial effect in cases where the medium is unevenly illuminated. We also show that the contribution of slopes become even more significant when the acquisition is sparse as it is generally the case with ocean-bottom seismometer surveys. The inferred models from such an extended time-attributes tomography will be used as initial guesses in a full-waveform inversion workflow context. The proposed strategy is benchmarked in 2-D media against a dip section of the SEG/EAGE overthrust model and then followed by a revisit of ocean bottom seismometers data from the eastern-Nankai subduction margin as a real deep crustal case study. [ABSTRACT FROM AUTHOR]

Full waveform inversion (FWI) is a data-fitting technique capable of generating high-resolution velocity models with a resolution down to half the seismic wavelength. FWI is applied typically to densely sampled seismic data. In this study, we applied FWI to 3-D wide-angle seismic data acquired using sparsely spaced ocean bottom seismometers (OBSs) from the Deep Galicia Margin west of Iberia. Our data set samples the S-reflector, a low-angle detachment present in this area. Here we highlight differences between 2-D, 2.5-D and 3-D-FWI performances using a real sparsely spaced data set. We performed 3-D FWI in the time domain and compared the results with 2-D and 2.5-D FWI results from a profile through the 3-D model. When overlaid on multichannel seismic images, the 3-D FWI results constrain better the complex faulting within the pre- and syn-rift sediments and crystalline crust compared to the 2-D result. Furthermore, we estimate variable serpentinization of the upper mantle below the S-reflector along the profile using 3-D FWI, reaching a maximum of 45 per cent. Differences in the data residuals of the 2-D, 2.5-D and 3-D inversions suggest that 2-D inversion can be prone to overfitting when using a sparse data set. To validate our results, we performed tests to recover the anomalies introduced by the inversions in the final models using synthetic data sets. Based on our comparison of the velocity models, we conclude that the use of 3-D data can partially mitigate the problem of receiver sparsity in FWI. [ABSTRACT FROM AUTHOR]

The 2011 M w 9.0 Tohoku-oki earthquake ruptured the subduction megathrust fault in the central Japan Trench. We investigated the aftershock activity in the southern Japan Trench to the south of the main rupture area using ocean bottom seismographs deployed both landward and seaward of the trench. In the trench-outer rise region seaward of the trench axis, we identified several ∼100-km-long linear earthquake trends both parallel and oblique to the southern Japan Trench. The earthquake trend oblique to the southern Japan Trench is a southward extension of the trench-parallel linear earthquake trend in the central to northern Japan Trench. The trench-parallel normal-faults in the trench-outer rise region could extend linearly, despite the change of the trench strike from N–S to NNE–SSW to the south of the main rupture area. Normal-faults oblique to the trench should be considered as substantial parts of large intraplate normal-faulting earthquakes. In addition, intraplate seismicity coinciding with the lower velocity oceanic mantle suggest that the structure heterogeneity would be indicative of normal-faults extending into the mantle. In the trench landward area, earthquake activity showed along-trench variations. Earthquakes along the shallow megathrust interface near the trench were observed south of 37°N. These shallow near-trench regular earthquakes, which are located close to the episodic tremors and temporally correlated with the tremor activities, suggest that the afterslip on the plate interface likely extended to the shallow plate interface close to the trench axis. Smaller spatial scale structure heterogeneity, such as the thickness variation in the channel-like low-velocity sedimentary unit, likely relate to the proximity of the regular earthquakes and slow slip which results in the formation of diverse slip behaviours in the shallow subduction zone of the southern Japan Trench. [ABSTRACT FROM AUTHOR]

Single-component ring lasers have provided high-resolution observations of Earth's rotation rate as well as local earthquake- or otherwise-induced rotational ground motions. Here, we present the design, construction and operational aspects of ROMY, a four-component, tetrahedral-shaped ring laser installed at the Geophysical Observatory Fürstenfeldbruck near Munich, Germany. Four equilateral, triangular-shaped ring lasers with 12 m side length provide rotational motions that can be combined to construct the complete vector of Earth's rotation from a point measurement with very high resolution. Combined with a classic broad-band seismometer, we obtain the most accurate 6 degree-of-freedom ground motion measurement system to date, enabling local and teleseismic observations as well as the analysis of ocean-generated Love and Rayleigh waves. The specific design and construction details are discussed as are the resulting consequences for permanent observations. We present seismic observations of local, regional and global earthquakes as well as seasonal variations of ocean-generated rotation noise. The current resolution of polar motion is discussed and strategies how to further improve long-term stability of the multicomponent ring laser system are presented. [ABSTRACT FROM AUTHOR]

This paper describes an efficient approach to high-resolution time-reversal source imaging simulation. Dense seismograph network records are backpropagated from stations to the hypocentre through a 3-D subsurface structure model to estimate the initial source wavefield at the earthquake initiation time. By assimilating high-density observational data into the time-reversal wave propagation, a clearer source image can be determined, even for deep and distant earthquakes, than is achievable with conventional source imaging. The effectiveness of data-assimilation-based source imaging by a time-reversal wave propagation simulation with a 3-D heterogeneous structural model was demonstrated using recordings from a nationwide strong-motion seismograph network during the 2007 Off Niigata, Japan, M w 6.6 earthquake, and the 2007 Off Ibaraki, Japan, M w 6.8 earthquake. Such data-assimilated-based simulations are also effective for early forecasting of strong ground motions caused by large earthquakes through fast time-advancing simulations based on the current assimilated wavefield. We will discuss the feasibility of a disaster prevention system for the early forecasting of strong motion disasters due to large earthquakes, based on repeatedly estimating source parameters and forecasting strong motions in future time based on the current assimilated wavefields. [ABSTRACT FROM AUTHOR]

A fraction of the acoustic wave energy (from the atmosphere) may couple into the ground, and it can thus be recorded as ground motion using seismometers. We have investigated this coupling, with two questions in mind, (i) how strong it is for small explosive sources and offsets up to a few tens of meters and (ii) what we can learn about the shallow subsurface from this coupling. 25 firecracker explosions and five rocket explosions were analysed using colocated seismic and infrasound sensors; we find that around 2 per cent of the acoustic energy is admitted into the ground (converted to seismic energy). Transfer coefficients are in the range of 2.85–4.06 nm Pa–1 for displacement, 1.99–2.74 μm s–1 Pa–1 for velocity, and 2.2–2.86 mm s−2 Pa–1 for acceleration. Recording dynamic air pressure together with ground motion at the same site allows identification of different waves propagating in the shallow underground, notably the seismic expression of the direct airwave, and the later air-coupled Rayleigh wave. We can reliably infer shallow ground properties from the direct airwave, in particular the two Lamé constants (λ and μ) and the Poisson ratio. Firecrackers as pressure sources allow constraining elastic parameters in the top-most layer. In this study, they provide frequency-dependent values of λ decreasing from 119 MPa for low frequencies (48 Hz) to 4.2 MPa for high frequencies (341 Hz), and μ values decreasing from 33 to 1.8 MPa. Frequency-dependent Poisson ratios ν are in the range of 0.336–0.366. [ABSTRACT FROM AUTHOR]

We present a new approach to simulate high-frequency seismic wave propagation in and under the oceans. Based upon AxiSEM3D, this method supports a fluid ocean layer, with associated water-depth phases and seafloor topography (bathymetry). The computational efficiency and flexibility of this formulation means that high-frequency calculations may be carried out with relatively light computational loads. A validation of the fluid ocean implementation is shown, as is an evaluation of the oft-used ocean loading formulation, which we find breaks down at longer periods than was previously believed. An initial consideration of the effects of seafloor bathymetry on seismic wave propagation is also given, wherein we find that the surface waveforms are significantly modified in both amplitude and duration. When compared to observed data from isolated island stations in the Pacific, synthetics which include a global ocean and seafloor topography appear to more closely match the observed waveform features than synthetics generated from a model with topography on the solid surface alone. We envisage that such a method will be of use in understanding the new and exciting ocean-bottom and floating seismometer data sets now being regularly collected. [ABSTRACT FROM AUTHOR]

Distributed acoustic sensing (DAS) acquisition is becoming more and more popular for its dense sampling at a lower cost than seismometers. However, data processing for DAS data is challenging, especially for surface-deployedfibers, in which only the horizontal component of strain variation is effectively recorded. Also, the coupling between the fiber and the Earth is usually poor and the recorded single-component data are noisy. Thus, we introduce data processing strategies dedicated to enhancing the ambient-noise and active-source seismic data recorded by a horizontally deployed tactical fiber-optics cable buried in a sand dune area in Saudi Arabia. We propose a similarity-weighted stacking of randomly selected short-time duration windows to generate virtual common shot gathers (CSG) from the recorded ambient noise. The similarity-weighted stacking only counts the primary contributions of coherent events, while a short-time correlation can suppress the crosstalk usually present in late arrivals. The stacking fold is preserved or even can be increased by generating plenty of random time segments compared to stacking the full recording time. For the recorded active-source data, we skip the interferometric step, but use the envelope of the CSG. The envelope is needed to mitigate the complexity of waveforms, while preserving the slopes of arrivals. Then, we use the wave-equation-based Rayleigh-wave dispersion spectrum inversion, which utilizes all the dispersion modes available and does not require picking the dispersion curve, in estimating the shallow S -wave velocities. The local cross-correlation objective function allows for additional freedom in matching the modelled and observed data, and thus, helps us avoid falling into a local minimum when starting with kinematically poor velocity models. [ABSTRACT FROM AUTHOR]

The Mid-Atlantic Ridge at 13°N is regarded as a type locality for oceanic core complexes (OCCs), as it contains, within ∼70 km along the spreading axis, four that are at different stages of their life cycle. The wealth of existing seabed observations and sampling makes this an ideal target to resolve contradictions between the existing models of OCC development. Here we describe the results of P -wave seismic tomographic modelling within a 60 × 60 km footprint, containing several OCCs, the ridge axis and both flanks, which determines OCC crustal structure, detachment geometry and OCC interconnectivity along axis. A grid of wide-angle seismic refraction data was acquired along a series of 17 transects within which a network of 46 ocean-bottom seismographs was deployed. Approximately 130 000 first arrival traveltimes, together with sparse Moho reflections, have been modelled, constraining the crust and uppermost mantle to a depth of ∼10 km below sea level. Depth slices through this 3-D model reveal several independent structures each with a higher P -wave velocity (Vp) than its surrounds. At the seafloor, these features correspond to the OCCs adjacent to the axial valley walls at 13°20′N and 13°30′N, and off axis at 13°25′N. These high- Vp features display dipping trends into the deeper crust, consistent with the surface expression of each OCC's detachment, implying that rocks of the mid-to-lower crust and uppermost mantle within the footwall are juxtaposed against lower Vp material in the hangingwall. The neovolcanic zone of the ridge axis has systematically lower Vp than the surrounding crust at all depths, and is wider between OCCs. On average, throughout the 13°N region, the crust is ∼6 km-thick. However, beneath a deep lava-floored basin between axial OCCs the crust is thinner and is more characteristically oceanic in layering and velocity–depth structure. Thicker crust at the ridge axis suggests a more magmatic phase of current crustal formation, while modelling of the sparse Moho reflections suggests the crust–mantle boundary is a transition zone throughout most of the 13°N segment. Our results support a model in which OCCs are bounded by independent detachment faults whose dip increases with depth and is variable with azimuth around each OCC, suggesting a geometry and mechanism of faulting that is more complicated than previously thought. The steepness of the northern flank of the 13°20′N detachment suggests that it represents a transfer zone between different faulting regimes to the south and north. We propose that individual detachments may not be linked along-axis, and that OCCs act as transfer zones linking areas of normal spreading and detachment faulting. Along ridge variation in magma supply influences the nature of this detachment faulting. Consequently, not only does magma supply control how detachments rotate and migrate off axis before finally becoming inactive, but also how, when and where new OCCs are created. [ABSTRACT FROM AUTHOR]

In late June 2016, the Harmalière clayey landslide (located 30 km south of the city of Grenoble, French Alps) was dramatically reactivated at the headscarp after a 35-yr-long period of continuous but limited activity. The total involved volume, which moved as sliding blocks of various sizes, was estimated to be about 2 × 10 6 m3. Two seismometers were installed at the rear of the main headscarp in August 2016, on both sides of a developing fracture delineating a block with a volume of a few hundred cubic metres. For 4 months, they continuously recorded seismic ambient vibrations and microearthquakes until the block broke. Five seismic parameters were derived from the monitoring: the cumulative number of microearthquakes (CNe), the seismic energy (SE), the block resonance frequency (fB), the relative variation in Rayleigh wave velocity (d V/V) deduced from noise cross-correlations between the two sensors and the associated correlation coefficient (CC). All parameters showed a significant precursory signal before the rupture, but at very different times, which indicates the complexity of the rupture mechanism in this clay material. [ABSTRACT FROM AUTHOR]

Breakthroughs in understanding the structure and dynamics of our planet will strongly depend upon instrumenting deep oceans. Progress has been made these last decades in ocean-bottom seismic observations, but ocean-bottom seismometer (OBS) temporary deployments are still challenging and face set-up limitations. Launched from oceanographic vessels, OBSs fall freely and may slightly drift laterally, dragged by currents. Therefore, their actual orientation and location on the landing sites are hard to assess precisely. Numerous techniques have been developed to retrieve this key information, but most of them are costly, time-consuming or inaccurate. In this work, we show how ship noise can be used as an acoustic source of opportunity to retrieve both the orientation and the location of OBSs on the ocean floor. To retrieve the OBS orientation, we developed a first method based on a combination of seismic and pressure data through the use of the acoustic intensity. This latter can be used to quantify the OBS orientation from the ship noise direction of arrival (DOA), which can then be compared with known ship trajectories obtained from the automatic identification system (AIS). To accurately relocate OBSs, we also developed a second method based on the hydrophone data which computes distances of acoustical sources by measuring time differences of arrival (TDOA) between direct and reverberated phases. The OBS location is then retrieved by fitting measured ship distances with known ship trajectories. In this study, a full network of OBSs deployed in the SW Indian Ocean was reoriented and a test station was relocated. We demonstrate that our new methods may quantify the OBS orientation with an accuracy of about one degree, and its location with an accuracy of a few tens of metres, depending on the number of ships used in the analysis. [ABSTRACT FROM AUTHOR]

Station noise levels play a fundamental limitation in our ability to detect seismic signals. These noise levels are frequency-dependent and arise from a number of physically different drivers. At periods greater than 100 s, station noise levels are often limited by the self-noise of the instrument as well as the sensitivity of the instrument to non-seismic noise sources. Recently, station operators in the Global Seismographic Network (GSN) have deployed several Streckeisen STS-6A very broad-band borehole seismometers. These sensors provide a potential replacement for the no-longer-produced Streckeisen STS-1 seismometer and the GeoTech KS-54 000 borehole seismometer. Along with showing some of the initial observational improvements from installing modern very broad-band seismometers at depth, we look at current limitations in the seismic resolution from earth tide periods 100 000 s (0.01 mHz) to Nyquist at most GSN sites (0.02 s or 50 Hz). Finally, we show the potential for improved observations of continuously excited horizontal Earth hum as well as the splitting of very long-period torsional modes. Both of these observations make use of the low horizontal noise levels which are obtained by installing very broad-band borehole seismometers at depth. [ABSTRACT FROM AUTHOR]

We use seismic noise cross-correlations to obtain a 3-D tomography model of SV -wave velocities beneath the western Indian Ocean, in the depth range of the oceanic crust and uppermost mantle. The study area covers 2000 × 2000 km2 between Madagascar and the three spreading ridges of the Indian Ocean, centred on the volcanic hotspot of La Réunion. We use seismograms from 38 ocean bottom seismometers (OBSs) deployed by the RHUM-RUM project and 10 island stations on La Réunion, Madagascar, Mauritius, Rodrigues, and Tromelin. Phase cross-correlations are calculated for 1119 OBS-to-OBS, land-to-OBS, and land-to-land station pairs, and a phase-weighted stacking algorithm yields robust group velocity measurements in the period range of 3–50 s. We demonstrate that OBS correlations across large interstation distances of >2000 km are of sufficiently high quality for large-scale tomography of ocean basins. Many OBSs yielded similarly good group velocity measurements as land stations. Besides Rayleigh waves, the noise correlations contain a low-velocity wave type propagating at 0.8–1.5 km s−1 over distances exceeding 1000 km, presumably Scholte waves travelling through seafloor sediments. The 100 highest-quality group velocity curves are selected for tomographic inversion at crustal and lithospheric depths. The inversion is executed jointly with a data set of longer-period, Rayleigh-wave phase and group velocity measurements from earthquakes, which had previously yielded a 3-D model of Indian Ocean lithosphere and asthenosphere. Robust resolution tests and plausible structural findings in the upper 30 km validate the use of noise-derived OBS correlations for adding crustal structure to earthquake-derived tomography of the oceanic mantle. Relative to crustal reference model CRUST1.0, our new shear-velocity model tends to enhance both slow and fast anomalies. It reveals slow anomalies at 20 km depth beneath La Réunion, Mauritius, Rodrigues Ridge, Madagascar Rise, and beneath the Central Indian spreading ridge. These structures can clearly be associated with increased crustal thickness and/or volcanic activity. Locally thickened crust beneath La Réunion and Mauritius is probably related to magmatic underplating by the hotspot. In addition, these islands are characterized by a thickened lithosphere that may reflect the depleted, dehydrated mantle regions from which the crustal melts where sourced. Our tomography model is available as electronic supplement. [ABSTRACT FROM AUTHOR]

We recently found the original Omori seismograms recorded at Hongo, Tokyo, of the 1922 Atacama, Chile, earthquake (MS  = 8.3) in the historical seismogram archive of the Earthquake Research Institute (ERI) of the University of Tokyo. These recordings enable a quantitative investigation of long-period seismic radiation from the 1922 earthquake. We document and provide interpretation of these seismograms together with a few other seismograms from Mizusawa, Japan, Uppsala, Sweden, Strasbourg, France, Zi-ka-wei, China and De Bilt, Netherlands. The 1922 event is of significant historical interest concerning the cause of tsunami, discovery of G wave, and study of various seismic phase and first-motion data. Also, because of its spatial proximity to the 1943, 1995 and 2015 great earthquakes in Chile, the 1922 event provides useful information on similarity and variability of great earthquakes on a subduction-zone boundary. The 1922 source region, having previously ruptured in 1796 and 1819, is considered to have significant seismic hazard. The focus of this paper is to document the 1922 seismograms so that they can be used for further seismological studies on global subduction zones. Since the instrument constants of the Omori seismographs were only incompletely documented, we estimate them using the waveforms of the observed records, a calibration pulse recorded on the seismogram and the waveforms of better calibrated Uppsala Wiechert seismograms. Comparison of the Hongo Omori seismograms with those of the 1995 Antofagasta, Chile, earthquake (M w = 8.0) and the 2015 Illapel, Chile, earthquake (M w = 8.3) suggests that the 1922 event is similar to the 1995 and 2015 events in mechanism (i.e. on the plate boundary megathrust) and rupture characteristics (i.e. not a tsunami earthquake) with M w = 8.6 ± 0.25. However, the initial fine scale rupture process varies significantly from event to event. The G1 and G2, and R1 and R2 of the 1922 event are comparable in amplitude, suggesting a bilateral rupture, which is uncommon for large megathrust earthquakes. [ABSTRACT FROM AUTHOR]

The ice cap covering Antarctica has long limited our understanding of the continental-scale crustal model due to its inaccessibility and the resulting logistical difficulties when executing geophysical field work, such as seismograph deployment. Resolving a high spatial resolution crustal model for Antarctica where seismographs are sparsely distributed stimulates scientific interest in this relatively less studied continent. In this study, we utilize satellite gravity observations from the global gravity model EIGEN-6C4 to create an alternative crustal thickness model of Antarctica. The gravity data were corrected for sediments, topography and ice cover. Furthermore, we considered the gravity effect due to vertical deformation of the lithosphere caused by ice load besides the earth's curvature in the modelling. We inverted the corrected gravity data using the regularized Bott's inversion method in spherical approximation and constrained the results by seismic observations. This crustal thickness model shows a thicker average crust in East Antarctica and a thinner one in West Antarctica. The thickest crust is in the Gamburtsev Subglacial Mountains with a Moho depth of over 40 km. The thicker crust is particularly evident along the Transantarctic Mountains and the Dronning Maud lands. Comparisons with existing models show a good correlation in gravity-constrained areas. Differences appear in the sedimentary basins and crust with thickness closer to seismic point observations. Overall, our crustal model is relatively improved than the existing gravity derived models. [ABSTRACT FROM AUTHOR]

We propose a new Bayesian method to reveal the Vs structure of the near surface of the earth using spatial autocorrelation (SPAC) functions and apply this new method to synthetic, broad-band and geophone data sets. The principle of SPAC is introduced, and an implementation of the Bayesian Monte Carlo inversion (BMCI) for modelling SPAC coherency functions is described. To demonstrate its effectiveness, BMCI is applied to synthetic tests, data from 14 SPAC array sites in the Salt Lake Valley (SLV), Utah, and two arrays (one broad-band and one geophone) located in south central Utah. The Vs models derived from previous SPAC analysis of the 14 SLV sites differ by 10 per cent at most from those determined by BMCI and lie within uncertainties determined for the BMCI models. These agreements demonstrate the effectiveness of the BMCI method. The synthetic tests and applications to the SLV SPAC data show BMCI has great potential to resolve Vs structure down to at least 400 m. To achieve resolution for deeper Vs structure, longer duration deployments, wider array apertures, and additional seismometers or geophones can be used. Additionally, when the target frequencies are greater than 0.1 Hz, there is no apparent disadvantage in using geophone data for BMCI compared to broad-band data. Most significantly, BMCI places a quantifiable constraint on the uncertainties of the Vs models as well as Vs30. [ABSTRACT FROM AUTHOR]

Long-period Rayleigh wave horizontal to vertical amplitude (H/V) ratios at a station provide information about local earth structure that is complementary to phase velocity. However, a number of studies have observed that significant scatter appears in these measurements making it difficult to use H/V ratio measurements to resolve earth structure. Some of the scatter in these measurements has been attributed to local geological structure while some has remained unaccounted for. Most Global Seismographic Network (GSN) stations contain two nearby high-quality broad-band seismometers (e.g. in the same vault, but on different piers or in different boreholes). For each broad-band sensor in the IRIS/USGS component of the GSN, we estimate H/V ratios of fundamental mode Rayleigh waves using M  > 6.5 earthquakes from 2001 to 2018 (around 19 000 measurements). We compute these ratios at a number of discrete periods (25, 50, 75, 100 and 150 s) and find that for well-isolated Rayleigh waves (windows where the correlation coefficients between radial and the phase-shifted vertical components are greater than 0.9) significant scatter in H/V ratios occurs between colocated sensors (greater than 25 per cent at 100 s period). This suggests the scatter in H/V ratio measurements can be at least partially attributed to extremely local phenomena such as sensor emplacement in the vault. We also find that H/V ratios can vary as a function of event backazimuth, indicating that care must be taken when computing average ratios for a station, as a large number of events from a given region could bias H/V ratio measurements at a station. [ABSTRACT FROM AUTHOR]

In the Sea of Marmara, areas of gas seepage or cold seeps are tightly related to the faults system and understanding the spatial and temporal dynamics in gas-related processes is crucial for geohazard mitigation. Although acoustic surveys proved to be efficient in detecting and locating cold seeps, temporal variability or trends in the gas-related processes are still poorly understood. Two arrays of 10 ocean bottom seismometers were deployed in the western part of the Sea of Marmara in 2011 and 2014, respectively. In addition to the local seismic events, the instruments recorded a large number of short duration events and long-lasting tremors. Short duration events are impulsive signals with duration <1 s, amplitude well above the noise level and a frequency spectrum with one or two narrow peaks. They are not correlated from one site to another, suggesting a very local source. Tremors consist of sequences of clustered impulsive signals lasting for minutes to more than an hour with a multipeak frequency spectrum. Based on evidence of known seepage and by analogy with volcanic and hydrothermal models, we suggest that short duration events and tremors are associated with gas migration and seepage. There is a relationship between tremors associated with gas emission and the local seismicity, although not systematic. Rather than triggering gas migration out of the seabed, locally strong earthquakes act as catalysts when gas is already present or gas emission is already initiated. [ABSTRACT FROM AUTHOR]

As the number of seismic sensors grows, it is becoming increasingly difficult for analysts to pick seismic phases manually and comprehensively, yet such efforts are fundamental to earthquake monitoring. Despite years of improvements in automatic phase picking, it is difficult to match the performance of experienced analysts. A more subtle issue is that different seismic analysts may pick phases differently, which can introduce bias into earthquake locations. We present a deep-neural-network-based arrival-time picking method called "PhaseNet" that picks the arrival times of both P and S waves. Deep neural networks have recently made rapid progress in feature learning, and with sufficient training, have achieved super-human performance in many applications. PhaseNet uses three-component seismic waveforms as input and generates probability distributions of P arrivals, S arrivals and noise as output. We engineer PhaseNet such that peaks in the probability distributions provide accurate arrival times for both P and S waves. PhaseNet is trained on the prodigious available data set provided by analyst-labelled P and S arrival times from the Northern California Earthquake Data Center. The data set we use contains more than 700 000 waveform samples extracted from over 30 yr of earthquake recordings. We demonstrate that PhaseNet achieves much higher picking accuracy and recall rate than existing methods when applied to the waveforms of known earthquakes, which has the potential to increase the number of S -wave observations dramatically over what is currently available. This will enable both improved locations and improved shear wave velocity models. [ABSTRACT FROM AUTHOR]

We apply receiver function (RF) methodology to map the geometry of a sedimentary basin along a ∼10-km-long profile of broadband seismometers that recorded continuously for approximately 3 months. For a subset of the stations, we apply the Neighbourhood Algorithm inversion scheme, to quantify the geometry of basin bounding fault directly beneath the stations. We compare our results with active reflection seismic data and with the lithostratigraphy from a well located along the profile. We find that the P -to-s conversions from the sediments–basement interface (SBI), recorded in RF data sets together with information on intrabasin structures, are useful for obtaining high resolution images of the basin. The depth of the SBI derived from RF inversion is consistent (within ∼0.4 km) with the estimates from active reflection seismic and the well data. This study highlights that analysis of teleseismic waveforms can retrieve relevant information on the structure of a sedimentary basin. [ABSTRACT FROM AUTHOR]

We formulate a full wavefield inversion (FWFI) for ambient seismic noise recorded by large and dense seismograph arrays. FWFI exploits the constraints on the gradients of the wavefield that array data inherently possess. We pose FWFI as a partial differential equation (PDE) constrained inverse problem resulting in a joint estimation of a reconstructed wavefield and the medium parameters. The inverse problem is solved by variable projection. We explicitly allow for non-unique solutions to the PDE system that is imposed as a constraint. The boundary conditions of the wavefield do not need to be specified, and can remain unknown. This makes the algorithm suitable for inverting observations of ambient seismic noise by dense arrays. The result is that the inverse problem for subsurface properties becomes insensitive to the character and distribution of the noise sources that excited the seismic wavefield. In principle the formulation holds equally for ambient noise wavefields and for wavefields excited by controlled sources. The theory is supported with examples in one dimension in the time domain, and in two dimensions in the frequency domain. The latter are of interest in the inversion of surface wave ambient noise for phase velocity maps. [ABSTRACT FROM AUTHOR]

Several days of passive seismic broad-band recordings (vertical component) from a dense 3 × 6 km array installed near Chémery (France), with about 100 seismometers, are analysed for wavefield characterization between 0.1 and 3 Hz. Backazimuth is determined by using the Multiple Signal Characterization (MUSIC) algorithm at frequencies below 1 Hz, and non-coherent cross-correlation beamforming above 1 Hz, since the latter is less sensitive to aliasing issues. A novel method of determining the wavefield velocity is introduced, consisting of processing a cross-correlation common-offset gather by the MUSIC algorithm. The fundamental and three higher modes of Rayleigh waves (R0, R1, R2 and R3) are identified under 1 Hz. Above 1.5 Hz, the L g phase is detected, while R0 and R1 are also present. Roughly between 1 and 1.5 Hz, a quicker phase, probably P g, is detected. Both P g and L g are dominant during night time, suggesting they have a natural origin, which is also consistent with their backazimuth pointing towards the Atlantic. Large scale 2-D spectral-element simulations using deep- and shallow-water ocean sources confirm the possibility of the L g phase excitation. Thus, even above 1 Hz, natural sources can explain the major part of the ambient noise energy during quiet time periods. [ABSTRACT FROM AUTHOR]

Many applications in seismology rely on the accurate absolute timing of seismograms. However, both seismological land stations and ocean bottom seismometers (OBSs) can be affected by clock errors, which cause the absolute timing of seismograms to deviate from a highly accurate reference time signal, usually provided by GPS satellites. Timing problems can occur in land stations when synchronization with a GPS signal is temporarily or permanently lost. This can give rise to complicated, time-dependent clock drifts relative to GPS time, due to varying environmental conditions. Seismometers at the ocean bottom cannot receive GPS satellite signals, but operate in more stable ambient conditions than land stations. The standard protocol is to synchronize an OBS with a GPS signal immediately before deployment and after recovery. The measured timing deviation, called ‘skew’, is assumed to have accumulated linearly over the deployment interval, an assumption that is plausible but usually not verifiable. In recent years, cross-correlations of ambient microseismic noise have been put to use for correcting timing errors, but have been limited to interstation distances of at most a few tens of kilometres without reducing the temporal resolution. We apply noise cross-correlations to the evaluation of clock errors in four broad-band land stations and 53 wideband and broad-band OBSs, which were installed on and around the island of La Réunion in the western Indian Ocean during the RHUM-RUM (Réunion Hotspot and Upper Mantle-Réunions Unterer Mantel) experiment. We correlate all three seismic components, plus a hydrophone channel in OBS stations. Daily cross-correlation functions are derived for intermediate distances (∼20 km) for land-to-land station pairs; stable, 10 d stacks are obtained for very large interstation distances up to >300 km for land-to-OBS and OBS-to-OBS configurations. Averaging over multiple station pairs, and up to 16 component pairs per station, improves the accuracy of the method by a factor of four compared to the single-channel approaches of prior studies. The timing accuracy of our method is estimated to be ∼20 ms standard deviation or one sample at a sampling rate of 50 Hz. In land stations, nonlinear clock drifts and clock jumps of up to 6 min are detected and successfully corrected. For 52 out of 53 OBSs, we successfully obtain drift functions over time, which validate the common assumption of linear clock drift. Skew values that were available for 29 of these OBSs are consistent with our independent estimates within their observational error bars. For 23 OBSs that lacked skew measurements, linear OBS clock drifts range between 0.2 and 8.8 ms d−1. In addition to linear drift, three OBSs are affected by clock jumps of ∼1 s, probably indicating a missing sample problem that would otherwise have gone undetected. Thus we demonstrate the routine feasibility of high-accuracy clock corrections in land and OBSs over a wide range of interstation distances. [ABSTRACT FROM AUTHOR]

We present a crustal-scale seismic profile in the Barents Sea based on new data. Wide-angle seismic data were recorded along a 600 km long profile at 38 ocean bottom seismometer and 52 onshore station locations. The modelling uses the joint refraction/reflection tomography approach where co-located multichannel seismic reflection data constrain the sedimentary structure. Further, forward gravity modelling is based on the seismic model.We also calculate net regional erosion based on the calculated shallow velocity structure. Our model reveals a complex crustal structure of the Baltic Shield to Barents shelf transition zone, as well as strong structural variability on the shelf itself.We document large volumes of pre-Carboniferous sedimentary strata in the transition zone which reach a total thickness of 10 km. A high-velocity crustal domain found below the Varanger Peninsula likely represents an independent crustal block. Large lower crustal bodies with very high velocity and density below the Varanger Peninsula and the Fedynsky High are interpreted as underplated material that may have fed mafic dykes in the Devonian. We speculate that these lower crustal bodies are linked to the Devonian rifting processes in the East European Craton, or belonging to the integral part of the Timanides, as observed onshore in the Pechora Basin. [ABSTRACT FROM AUTHOR]