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Seismic signals produced by wind turbines can have an adverse effect on seismological measurements up to distances of several kilometres. Based on numerical simulations of the emitted seismic wave field, we study the effectivity of seismic borehole installations as a way to reduce the incoming noise. We analyse the signal amplitude as a function of sensor depth and investigate effects of seismic velocities, damping parameters and geological layering in the subsurface. Our numerical approach is validated by real data from borehole installations affected by wind turbines. We demonstrate that a seismic borehole installation with an adequate depth can effectively reduce the impact of seismic noise from wind turbines in comparison to surface installations. Therefore, placing the seismometer at greater depth represents a potentially effective measure to improve or retain the quality of the recordings at a seismic station. However, the advantages of the borehole decrease significantly with increasing signal wavelength. [ABSTRACT FROM AUTHOR]

Seismic signals produced by wind turbines can have an adverse effect on seismological measurements up to distances of several kilometres. Based on numerical simulations of the emitted seismic wavefield, we study the effectivity of seismic borehole installations as a way to reduce the incoming noise. We analyse the signal amplitude as a function of sensor depth and investigate effects of seismic velocities, damping parameters and geological layerings in the subsurface. Our numerical approach is validated by real data from borehole installations affected by wind turbines. We demonstrate that a seismic borehole installation with an adequate depth can effectively reduce the impact of seismic noise from wind turbines in comparison to surface installations. Therefore, placing the seismometer at greater depth represents a potentially effective measure to improve or retain the quality of the recordings at a seismic station. However, the advantages of the borehole decrease significantly with increasing signal wavelength. [ABSTRACT FROM AUTHOR]

Records from ocean bottom seismometers (OBSs) are highly contaminated by noise, which is much stronger compared to data from most land stations, especially on the horizontal components. As a consequence, the high energy of the oceanic noise at frequencies below 1 Hz considerably complicates the analysis of the teleseismic earthquake signals recorded by OBSs. Previous studies suggested different approaches to remove low-frequency noises from OBS recordings but mainly focused on the vertical component. The records of horizontal components, which are crucial for the application of many methods in passive seismological analysis of body and surface waves, could not be much improved in the teleseismic frequency band. Here we introduce a noise reduction method, which is derived from the harmonic–percussive separation algorithms used in Zali et al. (2021), in order to separate long-lasting narrowband signals from broadband transients in the OBS signal. This leads to significant noise reduction of OBS records on both the vertical and horizontal components and increases the earthquake signal-to-noise ratio (SNR) without distortion of the broadband earthquake waveforms. This is demonstrated through tests with synthetic data. Both SNR and cross-correlation coefficients showed significant improvements for different realistic noise realizations. The application of denoised signals in surface wave analysis and receiver functions is discussed through tests with synthetic and real data. [ABSTRACT FROM AUTHOR]

Distributed Acoustic Sensing (DAS) is an emerging technology for acquiring seismic data on virtual sensors densely distributed along an optical fiber. The broadband response of the sensors, associated with the possibility of deploying fiber optic cables in harsh conditions and the relatively moderate cost of this sensing element gives clear perspectives for DAS in geothermal wells to contribute to the monitoring operations of geothermal plants. However, the technical feasibility of managing the large flow of data generated by the DAS and the suitability of the strain-rate acquisitions to monitor locally induced seismicity was yet to be assessed. We propose a monitoring concept establishing DAS as an effective component of the seismic monitoring of the Schäftlarnstraße geothermal plant (Munich, Germany). The underlying data management system links the existing on-site infrastructure, including the fiber optic cable deployed in one of the site's injection wells and the associated DAS recorder, to a cloud Internet-of-Things (IoT) platform designed to deliver both a secure storage environment for the DAS acquisitions and optimized computing resources for their processing. The proposed solution was tested over a period of six months and showed the feasibility of efficiently acquiring and processing the large flow of continuous DAS data. For seismic risk mitigation purposes, we additionally investigate the potential of the monitoring concept to tend towards real-time monitoring. The processing outcomes, focusing especially on two detected local seismic events, demonstrates the relevance of DAS from geothermal wells for the (micro)seismic monitoring of the geothermal site. Despite the noisy operational conditions, the applied processing workflow takes advantage of the sensors' high spatial density for data denoising and event triggering and highlights that higher detection sensitivity than conventional seismometers can be achieved. From a different perspective, further analyses of the DAS records confirm the logging capabilities of the technology, especially regarding well completion integrity. The 6-months test period shows that permanent DAS can be integrated as a routine seismic monitoring component of geothermal plants and advantageously complement surface seismometer based networks, especially in urban environments. [ABSTRACT FROM AUTHOR]

The Liguro-Provençal basin was formed as a back-arc basin of the retreating Calabrian–Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica–Sardinia block is associated with rifting, shaping the Ligurian Basin. It is still debated whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We perform ambient noise tomography, also taking into account teleseismic events, using an amphibious network of seismic stations, including 22 broadband ocean bottom seismometers (OBSs), to investigate the lithospheric structure of the Ligurian Basin. The instruments were installed in the Ligurian Basin for 8 months between June 2017 and February 2018 as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we carefully pre-process the data, including corrections for instrument tilt and seafloor compliance and excluding higher modes of the ambient-noise Rayleigh waves. We calculate daily cross-correlation functions for the AlpArray OBS array and surrounding land stations. We also correlate short time windows that include teleseismic earthquakes, allowing us to derive surface wave group velocities for longer periods than using ambient noise only. We obtain group velocity maps by inverting Green's functions derived from the cross-correlation of ambient noise and teleseismic events, respectively. We then used the resulting 3D group velocity information to calculate 1D depth inversions for S-wave velocities. The group velocity and shear-wave velocity results compare well to existing large-scale studies that partly include the study area. In onshore France, we observe a high-velocity area beneath the Argentera Massif, roughly 10 km below sea level. We interpret this as the root of the Argentera Massif. Our results add spatial resolution to known seismic velocities in the Ligurian Basin, thereby augmenting existing seismic profiles. In agreement with existing seismic studies, our shear-wave velocity maps indicate a deepening of the Moho from 12 km at the south-western basin centre to 20–25 km at the Ligurian coast in the north-east and over 30 km at the Provençal coast. The maps also indicate that the south-western and north-eastern Ligurian Basin are structurally separate. The lack of high crustal vP/vS ratios beneath the south-western part of the Ligurian Basin preclude mantle serpentinisation there. [ABSTRACT FROM AUTHOR]

The northern margin of the Ligurian Basin shows notable seismicity at the Alpine front, including frequent magnitude 4 events. Seismicity decreases offshore towards the Basin centre and Corsica, revealing a diffuse distribution of low-magnitude earthquakes. We analyse data of the amphibious AlpArray seismic network with focus on the offshore component, the AlpArray ocean bottom seismometer (OBS) network, consisting of 24 broadband OBSs deployed for 8 months, to reveal the seismicity and depth distribution of micro-earthquakes beneath the Ligurian Sea. Two clusters occurred between ∼ 10 km to ∼ 16 km depth below the sea surface, within the lower crust and uppermost mantle. Thrust faulting focal mechanisms indicate compression and an inversion of the Ligurian Basin, which is an abandoned Oligocene–Miocene rift basin. The basin inversion is suggested to be related to the Africa–Europe plate convergence. The locations and focal mechanisms of seismicity suggest reactivation of pre-existing rift-related structures. Slightly different striking directions of presumed rift-related faults in the basin centre compared to faults further east and hence away from the rift basin may reflect the counter-clockwise rotation of the Corsica–Sardinia block. High mantle S-wave velocities and a low Vp/Vs ratio support the hypothesis of strengthening of crust and uppermost mantle during the Oligocene–Miocene rifting-related extension and thinning of continental crust. [ABSTRACT FROM AUTHOR]

Icequakes are the result of processes occurring within the ice mass or between the ice and its environment. Studying icequakes provides a unique view on ice dynamics, specifically on the basal conditions. Changes in conditions due to environmental or climate changes are reflected in icequakes. Counting and characterizing icequakes is thus essential to monitor them. Most of the icequakes recorded by the seismic station at the Belgian Princess Elisabeth Antarctica Station (PE) have small amplitudes corresponding to maximal displacements of a few nanometres. Their detection threshold is highly variable because of the rapid and strong changes in the local seismic noise level. Therefore, we evaluated the influence of katabatic winds on the noise measured by the well-protected PE surface seismometer. Our purpose is to identify whether the lack of icequake detection during some periods could be associated with variations in the processes generating them or simply with a stronger seismic noise linked to stronger wind conditions. We observed that the wind mainly influences seismic noise at frequencies greater than 1 Hz. The seismic noise power exhibits a bilinear correlation with the wind velocity, with two different slopes at a wind velocity lower and greater than 6 m s -1 and with, for example at a period of 0.26 s, a respective variation of 0.4 dB (m -1 s) and 1.4 dB (m -1 s). These results allowed a synthetic frequency and wind-speed-dependent noise model to be presented that explains the behaviour of the wind-induced seismic noise at PE, which shows that seismic noise amplitude increases exponentially with increasing wind speed. This model enables us to study the influence of the wind on the original seismic dataset, which improves the observation of cryoseismic activity near the PE station. [ABSTRACT FROM AUTHOR]

The versatility and cost efficiency of fibre-optic distributed acoustic sensing (DAS) technologies facilitate geophysical monitoring in environments that were previously inaccessible for instrumentation. Moreover, the spatio-temporal data density permitted by DAS naturally appeals to seismic array processing techniques, such as beamforming for source location. However, the measurement principle of DAS is inherently different from that of conventional seismometers, providing measurements of ground strain rather than ground motion, and so the suitability of traditional seismological methods requires in-depth evaluation. In this study, we evaluate the performance of a DAS array in the task of seismic beamforming, in comparison with a co-located nodal seismometer array. We find that, even though the nodal array achieves excellent performance in localising a regional ML 4.3 earthquake, the DAS array exhibits poor waveform coherence and consequently produces inadequate beamforming results that are dominated by the signatures of shallow scattered waves. We demonstrate that this behaviour is likely inherent to the DAS measurement principle, and so new strategies need to be adopted to tailor array processing techniques to this emerging measurement technology. One strategy demonstrated here is to convert the DAS strain rates to particle velocities by spatial integration using the nodal seismometer recordings as a reference, which dramatically improves waveform coherence and beamforming performance and warrants new types of "hybrid" array design that combine dense DAS arrays with sparse seismometer arrays. [ABSTRACT FROM AUTHOR]

We demonstrate the capability of distributed acoustic sensing (DAS) to record volcano-related dynamic strain at Etna (Italy). In summer 2019, we gathered DAS measurements from a 1.5 km long fibre in a shallow trench and seismic records from a conventional dense array comprised of 26 broadband sensors that was deployed in Piano delle Concazze close to the summit area. Etna activity during the acquisition period gives the extraordinary opportunity to record dynamic strain changes (∼ 10 -8 strain) in correspondence with volcanic events. To validate the DAS strain measurements, we explore array-derived methods to estimate strain changes from the seismic signals and to compare with strain DAS signals. A general good agreement is found between array-derived strain and DAS measurements along the fibre optic cable. Short wavelength discrepancies correspond with fault zones, showing the potential of DAS for mapping local perturbations of the strain field and thus site effect due to small-scale heterogeneities in volcanic settings. [ABSTRACT FROM AUTHOR]

The Liguro-Provençal basin was formed as a back-arc basin of the retreating Calabrian-Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica-Sardinia block is associated with rifting, shaping the Ligurian Sea. It is still debated whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We invert velocity models using an amphibious network of seismic stations, including 22 broadband Ocean Bottom Seismometers (OBS) to investigate the lithospheric structure of the Ligurian sea. The instruments were installed in the Ligurian Sea for eight months between June 2017 and February 2018 as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we attentively pre-process the data, including corrections for instrument tilt and seafloor compliance. We took extra care to exclude higher modes of the ambient-noise Rayleigh waves. We calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. Additionally, we correlate short time windows that include teleseismic earthquakes that allow us to derive surface wave group velocities for longer periods than using ambient noise only. Group velocity maps are obtained by inverting Green's functions derived from the cross-correlation of ambient noise and teleseismic events, respectively. We then used the resulting 3D group velocity information to calculate 1D depth inversions for S-wave velocities. The shear-wave velocity results show a deepening of the Moho from 12 km at the southwestern basin centre to 20-25 km at the Ligurian coast in the northeast and over 30 km at the Provençal coast. We find no hint on mantle serpentinisation and no evidence for an Alpine slab, at least down to depths of 25 km. However, we see a separation of the southwestern and northeastern Ligurian Basin that coincides with the promoted prolongation of the Alpine front. [ABSTRACT FROM AUTHOR]

Seismometers have detected the social response to lockdown measures implemented following the onset of the COVID-19 pandemic in cities around the world. This long-lasting pandemic has been a particular challenge in countries such as Mexico, where the informal economy constitutes most of the working population. This context motivated the monitoring of the mobility of populations throughout the various phases of lockdown measures independently of people's access to the internet and mobile technology. Here we use the variation of anthropogenic seismic noise in the city of Querétaro (central Mexico) recorded by a network of low-cost Raspberry Shake seismic stations to study the spatial and temporal variation of human activity in the city throughout the pandemic and during sporting events. The results emphasize the importance of densifying urban seismic networks and of tracking human activities without the privacy concerns associated with mobile technology. [ABSTRACT FROM AUTHOR]

The city of Auckland, New Zealand (Tāmaki Makaurau, Aotearoa), sits on top of an active volcanic field. Seismic stations in and around the city monitor activity of the Auckland Volcanic Field (AVF) and provide data to image its subsurface. The seismic sensors – some positioned at the surface and others in boreholes – are generally noisier during the day than during nighttime. For most stations, weekdays are noisier than weekends, proving human activity contributes to recordings of seismic noise, even on seismographs as deep as 384 m below the surface and as far as 15 km from Auckland's Central Business District. Lockdown measures in New Zealand to battle the spread of COVID-19 allow us to separate sources of seismic energy and evaluate both the quality of the monitoring network and the level of local seismicity. A matched-filtering scheme based on template matching with known earthquakes improved the existing catalogue of five known local earthquakes to 35 for the period between 1 November 2019 and 15 June 2020. However, the Level-4 lockdown from 25 March to 27 April – with its drop in anthropogenic seismic noise above 1 Hz – did not mark an enhanced detection level. Nevertheless, it may be that wind and ocean swell mask the presence of weak local seismicity, particularly near surface-mounted seismographs in the Hauraki Gulf that show much higher levels of noise than the rest of the local network. [ABSTRACT FROM AUTHOR]

The northern margin of the Ligurian Basin shows notable seismicity at the Alpine front, including frequent magnitude 4 events. Seismicity decreases offshore towards the Basin centre and Corsica, revealing a diffuse distribution of low magnitude earthquakes. We analyse data of the amphibious AlpArray seismic network with focus on the offshore component, the AlpArray OBS network, consisting of 24 broadband ocean bottom seismometers deployed for eight months, to reveal the seismicity and depth distribution of micro-earthquakes beneath the Ligurian Sea. Two clusters occurred between ~10 km to ~16 km depth below sea surface, within the lower crust and uppermost mantle. Thrust faulting focal mechanisms indicate compression and an inversion of the Ligurian Basin, which is an abandoned Oligocene rift basin. The Basin inversion is suggested to be related to the Africa-Europe plate convergence. The locations and focal mechanisms of seismicity suggest reactivation of pre-existing rift structures. Slightly different striking directions of faults in the basin centre compared to faults further east and hence away from the abandoned rift may mimic the counter-clockwise rotation of the Corsica-Sardinia block during ~20-16 Ma. The observed cluster events support the hypothesis of strengthening of crust and uppermost mantle during rifting related extension and thinning of continental crust. [ABSTRACT FROM AUTHOR]

We demonstrate the capability of Distributed Acoustic Sensing (DAS) in recording volcano related dynamic strain at Etna (Italy). In summer 2019, we gathered DAS measurements from a 1.5 km long fibre in a shallow trench and seismic records from a conventional dense array comprising 26 broadband sensors deployed in Piano delle Concazze close to the summit area. The multifaceted style of Etna activity during the acquisition period gives the extraordinary opportunity to record and detect tiny strain changes (few 10-8 strain) in correspondence with volcanic events. To validate the DAS strain measures, we explored array-derived methods to estimate strain changes from the seismic signals and to compare with strain DAS signals. A general good agreement was found between array-derived strain and DAS measures along the fibre optic cable. Short wavelength discrepancies correspond with fault zones, showing the potential of DAS in mapping local perturbations of the strain field, and thus site effect, due to small-scale heterogeneities in volcanic settings. [ABSTRACT FROM AUTHOR]

Seismometers have detected the social response to lockdown measures implemented following the onset of the COVID-19 pandemic in cities around the world. This long-lasting lockdown has been a particular challenge in countries such as Mexico, where the informal economy constitutes most of the working population. This context motivated the monitoring of the mobility of populations throughout the various phases of lockdown measures, independently from people's access to the internet and mobile technologies. Here we use the variation of anthropogenic seismic noise in the city of Querétaro (central Mexico) recorded by a network of low-cost Raspberry Shake seismic stations to study the spatial and temporal variation of human activity in the city throughout the pandemic and during sportive events. The results emphasize the importance of densifying urban seismic networks and tracking human activities without the privacy concerns associated with mobile technologies. [ABSTRACT FROM AUTHOR]

The versatility and cost-efficiency of fibre-optic Distributed Acoustic Sensing (DAS) technologies facilitate geophysical monitoring in environments that were previously inaccessible for instrumentation. Moreover, the spatio-temporal data density permitted by DAS naturally appeals to seismic array processing techniques, such as beamforming for source location. However, the measurement principle of DAS is inherently different from that of conventional seismometers, providing measurements of ground strain rather than ground motion, and so the suitability of traditional seismological methods requires in-depth evaluation. In this study, we evaluate the performance of a DAS array in the task of seismic beamforming, in comparison with a co-located nodal seismometer array. We find that, even though the nodal array achieves excellent performance in localising a regional ML 4.3 earthquake, the DAS array exhibits poor waveform coherence and consequently produces inadequate beamforming results that are dominated by the signatures of shallow scattered waves. We demonstrate that this behaviour is likely inherent to the DAS measurement principle, and so new strategies need to be adopted to tailor array processing techniques to this emerging measurement technology. [ABSTRACT FROM AUTHOR]

A shallow Mw 5.3 earthquake near Lake Muir in the stable continental region (SCR) crust of southwest Western Australia on the 16 September 2018 was followed on the 8 November by a proximal Mw 5.2 event. Focal mechanisms produced for the events suggest reverse and strike-slip rupture, respectively. Field mapping, guided by Sentinel-1 InSAR data, reveals that the first event produced an approximately 3 km long and up to 0.4–0.6 m high west-facing surface rupture, consistent with reverse slip on a moderately east-dipping fault. The InSAR data also show that the surface scarp relates to a subsurface rupture ∼ 5 km long, bound at its northern and southern extremities by bedrock structures. The November event produced a surface deformation envelope that is spatially coincident with that of the September event but did not result in discrete surface rupture. Almost 900 aftershocks were recorded by a temporary seismometer deployment. Hypocentre locations correlate poorly with the rupture plane of their respective mainshocks but correlate well with regions of increased Coulomb stress. The spatial and temporal relationships between the Mw>5.0 events and their aftershocks reveals dependencies with implications for how other less well-documented SCR earthquake sequences could be interpreted. Furthermore, the September Mw 5.3 Lake Muir earthquake was the ninth event documented to have produced surface rupture in Australia in historical times. These nine ruptures are located exclusively in the Precambrian non-extended SCR rocks of central and western Australia, and none could have been identified and mapped using topographic signature prior to the historical event. Consistent, though fragmentary, evidence exists from analogous regions worldwide. Our analysis of the Lake Muir earthquake sequence therefore provides constraint on models describing mechanisms for strain accumulation and localized release as earthquakes in non-extended SRC crust. [ABSTRACT FROM AUTHOR]

Joint marine electromagnetic (EM) and seismic interpretations are widely used for offshore gas hydrate and petroleum exploration to produce improved estimates of lithology and fluids and to decrease the risk of low gas saturation. However, joint data acquisition is not commonly employed. Current marine EM data acquisition depends on an ocean bottom electromagnetic receiver (OBEM), and current seismic exploration methods use seismometers. Joint simultaneous data acquisition can decrease costs and improve efficiency, but conventional independent data receivers have several drawbacks, including a large size, high costs, position errors, and low operational efficiencies. To address these limitations, we developed a compact ocean bottom electromagnetic receiver and seismometer (OBEMS). Based on existing ocean bottom E -field receiver (OBE) specifications, including low noise levels, low power consumption, and low time drift errors, we integrated two induction coils for the magnetic sensor and a three-axis omnidirectional geophone for the seismic sensor to assemble an ultra-short baseline (USBL) transponder as the position sensor, which improved position accuracy and operational efficiency while reducing field data acquisition costs. The resulting OBEMS has a noise level of 0.1 nV m -1 rt -1 (Hz) at 1 Hz in the E -field, 0.1 pT rt -1 (Hz) at 1 Hz in the B -field, and a 30 d battery lifetime. This device also supports a Wi-Fi interface for the configuration of data acquisition parameters and data download. Offshore acquisition was performed to evaluate the system's field performance during offshore gas hydrate exploration. The OBEMS operated effectively throughout the operation and field testing. Therefore, the OBEMS can function as a low-cost, compact, and highly efficient joint data acquisition method. [ABSTRACT FROM AUTHOR]

Joint marine electromagnetic (EM) and seismic interpretation are widely used for offshore gas hydrate and petroleum exploration, produce better estimates of lithology and fluids, and decrease the risk of low gas saturation. However, joint data acquisition is not commonly employed. Current marine EM data acquisition depends on an ocean bottom electromagnetic receiver (OBEM) and current seismic exploration methods use seismometers. Joint simultaneous data acquisition can decrease costs and improve efficiency; yet conventional independent data receivers have several drawbacks, including large size, high costs, position errors, and low operational efficiency. To address these limitations, we developed a compact ocean bottom electromagnetic receiver and seismometer (OBEMS). Based on existing ocean bottom E-field receiver (OBE) specifications, including low noise levels, low power consumption, and low clock-drift error, we integrated two induction coils for the magnetic sensor and a three-axis omnidirectional geophone for the seismic sensor and assembled an ultra-short base line (USBL) transponder as the position sensor, which improved position accuracy and operational efficiency while reducing field data acquisition costs. The resulting OBEMS has a noise level of 0.1 nV/m/rt (Hz) at 1 Hz in E-field and 0.1 pT/rt (Hz) at 1 Hz in B-field and a 30 day battery lifetime. It also supports a WiFi interface for configuring data acquisition parameters and data download. Offshore acquisition was performed to evaluate the system's field performance during offshore gas hydrate exploration. The OBEMS functioned effectively throughout operation and field testing. The OBEMS therefore functions as a low cost, compact, and highly efficient joint data acquisition method. [ABSTRACT FROM AUTHOR]

Imaging via pre-stack depth migration (PSDM) of reflection towed-streamer multichannel seismic (MCS) data at the scale of the whole crust is inherently difficult. This is because the depth penetration of the seismic wavefield is controlled, firstly, by the acquisition design, such as streamer length and air-gun source configuration, and secondly by the complexity of the crustal structure. Indeed, the limited length of the streamer makes the estimation of velocities from deep targets challenging due to the velocity–depth ambiguity. This problem is even more pronounced when processing 2-D seismic data due to the lack of multi-azimuthal coverage. Therefore, in order to broaden our knowledge about the deep crust using seismic methods, we present the development of specific imaging workflows that integrate different seismic data. Here we propose the combination of velocity model building using (i) first-arrival tomography (FAT) and full-waveform inversion (FWI) of wide-angle, long-offset data collected by stationary ocean-bottom seismometers (OBSs) and (ii) PSDM of short-spread towed-streamer MCS data for reflectivity imaging, with the former velocity model as a background model. We present an application of such a workflow to seismic data collected by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER) in the eastern Nankai Trough (Tokai area) during the 2000–2001 Seize France Japan (SFJ) experiment. We show that the FWI model, although derived from OBS data, provides an acceptable background velocity field for the PSDM of the MCS data. From the initial PSDM, we refine the FWI background velocity model by minimizing the residual move-outs (RMOs) picked in the pre-stack-migrated volume through slope tomography (ST), from which we generate a better-focused migrated image. Such integration of different seismic datasets and leading-edge imaging techniques led to greatly improved imaging at different scales. That is, large to intermediate crustal units identified in the high-resolution FWI velocity model extensively complement the short-wavelength reflectivity inferred from the MCS data to better constrain the structural factors controlling the geodynamics of the Nankai Trough. [ABSTRACT FROM AUTHOR]

The Sumatran subduction zone exhibits strong seismic and tsunamogenic potential with the prominent examples of the 2004, 2005 and 2007 earthquakes. Here, we invert travel-time data of local earthquakes for vp and vp=vs velocity models of the central Sumatran forearc. Data were acquired by an amphibious seismometer network consisting of 52 land stations and 10 ocean-bottom seismometers located on a segment of the Sumatran subduction zone that had not ruptured in a great earthquake since 1797 but witnessed recent ruptures to the north in 2005 (Nias earthquake, Mw D 8:7) and to the south in 2007 (Bengkulu earthquake, Mw D 8:5). The 2-D and 3-D vp velocity anomalies reveal the downgoing slab and the sedimentary basins. Although the seismicity pattern in the study area appears to be strongly influenced by the obliquely subducting Investigator Fracture Zone to at least 200 km depth, the 3-D velocity model shows prevailing trench-parallel structures at depths of the plate interface. The tomographic model suggests a thinned crust below the basin east of the forearc islands (Nias, Pulau Batu, Siberut) at ~180 km distance to the trench. vp velocities beneath the magmatic arc and the Sumatran fault zone (SFZ) are around 5 km s-1 at 10 km depth and the vp=vs ratios in the uppermost 10 km are low, indicating the presence of felsic lithologies typical for continental crust. We find moderately elevated vp=vs values of 1.85 at ~150 km distance to the trench in the region of the Mentawai Fault. vp=vs ratios suggest an absence of large-scale alteration of the mantle wedge and might explain why the seismogenic plate interface (observed as a locked zone from geodetic data) extends below the continental forearc Moho in Sumatra. Reduced vp velocities beneath the forearc basin covering the region between the Mentawai Islands and the Sumatra mainland possibly reflect a reduced thickness of the overriding crust. [ABSTRACT FROM AUTHOR]

This paper focuses on major issues related to the data reliability and network performance of 20 broadband (BB) stations of the Czech (CZ) MOBNET (MOBile NETwork) seismic pool within the AlpArray seismic experiments. Currently used high-resolution seismological applications require high-quality data recorded for a sufficiently long time interval at seismological observatories and during the entire time of operation of the temporary stations. In this paper we present new hardware and software tools we have been developing during the last two decades while analysing data from several international passive experiments. The new tools help to assure the high-quality standard of broadband seismic data and eliminate potential errors before supplying data to seismological centres. Special attention is paid to crucial issues like the detection of sensor misorientation, timing problems, interchange of record components and/or their polarity reversal, sensor mass centring, or anomalous channel amplitudes due to, for example, imperfect gain. Thorough data quality control should represent an integral constituent of seismic data recording, preprocessing and archiving, especially for data from temporary stations in passive seismic experiments. Large international seismic experiments require enormous efforts from scientists from different countries and institutions to gather hundreds of stations to be deployed in the field during a limited time period. In this paper, we demonstrate the beneficial effects of the procedures we have developed for acquiring a reliable large set of high-quality data from each group participating in field experiments. The presented tools can be applied manually or automatically on data from any seismic network. [ABSTRACT FROM AUTHOR]

We conducted controlled releases of single blocks within a soft-rock (black marls) gully of the Rioux Bourdoux torrent (French Alps). 28 blocks, with masses ranging from 76 kg to 472 kg, were used for the experiment. An instrumentation combining video cameras and seismometers was deployed along the traveled path. The video cameras allow to reconstruct the trajectories of the blocks and to estimate their velocities at the time of the different impacts with the slope. These data are compared to the recorded seismic signals. As the distance between the falling block and the seismic sensors at the time of each impact is known, we were able to determine the associated seismic signal amplitude corrected from propagation and attenuation effects. We compared the velocity, the loss of potential energy, the kinetic energy and the momentum of the block at each impact to the true amplitude and the energy of the corresponding part of the seismic signal. Our results suggest that the amplitude of the seismic signal scales with the momentum of the block at the impact. We also found a scaling law between the potential energy lost, the kinetic energy and the energy of the seismic radiation generated by the impacts. By combining these scaling laws, we inferred the mass and the velocity before impact of each block directly from the seismic signal. Despite high uncertainties, the values found are close to the true values of the mass and the velocities of the blocks. These relationships also provide new insights to understand the source of high-frequency seismic signals generated by rockfalls. [ABSTRACT FROM AUTHOR]

Eastern Austria is a region of low to moderate seismicity, and hence the seismological network coverage is relatively sparse. Nevertheless accurate earthquake location is very important, as the area is one of the most densely populated and most developed areas in Austria. In 2013 a series of earthquakes with magnitudes up to 4.2 was recorded in the Southern Vienna Basin. With portable broadband, semi-permanent, and permanent installed seismic sensors from different institutions it was possible to record the main- and aftershocks with an unusual multitude of closeby seismic stations. In this study we combine records from all available stations up to 240 km distance in one dataset. First, we stabilize the location with three stations deployed in the epicentral area. The higher network density moves the location of smaller magnitude events closer to the main shocks, with respect to preliminary locations achieved by permanent and semi-permanent networks. Then we locate with NonLinLoc using consistent picks, a 3-D velocity model and apply station corrections. This second approach results in stable epicenters, for limited and even changing station availability This dataset can then be inspected more closely for the presence of regional phases, which then can be used for more accurate localizations and especially depth estimation. Further research will address directivity effects and the asymmetry in earthquake intensity observed throughout the area, using double differences and cross-correlations. [ABSTRACT FROM AUTHOR]

We plan to deploy in the Taranto Gulf some Ocean Bottom broadband Seismometer with Hydrophones. Our aim is to investigate the offshore seismicity of the Sibari Gulf. The seismographic network optimization consists in the identification of the optimal sites for the installation of the offshore stations, which is a crucial factor for the success of the monitoring campaign. In this paper, we propose a two steps automatic procedure for the identification of the best stations geometry. In the first step, based on the application of a set of a priori criteria, the suitable sites to host the ocean bottom seismic stations are identified. In the second step, the network improvement is evaluated for all the possible stations geometries by means of numerical simulation. The application of this procedure allows us to identify the best stations geometry to be achieved in the monitoring campaign. [ABSTRACT FROM AUTHOR]

The lateral variation of the stress field in the southern Aegean plate and the subducting Hellenic slab is determined from recordings of seismicity obtained with the CYCNET and EGELADOS networks in the years from 2002 to 2007. First motions from 7000 5 well-located earthquakes were analysed to produce 540 well-constrained focal mechanisms. They were complemented by another 140 derived by waveform matching of records from larger events. Most of these earthquakes fall into 16 distinct spatial clusters distributed over the southern Aegean region. For each cluster, a stress inversion could be carried out yielding consistent estimates of the stress field and its spatial varia10 tion. At crustal levels, the stress field is generally dominated by a steeply dipping compressional principal stress direction except in places where coupling of the subducting slab and overlying plate come into play. Tensional principal stresses are generally subhorizontal. Just behind the forearc, the crust is under arc-parallel tension whereas in the volcanic areas around Kos, Columbo and Astypalea tensional and intermedi15 ate stresses are nearly degenerate. Further west and north, in the Santorini-Amorgos graben and in the area of the islands of Mykonos, Andros and Tinos, tensional stresses are significant and point around the NW-SE direction. Very similar stress fields are observed in western Turkey with the tensional axis rotated to NNE-SSW. Intermediate depth earthquakes below 100 km in the Nisyros region indicate that the Hellenic slab 20 experiences slab-parallel tension at these depths. The direction of tension is close to east-west and thus deviates from the local NW-oriented slab dip presumably owing to the segmentation of the slab. Beneath the Cretan sea, at shallower levels, the slab is under NW-SE compression. The lateral and depth variations of the stress field reflect the various agents that influence tectonics in the Aegean: subduction of the Hellenic 25 slab, incipient collision with continental African lithosphere, roll back of the slab in the south-east, segmentation of the slab, arc volcanism and extension of the Aegean crust. [ABSTRACT FROM AUTHOR]

In recent years many devastating earthquakes with Mw ≥ 7.5 occurred during 2017-2018 asa result of active plate interactions near the major mega-thrust subduction zones. Theyreveal tectonic complexities and astonishing deformation styles associated with theplate movements. In this study, we determined source mechanism parameters andfinite-fault slip distribution models of four destructive earthquakes (Mw ≥ 7.5) byperforming point-source and finite-fault slip inversions. The shapes and amplitudesof long-period and broad-band P- and SH-waveforms recorded by the Federationof Digital Seismograph Networks (FDSN) and the Global Digital SeismographNetwork (GDSN) stations in the distance range of 30˚ - 90˚ are compared withsynthetic waveforms. Finite-fault slip models of earthquakes are estimated by applyinga hybrid back-projection method that uses teleseismic P-waveforms to integratethe direct P-phase with reflected phases from structural discontinuities near thesource. Overall results indicate various faulting mechanisms at relatively shallowfocal depths (h<70 km). For example, the February 25, 2018 Papua New Guinea(Mw: 7.5) earthquake indicates a thrust faulting mechanism associated with theconvergence of Australia and Pacific Plates. The September 28, 2018 Sulawesi (Indonesia)earthquake (Mw: 7.5), occurred within the Molucca Sea micro-plate at the eastern part ofIndonesia shows a strike-slip faulting mechanism reflecting the complex deformationsrelated to the interactions between Australia, Sunda, Pacific and Philippine Seaplates. Regardless strike-slip mechanism involved, this large earthquake producedunexpected tsunami waves along the coastal planes of Sulawesi. Finite-fault slipmodels reveal several heterogeneous rupture propagations and slip distributions onfault planes of those earthquakes. In addition, distributions of P- wave first motionpolarities recorded at near-field and regional seismic stations are consistent withthe best-fitting minimum misfit source mechanism solutions of earthquakes. Thisstudy is partially supported by the Turkish Academy of Sciences (TÜBA-GEBIP). [ABSTRACT FROM AUTHOR]

Despite its ground installation at about 1.5 meter from the lander, SEIS (Seismic Experiment for Interior Structure) will be sensitive to the ground displacement generated by the lander, in addition to direct forces from the lander propagating along the tether. This lander noise is also expected in the VBB (Very Broad Band seismometers) bandwidth, including below the gravest resonance mode of the lander, expected close or below 10 Hz. In the VBB bandwidth, the lander deck acceleration has been monitored by the SPs (Short Period seismometers) when the latter were operating on it. These measurements provide key constrains on the level of deck acceleration and possibly deck rotation and can therefore be used to predict the forces generated by the lander on the ground and to compare them with the observations made by the SEIS VBBs on the ground. We use formalism developed by Fayon et al. (2017) to model the lander and its 3 feet and complete the approach of Murdoch et al (2017) by adding the torques generated on each foot by the lander rotations, which might likely be important in the bandwidth of the VBB sensors, e.g. [0-10 Hz]. We then identify on the ground VBB signals the largest resonances in the 0-10 Hz bandwidth, including in term of polarized signals on the 3 axes components and analyze their amplitude as a function of wind level and pressure fluctuation. For all the identified VBB resonances, we repeat the analysis using the SP data on the deck. We present preliminary results and tests on a noise rejection algorithm, in order to improve the capability of the VBBs to detect non-lander atmospheric signals and seismic signals in the DC-10 Hz bandwidth. We finally conclude by discussing the opportunity offered by this lander signal to better characterize the wind flow on the lander. [ABSTRACT FROM AUTHOR]

The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission landed successfully on Mars on 26 November, 2018 at 4.51°N, 135.97°E in Elysium Planitia. InSight's seismic payload, SEIS, includes the short-period (SP) sensors optimized to investigate the martian seismic signal above a few Hz [1, 2]. In particular, the three axes of the short-period (SP) sensor contained within the SEIS payload have survived the rigors of launch, cruise, EDL (entry descent and landing) and, finally, the pyrotechnic shocks during the deployment of the assembly into its final configuration as a seismic station on Mars. The SPs operated for a total of 210 hours from landing to sol 40. The first data was acquired on Sol 4, with six additional sols of on-deck observations. SEIS was deployed on to the surface of Mars on Sol 24. Further SP data was recorded on the ground, with the very-broad-band sensor (VBB) joining SP operations after SEIS levelling on sol 35. The SP has generally operated whenever thermal constraints have allowed, usually in the second half of each sol, after system observations confirmed that the UHF-induced currents in the tether did not pose an operational risk. In doing so, the sensor has satisfied its performance, thermal and shock requirements for the mission [2]. Prior to landing, the two horizontal SPs acquired data during the cruise to Mars on 16 and 19 July and 29-30 August with the free-fall environment allowing the performance of the sensors to be determined and validating their noise requirements. Moreover, the sensor has operated on the InSight lander deck from sol 4 of the mission, owing to its ability to function over a wide range of tilts [1]. This has enabled the dynamical characterization of the lander up to 40Hz with a sensitivity below 10 nm/s2/rtHz [3]. Off the deck, the ground motion response is modified only by the LVL mount [4] and the direct coupling of environmental sources [5]. These observations have allowed the characterization of the environmental injections from around noon to 22:00 LTST, showing noisy and quiet regimes. The analysis of the on and off deck observations, therefore, determines the propagation of lander generated and atmospheric sources to the sensor assembly. This is essential for the comprehension of the recorded seismograms [3, 4]. We will present a summary of the SP's initial results on and off the deck.[1] Pike et al. Proc. IEEE MEMS, (2018) [2] Lognonné et al., in press, Space Science review, 2019.[3] Murdoch et al. Space Sci Rev (2017) 211: 457. [4] Knapmeyer-Endrun et al. Space Sci Rev (2018) 214: 94[5] Mimoun et al. Space Sci Rev (2017) 211: 383. [ABSTRACT FROM AUTHOR]

Earthquake early warning (EEW) plays an important role in earthquake disaster mitigation. There are two types of EEW system, regional and on-site. On-site EEW systems analyze initial part of the seismic waves from the P-waves to predict later ground motion from the S-waves and surface waves. In recent years, an on-site EEW method based on fixed peak displacement (Pd) threshold is developed. Although the method consistently provides effective warnings in Taiwan, several studies suggest that its strong filter dependence might introduce extra biases to the system. Also, the fixed Pd threshold suffers from the inevitable trade-off between a false alarm and a missed alarm. In order to overcome the abovementioned problems of fixed Pd threshold method, we utilize techniques in machine learning and develop a new method of on-site early warning. Owing to the property that convoluted-neural-network (CNN) will automatic sampling on different frequencies, the unfiltered seismic signal itself is sufficient to derive a warning threshold. Also, multi-layered neural-network is capable of forming a complex non-linear threshold to reduce both the false alarms and missed alarms. As an example, we collect 21 medium to large inland earthquakes in Taiwan. We compare the performance between the proposed method and the fixed Pd threshold method. The result shows that the proposed method outperforms in both false alarm rate (0.01 percent) and missed alarm rate (30 percent). The proposed method can provide not only significant improvements for on-site EEW but also a window into the initial P-waves. Studying the trained models might even reveal the hidden indicators inside the initial P-waves.Keywords: earthquake disaster mitigation, on-site type earthquake early warning, machine learning [ABSTRACT FROM AUTHOR]

We examine records obtained in the regional-to-far field ofthe catastrophic volcanic landslide and tsunami atKrakatau on 22 December 2018. In the absence of operationalDART-type sensors in that part in the Indian Ocean, we userecords on horizontal long-period seismometers at a number ofcoastal sites: this methodology, pioneered by Yuan et al. [2005]has been successfully applied to several tsunamis, both in the far fieldand more recently at close distance during the 2017 Karrat Fjord, Greenlandlandslide. Using records at Cocos Island (COCO; 1160 km), we reconstructan equivalent amplitude in deep water of 0.15 mm peak-to-peak,with energy peaked around 2.5 mHz.We fail to observe hydroacoustic T phases on hydrophones of the IMSoff Diego Garcia, but a weak signal is present on the seismic records atCOCO and at one of the two stations of the Australian network on ChristmasIsland (490 km), the latter featuring energy in the 7-t0-20 Hz band.Finally, the eruption is recorded as an exceptionally crisp signal by the infrasound arrayI06 of the IMS at Cocos Islands, with a signal peaked around 0.25 Hz.Quantified modeling of these various signals will be presented, based inparticular on a combined dynamic model of the landslide and tsunami, coupled toa simulation of the water waves using a Boussinesq approximation, whoseresults will be compared to the deconvolved seismic record at COCO. [ABSTRACT FROM AUTHOR]

A research project with an exciting field component presents a unique opportunity for broad public engagement. Outreach with schools is especially valuable in that it can have a profound, lasting impact. It shows the students how science works, encourages them to study science, and broadens their career perspectives – to quote from the feedback survey for the outreach programme that we present here as an example. The project SEA-SEIS (Structure, Evolution and Seismicity of the Irish Offshore) kicked off in 2018 with a 3-week expedition on the research ship Celtic Explorer. For the first time, broadband ocean-bottom seismometers were deployed across the vast Irish offshore, with a few instruments also in the UK and Iceland waters. Prior to the expedition, we asked schools across Ireland to help the scientists to give each seismometer a name. Participating in a real, exciting research project got the students enthusiastically engaged. Many of the classes participating in the naming competition then took part in our live, ship-to-class video link-ups. After a brief introduction to the project and meeting the team, the students asked numerous questions about the research, life on the ship, and how one becomes a scientist. The follow-up survey responses from the teachers confirmed that the engagement was not only exciting ("it was like speaking to Indiana Jones!") but had a lasting impact. With most of our scientists on the ship being female, the female students in the classrooms were particularly encouraged. The SEA-SEIS outreach programme is sustainable and now continues with follow-up national competitions (a seismology song-and-rap one for secondary schools and a drawing one for primary schools – we shall present some remarkable entries). The programme's success in engaging students across the country and sharing with them the excitement of research demonstrates the value of active research projects as a means of broad outreach and education. Obviously, the outreach programme could not succeed without the scientists on the project putting in the work. Fortunately, researchers see quickly the very real impact of the outreach and get involved with enthusiasm and commitment. The programme's outcomes thus include not only school students getting more interested in science but also young researchers getting experience and appreciation of the education and outreach work. Website: www.sea-seis.ie [ABSTRACT FROM AUTHOR]

InSight landed on Mars on November 26th, 2018 with several geophysical instruments including a short-period seismometer and a broadband seismometer (SEIS, Seismic Experiment for Interior Structure). Both seismometers are now installed directly on the Mars surface and enable to analyze the continuous seismic signal. The purpose of this study is to analyze autocorrelations of Mars continuous seismic signal. Seismic interferometry by ambient noise autocorrelations is a special case of Green's function retrieval for single-station analysis. High-frequency noise autocorrelations can be used to extract the zero-offset reflectivity and basin resonances beneath of the landing site while low-frequency noise autocorrelations contain mainly orbiting surface waves and are useful to extract normal modes. We analyze the signal by using both the classical and phase autocorrelations. Correlograms are stacked using the phase weighted stack method in order to enhance the signal to noise ratio.The method was tested also on the two blindtest datasets provided by the Marsquake Service (MQS) and by the Mars Structure Service (MSS). It is further applied to analyze the first data recorded on Mars by the co-located broadband and short period seismometers. [ABSTRACT FROM AUTHOR]

At convergent plate boundaries, the nature and spatial distribution of volcanism is largely controlled by major, margin-parallel and transverse fault systems that act as magmatic and hydrothermal fluid conduits through the crust. In the south-central Chilean Andes, the volcanically and seismically active Tinguiririca and Planchon-Peteroa volcanoes are considered to be tectonically related to the major Miocene-Pliocene El Fierro thrust fault system due to their alignment along strike of the fault trace. This large scale reverse fault system is characterized by hydrothermally altered fault cores, which exhibit signatures of copper-porphyry mineral haloes within the fracture damage zone, and geothermally heated springs that occur along the outcropping fault traces. These surficial features support the hypothesis that hydrothermal fluid propagates up to the surface in the hanging wall damage zone, below an impermeable fault core. This study aims to constrain the 3D architecture of these tectono-magmatic features and their interaction with the active geothermal system. This was achieved by combining a 24 station Metronix magnetotelluric and 12 station ESPD and 6TD seismic network in a 40 x 40km- grid within the selected study area. Data from the magnetotelluric survey were processed with robust remote referencing techniques with BIRRP to produce reliable impedance values, followed by a 3D inversion to produce the presented regional subsurface conductivity signatures. The local seismic hypocentres picked from SAC from the first six months are also presented here, together with the magnetotelluric results. The combined results show a distinct coherence of high conductivity regions and seismic clusters in the top 10km of the crust. These conductors show a westward dipping trend in the region of the El Fierro fault, and also occur as large conductive regions beneath the Planchon-Peteroa volcanic complex. As the seismic clusters occur on the eastern edge of their correlating conductors, it is also interpreted that the ambient tectonic stresses are causing the seismic events to occur on the footwall of the large-scale reverse fault systems. As the hydrothermal system in this region is still volcanically active, evident from the minor stratovolcanic eruption that occurred in November 2018 within the duration of the seismometer deployment, the results from this study provide evidence of how lithospheric weakening from the circulation of meteoric and magmatic fluids within the crust promote tectonic rupture. The fluids are attributed to meteoric and magmatic sources in circulation within the active hydrothermal system, the composition and temperature gradients of which will be constrained with finalised 3D inversions. We also show strong evidence of a north-west oriented seismic cluster that may indicate the presence of a margin-oblique second-order NW fault, commonly known as Andean Transverse faults (ATF), which can also be correlated with NW lineaments shown in preliminary resistivity maps. This study will ultimately provide insight into the interaction of geothermal fluids with concealed/blind fault systems, and their influence on orogenic evolution of this region of the South Central Andes [ABSTRACT FROM AUTHOR]

The new Trillium Slim Borehole 120 seismometer is a 104 mm diameter instrument with Trillium 120QA/PH class performance having significantly improved SWaP (size, weight and power). It is designed for smaller holes down to 4.5" or 115 mm diameter, in shallow or deep deployment, using a simple passive holelock or sand installation. The small diameter permits deployment in existing small boreholes, and facilitates construction of new lower cost boreholes, minimizing disturbance of the surrounding rock and improving instrument coupling.We present initial test results for this instrument in small and large diameter cased holes, direct burial, and side-by-side pier testing. High performance with simpler logistics makes the Trillium Slim Borehole well suited for many applications, including new higher-density arrays for full waveform analysis and detection of earthquake gravity signals. In conclusion we present a proposed array design for optimal measurement of earthquake gravity signals on a regional scale. [ABSTRACT FROM AUTHOR]

The InSight mission landed on Mars on November 26th, 2018 with several geophysical instruments including a short-period seismometer and a broadband seismometer (SEIS, Seismic Experiment for Interior Structure). Both seismometers are now installed directly on Mars surface and enable to analyze the continuous seismic signal. The purpose of this study is to analyze, quantify and characterize the frequency-dependent polarization of Mars continuous seismic signal. On Earth, the secondary microseisms are the dominant signals in the absence of earthquakes. Secondary microseisms are dominantly Rayleigh waves, with elliptical polarization in the vertical plane. We do not expect to record microseisms on Mars because of the absence of oceans. We adapted the method developed for Earth data by Schimmel et al. (2011) to analyze the frequency-dependent polarization of continuous Mars data. The method enables to detect elliptically polarized signal such as Rayleigh waves and also linearly polarized signals that could be either Love waves or body waves. The method was tested on the two blindtest datasets provided by the Marsquake Service (MQS) and by the Mars Structure Service (MSS). It is then applied to analyze the first data recorded on Mars by the broadband and the short period seismometers. [ABSTRACT FROM AUTHOR]

A key goal of the NASA InSight mission to Mars is the detection, discrimination, location and characterization of Marsquakes. InSight landed on Mars on November 26, 2018, and the SEIS seismometer package was deployed on the surface over Christmas. Over its nomination mission duration of one martian year, SEIS is expected to register a few global events of magnitude larger than 4 and a few tens of events at local and regional distances of smaller magnitudes, in addition to a few impacts.We have calibrated single-station procedures for: the discrimination of non-seismic pulses, by correlating with measurements of pressure, wind and magnetic anomalies; the discrimination of possible meteoritic impacts, by depth estimation (using depth phases such as pP and sP) and spectral analysis; event location based on the use of differential travel-times of body waves and surface waves, and of multiple Rayleigh and Love-wave orbits; accounting for both aleatoric (i.e. picking error) and epistemic (i.e. phase identification and model uncertaienty) uncertainties; event quantification and characterization, including magnitude assessment with a new suite of Martian magnitudes and moment-tensor inversion; the iterative refinement of the model suite and event locations.These procedures have been tested in blind tests and operation readiness tests using realistic Mars conditions and we expect confirmation and calibration as part of the Marsquake Service operations, using data recorded on Mars, in conjunction with the Mars Structural Service (MSS). [ABSTRACT FROM AUTHOR]

With the deployment of the SEIS seismometer package by the Insight lander on the surface of Mars, the Mars Quake Service (MQS) has started its work to detect, locate and catalogue Martian seismicity. In preparation, a number of methods to locate seismic events based on polarization (azimuth) and traveltime (distance) have been presented and tested in a public blind test. However, both the location methods and synthetic data assumed a spherically symmetric planet.In a second stage, we computed a synthetic data set in a 3D Mars model including ellipticity, topography, crustal thickness and lateral velocity variations in the crust. This test revealed, that 1D is likely a good approximation for body wave and long period surface waves (T < 80s) traveltimes, but traveltimes can be off by as much as a few hundred seconds for shorter period surface waves (50s < T < 15s) with the crustal thickness being the most relevant source of error. For events that only produce a single body wave phase additional to the surface waves, this translates into a significant error in the distance estimation.In this presentation we argue how we can mitigate this problem by adding a 3D crust on top of the 1D models that fits the gravity data and hence approximate the crustal thickness, how to efficiently compute surface wave traveltimes for these models and demonstrate the efficacy of this approach in a 3D blind test. Furthermore, we show how we select a reasonable number of models for probabilistic location methods from a very large set of a priori models based on a clustering technique and how this selection can be updated as the number of events grows. [ABSTRACT FROM AUTHOR]

InSight landed on Mars in late November 2018, and the SEIS seismometer package was deployed on the surface over Christmas. SEIS is expected to operate for at least 1 Martian year, or two Earth years. The Marsquake Service (MQS) has been setup to create and curate a seismicity catalogue for Mars during the lifetime of the InSight mission. In preparation for operations, the MQS team have developed single station location and characterisation algorithms, created a software framework that manages the data flow that includes these approaches, and tested methods and operational procedures via a suite of blind tests. The MQS approach includes use of an a priori set of plausible martian models that can be refined once constraints from observed signals are included. The MQS works in conjunction with the Mars Structural Service (MSS) on building and adopting updated models. Mars has strong topographic variations, and 3D effects on surface waves can be taken into account. The MQS consists of an international team of seismologists that, in turn, screen incoming data to identify and characterise any seismicity. The MQS has been operational since the first seismic signals started arriving from on deck. In this presentation, we present the MQS, show initial results and describe the challenges we face dealing with the true Martian dataset. [ABSTRACT FROM AUTHOR]

Multiple sets of events with high waveform similarity (multiplets) have often occurred prior to different types of catastrophic failure events, e.g. large earthquakes, landslides and volcanic eruptions. This similarity indicates the source locations are restricted to a very localised zone. Hence, analysing multiplets has the potential to reveal underlying processes such as the nucleation of large events, or stable repeated slip, and hence could improve probabilistic forecasts of the likelihood of different catastrophic events. By resolving the processes taking place, we can determine their relationship to the catastrophic failure event as foreshocks, aftershocks or independent processes. The problem with finding multiplets is that they are often small, obscured in ambient noise, and sometimes only picked up by one seismometer. We have developed an optimised detection and analysis technique to extract a catalogue of multiplets and determine their temporal evolution in different seismic signals. By enhancing the short-time-average/long-time-average (STA/LTA) approach for finding events with a moving cross-correlation window, we discover new events automatically in a dedicated high Random-Access Memory computational engine. Subsequent temporal tracking and statistical analysis of these events then allows us to examine their occurrence and resolve the processes taking place. The algorithm's success in finding events is evaluated statistically with hits, false alarms and misses, analogous to clinical trials in medicine. Our method works significantly more successfully than the STA/LTA approach on its own, with more hits and more accurate pick times in different catastrophic failure settings. We demonstrate the success of our algorithm in real cases of past earthquake sequences and other catastrophic failure type events. From this, we can identify large earthquakes which have had (or not had) nucleating foreshocks in the form of multiplets, aiding to our understanding of the nucleation process. We can also use this technique for other types of catastrophic failure type events, such as volcanic eruptions, to determine the underlying processes taking place based on the evolution of multiplets prior to failure. Our current analysis on large earthquakes show no clear nucleation pattern between different cases; however, we do obtain a more complete view of the pre-mainshock seismicity taking place, helping to better understand pre-failure processes. [ABSTRACT FROM AUTHOR]

Located in the Mauleon basin (SW France) the MAUPASACQ experiment is a passive seismic network consisting of 435 stations (197 short-period seismographs, 190 geophone nodes and 48 broadband stations) over an area of 1500km² and recording for a period of 6 months. The complexe combination of differents seismic devices and mostly the use of geophones in a such large area lead to a complex ambient noise dataset. In order to take fully advantage of this unique dataset we computed both ambient noise correlation (c1) and correlation of correlation (c2). The combination between the c1's and c2's allow us to increase surface wave dispersion measurements and the ray coverage inside the array. From this new methodology preliminary ambient noise tomographic results are shown and discussed. [ABSTRACT FROM AUTHOR]

The history of seismography in Iceland began in 1909 with the installation of one horizontal Mainka seismograph in Reykjavík. Following a period of intermittent operation regular operation was initiated in1925 with the establishment of the Icelandic Meteorological Office. The number of stations increased gradually over the following decades, and in the sixties four stations were in operation. The number of permanent stations proliferated following the Heimaey eruption in 1973, and during most of the eighties, the number of stations was 40-50. The first digital seismograph stations were installed in 1990 and the analog seismic network was gradually replaced by digital stations over the next two decades. The number of analog seismograms in the Icelandic archives is estimated to be around 300 000 covering the period 1910-2010. A four-year project to make this record collection accessible on the internet has been initiated and funded. So far around 100 000 seismograms have been scanned and made available for download on the open website https://seismis.hi.is.This database will eventually be merged with a general Icelandic science data repository currently being established by the University of Iceland and various research institutes.The seismograms are scanned with a resolution of 300 dpi and presented for examination on the website as low resolution.png files but high-resolution jpg files can also be selected for download. The high-resolution files are on the order of 4-8 Mb each. Digitization of the seismic traces has not been attempted, since most of the seismograms are from short-period instruments and the waveforms are already lost.In addition to numerous teleseismic body wave phases the record collection contains primary data from various tectonic and magmatic events in Iceland during the last century. This includes eruptions of Hekla in 1947, 1970, 1980-81, 1991 and 2000, Surtsey in 1963-67, Heimaey in 1973, Askja in 1961, Grímsvötn in 1934, 1983, 1998, and 2004, Gjálp in 1996, rifting episode at Krafla in 1975-1984, persistent seismic activity of the Bárðarbunga and Katla volcanoes, numerous suspected subglacial magmatic events, earthquake swarms on the Reykjanes Peninsula Oblique Rift and within the Tjörnes Fracture Zone, and earthquake sequences in the transform zones of South and North Iceland. [ABSTRACT FROM AUTHOR]

The North Atlantic Ocean and its margins present important, unanswered geodynamics questions, both in the continental and oceanic domains. To better understand the structure and evolution of this area, a set of 18 state-of-the-art ocean-bottom seismometers (OBSs) have been deployed in the North Atlantic Ocean in September 2018 as part of the project SEA-SEIS (Structure, Evolution And Seismicity of the Irish offshore). The network covers the entire Irish offshore and parts of the UK and Iceland waters. Over a period of 15 months, they will record seismic data at 1-4 km depths on the ocean bottom, with the retrieval planned for April, 2020. With this new broadband data, together with all other globally available broadband data, we will improve the tomographic resolution of the North Atlantic region, including the Iceland hotspot. To image the lithosphere and mantle, structural information will be extracted from the large amount of vertical- and transverse-component waveform fits, including the unique new OBS data, using the Automated Multimode Inversion (AMI) of surface, S- and multiple S-waves. Realistic body-wave sensitivity kernels will be developed and integrated into the inversion. The improved tomographic models of the North Atlantic region should yield new insights into the origins of the enigmatic North Atlantic Igneous Province volcanism, the evolution and deep structure of the Iceland Hotspot and its effect on the evolution of the region. To minimize the effect of data errors on the model, extensive quality control has been applied to the available global and regional network data, including extensive checks of the correctness of the instrument response information, which can influence the accuracy of the structural information yielded by seismograms. The quality control revealed a number of problems with the response files at stations from various of networks. The problems are being fixed in joint efforts with the data centers and network operators. [ABSTRACT FROM AUTHOR]

Both seismic sensors of the SEIS (Seismic Experiment for Interior Structure) instrument (Lognonné et al. 2019) of the NASA InSight mission, i.e. the VBBs (Very Broad Band seismometers) and SPs (Short Period seismometers), are mounted on a mechanical leveling system, the LVL. Its purpose is first to provide the mechanical coupling between the instrument and the ground and then to ensure the sensors' level placement on the Martian ground. We developed a simplified analytical model of the LVL (Bagaini and Barajas-Olalde 2007, Fayon et al. 2019) in order to reproduce its mechanical behavior by predicting its resonances and transfer function. After calculation of the modeled LVL modes, two resonances were obtained between 35 and 50 Hz in both horizontal components, which is in good agreement with laboratory measurements performed on the LVL flight model. As these frequencies are comprised in the seismic measurements bandwidth, their study is crucial. A few parameters are adjustable in the model and after some simulations we noticed that the horizontal ground stiffness and the torque induced by the ground on the LVL's feet seem to change these both horizontal mode frequencies. This means that the LVL resonances depend on the mechanical coupling between it and the ground. For this reason, this resonances, as well as the relative comparison between SPs and VBBs signals, can be used to assess both the ground structure just beneath the feet and the quality of the transfer function. Whereas the InSight lander contact with the ground has been imaged by the lander cameras (Maki et al. 2019), this has not been possible for the contact of the SEIS feet, which remains to be characterized by indirect methods. We show the preliminary analysis of SEIS data on the ground as well as some results from updated SEIS models and discuss constraints on the depth of penetration of the cone on each foot, and on the geometry of feet penetration. We conclude by discussing if the mechanical properties of the surface layer or duricrust can be inferred. [ABSTRACT FROM AUTHOR]

The InSight lander successfully delivered geophysical instrument package on the Martian surface on November 26th, 2018, including a broadband and a short-period seismometer (Seismic Experiment for Interior Structure, SEIS). The seismic instrument package is specifically designed to record marsquakes and meteoritic impacts in Martian conditions. Routine operations are split into two services: the Mars Structure Service (MSS) and the Marsquake Service (MQS), which are responsible for defining structure models and seismicity catalogs, respectively. The first "deliverable" of the MSS will be a model based on the events detected during the first 3 months of seismic monitoring of the mission, for which only a few quakes might be expected based on current estimates of Mars seismic activity. To test our approach of determining the interior model of Mars and to prepare the InSight science team for data return, we made use of a "blind test" time series for which the Marsquake parameters (location, depth, origin time, and moment tensor) and interior model were unknown to the group at large.In preparation for the mission, the goal was to develop mature algorithms to handle the data as efficiency as possible. Synthetic seismic waveforms were computed in a 1D mantle model with a 3D crust on top using AxiSEM and Salvus. The time series were created by adding seismic noise that relies on pre-landing estimates of noise generated by the sensors, electronic system, environment, and nearby lander. To characterize what we could learn about Mars interior structure with only one station and with the first seismic event, we performed inversions of a synthetic data following a blind test process, where the interior model was unknown to all team members carrying out data analysis and inversion. We detail and compare the results of this "blind test" using different methods including inversion of surface wave dispersion data, body waves travel times, and the waveforms themselves. We have used mainly Bayesian techniques to obtain robust probability density functions of interior structure parameters. The effects on the retrieved model distributions of fixing mars quake location and origin time are investigated, as is the effect of using fixed Vs flexible parameterizations. To allow for tighter constraints, we also test the use of priors based on thermodynamically-constrained models together with seismic observations, as well as seismic confirmation/rejection of models purely based on thermodynamical modelling. These techniques considered here form a large part of the planned modeling of the MSS that will be ultimately employed with the first recording of a seismic event by InSight. [ABSTRACT FROM AUTHOR]

Here we develop a new tool in which teleseismic P-to-S converted waves are used to construct a fully 3-D shear-wave velocity model of the crust. Our method is fundamentally different from ambient noise and local earthquake tomography, as it uses sub-vertical rays. This approach is based on receiver functions (RF) and requires a dense seismological network to investigate the less-studied S-wave velocities.For the benchmark study we focus on the Central Alps, performing the analysis of RFs using a dataset composed of the last 20 years of high-quality data from permanent stations as well as temporary AlpArray stations, in order to get a homogeneous coverage of our zone.For the forward model, we implement an accurate ray-propagator which respects Snell's law in 3-D at any interface geometry. This approach can be summarized in 3 steps:1. An existing ray-shooting tool (Knapmeyer, 2004) computes P-ray geometry in a global velocity model (iasp91) to arrive at the station.2. Considering the true local dip, a shear-wave is shot from the piercing point at the local Moho to the surface. This S-wave usually arrives several km away from the station.3. We adjust the ray-parameter to make the corresponding crustal S-wave arrive at the station.For the Alps case, using a local 3-D velocity structure (Diehl et al., 2009) and a complex Moho geometry (Spada et al., 2013), the mean distance between the arriving S-wave and the station is about 150 m (median ca. 40 m).Based on the ray coverage of the actual dataset (almost 300'000 traces), we parameterize the model grid using square cells in X and Y directions, typically 25x25 km. In depth, we parameterize a multi-layer model defining an S-wave velocity above and below each discontinuity. The introduction of this flexibility allows accommodating a velocity gradient within each layer and investigate velocity jumps between them. The individual velocity model corresponding to each RF is extracted from the 3-D initial model along its ray path and the velocity structure is updated using a weighted mean at the enclosing mesh nodes. We envisage to manage the inversion process by the stochastic Neighbourhood Algorithm approach (Sambridge, 1999), looking at the ensemble of models that sample the good data-fitting model for a multidimensional parameter space. We first plan to create and test a 2-layer Vs model (Moho and intra-crustal discontinuity) for the Central Alpine region. Later, we plan to extend our dataset to the entire Alpine domain, and apply this tool to other areas with a dense network of broadband seismometers. [ABSTRACT FROM AUTHOR]

The extreme climatic patterns such as strong and prolonged rainfall frequently occurred inTaiwan during summer season, which are major triggered factors to hazardous large-scalecollapse events. Thus, it is important to explore the possible failure areas before the collapseoccurrence. Taiwan is located at the plate boundary, resulting the high seismicity to providethe high quality seismic records. Aforementioned condition provides a good opportunity totest the site-reference-spectral ratio (SRSR) method for the characterization of site stability.First, we installed five seismometers around the high failure potential landslide area.During the monitoring period, there are a total of 32 crustal (focal depth of 3-35km) earthquakes (local magnitude (ML) greater than 4.0) resulting the shackingintensity ranging from 1 to 5 (intensity scale of Central Weather Bureau). The farthestepicentral distance is 150 km. Station CS03 was selected to be a reference station,which generally recorded the smaller peak ground acceleration (PGA) for eachearthquake. Results show the SRSR value with a range of 0.3-30. The amplificationeffect related to reference station can be observed at higher frequency (&#x003E; 12 Hz)portion and mainly concentrated on the azimuth between 100 and 160 degrees, whichprobably corresponding to subsurface structure with northwest-to-southeast trend.Furthermore, we also applied SRSR analysis to other sites to provide comprehensivetest of method feasibility. In practical, SRSR method is useful and invasive tool tomap the possible landslide area by using dense and averaged-distribution seismicarray. Keywords: Site-reference-spectral ratio (SRSR), Stability, Peak ground acceleration(PGA) [ABSTRACT FROM AUTHOR]

Commercial Lunar Payload Services (CLPS) is a NASA program to acquire end-to-end commercial pay-load services between the Earth and lunar surface. The competitive nature of the program is expected to reduce the cost of lunar exploration. Proposals from commercial partners are currently being solicited. Here, we consider the advantages of including MEMS-type seismometers on one or more landed missions. Micro-Electro-Mechanical Systems (MEMS) sensors are small, lightweight sensors etched on a silicon wafer. A critical question is whether the instrument is likely to detect sufficient moonquakes given missions that may only extend during a single lunar day (approximately two weeks in Earth-time). In order to assess the deployment quantitatively, we can com-pare estimated instrument response and noise characteristics to documented lunar seismicity from Apollo data.The SEIS-SP sensor is a low-mass, but still relatively broadband seismometer that uses a silicon MEMS sensor. It was developed by Imperial College, the University of Oxford and Kinemetrics. The sensor formed part of the SEIS package on the InSight mission, and was initially deployed on Mars in late 2018. The sensor is a small and robust instrument. It is through-wafer etched and patterned in single-crystal silicon, with a 25 mm die size. Three sensors measure all three components of translational motion. The combined package for InSight had a total mass of 635 g for the sensor heads, electronics board and associated connectors and cabling, making such a system potentially very promising for deployment on future commercial lunar landers. The performance of the current generation of SEIS-SP sensor is limited by the thermal noise floor from gas damping within the package. The noise floor can be further improved by evacuating the chamber. The lunar environment reduces the risks of leaks, making this a low-risk option for the Moon, and potentially improving detection rates. We investigate the effectiveness of deploying the SEIS-SP or other lightweight instruments with similar self-noise and response characteristics for missions of varying lengths.The Apollo missions included surface deployments of seismometers that ran at multiple landing sites over periods between 1969 and 1977. Using the seismic data available from the seismometers deployed by astronauts during these missions, we investigate the magnitude of moonquakes which would be above the detection thresholds for various MEMS seismometers. Instruments deployed during the Apollo era detected several types of events, including artificial impacts, meteoroid strikes, shallow and deep moonquakes and thermal events. Given several years of operations for the Apollo instruments, we have excellent constraints on the lunar seismicity rate over the multi-year catalog. We also have constraints on diurnal variations for thermal events over the course of the lunar day/night cycle. Thermal quakes occur periodically, with a sharp double-peak at sunrise and a broad single-peak at sunset. Deep moonquakes have a clear correlation with the tidal phase of the Moon, with monthly periodicity of their occurrence times as well as tidally dependent variations in amplitude. We investigate which types of moonquakes could be detected. Finally, we estimate detection levels for a short-duration mission. [ABSTRACT FROM AUTHOR]

On April 2014 the Iquique Mw 8.1 earthquake ruptured the interpolate contact between the oceanic Nazca and continental South American plates offshore northern Chile between 19.5°S to 21°S in April 2014. This earthquake did not fully release the strain accumulated since the last great megathrust (Mw 8.8) in 1877 and had left an unbroken segment in the south. From December 2014 to November 2016, we deployed an offshore network of 15 Ocean Bottom Seismometers (OBS) that covered the rupture area and the unbroken southern segment using the Chilean Navy ship OPV Toro and R/V Sonne. That data set is supplemented by five weeks of data from 67 OBS installed for a controlled source seismic experiment during cruise MGL1610 of the R/V Marcus Langseth in late 2016. Data acquired onshore by stations from of the IPOC (Integrated Plate Boundary Observatory Chile) and CSN (Chilean Seismological Service) networks are also included.We present first results of this ongoing project, which include double-difference hypocenter relocations based on waveform cross-correlation. Most of the seismicity occurs between 19.5 and 21°S up-dip of the patch of maximum coseismic slip during the 2014 earthquake, while the seismicity in seismogenic depths is highly concentrated forming well-defined clusters. The observed seismicity provides constraints on the structure of the marine forearc and enables us to relate the seismicity distribution to the background seismicity, seafloor morphology, and regional tectonics. [ABSTRACT FROM AUTHOR]

Studying the mechanical response of the crust to large earthquakes provides unique insight into the processes of deformation in preparation for future earthquakes. Noise-based seismic velocity monitoring can directly probe the mechanical state of the crust at depth continuously in time. In this work, we study the deformation of the crust concerning the Mw 9.0, 2011 Tohoku-oki earthquake. In addition to the Hi-net short period sensors, we employ here for the first time for the noise-based monitoring the particular dense network of Hi-net tiltmeters as long period (8 – 50 s) seismometers to sample the crust below 5 km depth. Spatial distribution of strong velocity decreases at short periods can be limited to strong ground shaking induced by the 2011 Tohoku-oki earthquake, while the long period velocity changes correlate well with modeled static strain induced by viscoelastic relaxation and afterslip at depth. This observation indicates that the variations at depth are not a spurious effect of changes in the shallow layers. The amplitudes of coseismic velocity changes diminish with increasing depth. The temporal evolution of velocity changes at different depth shows that the maximum drops of velocity at greater depth are delayed in time with respect the date of the earthquake. This delay could find its origin in both the visco-elastic response of the crust to substantial strain changes or a complex response of crustal seismic velocities to transient deformation. [ABSTRACT FROM AUTHOR]