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Rapid and accurate earthquake magnitude estimations are essential for earthquake early warning (EEW) systems. The distance information between the seismometers and the earthquake hypocenter can be important to the magnitude estimation. We designed a deep-learning, multiple-seismometer-based magnitude estimation method using three heterogeneous multimodalities: three-component acceleration seismograms, differential P-arrivals, and differential seismometer locations, with a specific transformer architecture to introduce the implicit distance information. Using a data-augmentation strategy, we trained and selected the model using 5365 and 728 earthquakes. To evaluate the magnitude estimation performance, we use the root mean square error (RMSE), mean absolute error (MAE), and standard deviation error (ϭ) between the catalog and the predicted magnitude using the 2051 earthquakes. The model could achieve RMSE, MAE, and ϭ less than 0.38, 0.29, and 0.38 when the passing time of the earliest P-arrival is 3 s and stabilize to the final values of 0.20, 0.15, and 0.20 after 14 s. The comparison between the proposed model and model ii, which is retrained without the specific architecture, indicates that the architecture contributes to the magnitude estimation. The P-arrivals picking error testing indicates the model could provide robust magnitude estimation on EEW with an absolute error of less than 0.2 s. [ABSTRACT FROM AUTHOR]

Mimoun et al. (Space Sci Rev 211(1–4):383–428, 2017) developed a pre-landing noise model of the Martian seismometer package SEIS onboard InSight that analysed all the external and internal noise sources. We updated the environmental and instrumental parameters of the model as well as the ground properties with InSight mission data. We compared the output of the in situ noise model to the actual noise measured during the full mission for each individual noise source as well as for the full noise model. We evaluate in detail the efficiency of the model to fit the measured data and discuss the transient noise and other sources that were not included in the model. The main noise sources in the seismic bandwidth are the pressure noise and the lander noise, which is increased from the pre-landing model and overestimated when compared to the data; the magnetic field noise was overestimated in the pre-landing model and is now found to be negligible. The conclusions and models from this study could benefit future space missions. [ABSTRACT FROM AUTHOR]

On September 5, 2019, a moderate earthquake of Mw5.4 unexpectedly occurred in the apparently quiescent central basin of the South China Sea. We immediately carried out a seismicity monitoring experiment around the epicenter by using broadband ocean bottom seismometers (OBS) for the following three scientific targets. The first is knowing the earthquake seismogenic mechanism, fault structure and further development. The second is finding the role of the residual spreading ridge playing in earthquake processes and further revealing the deep structures of the ridge directional turning area. The third is confirming the existence and significance of the so called "Zhongnan fault". This paper reports the preliminary results of the first phase experiment. Five OBSs were deployed for seismicity monitoring with a duration of 288 days, but only three were recovered. Micro-earthquakes were firstly detected by an automatic seismic phase picking algorithm and then were verified by analyzing their seismic phases and time-frequency characteristics in detail. A total of 21, 68 and 89 micro-earthquakes were picked out from the three OBSs respectively within the distance of 30 km. The dominant frequency of these micro-earthquakes is 12–15 Hz, indicating tectonic fracturing. During the first two months after the mainshock the seismicity was relatively stronger, and micro-earthquakes were still occurring occasionally till the end of observation, indicating the epicenter area is active. We used Match&Locate method to locate 57 micro-earthquakes preliminarily. Their spatial distribution shows that the seismicity is developed mainly along the NE direction roughly parallel to the residual ridge with depth variations between 10–20 km. [ABSTRACT FROM AUTHOR]

Scientific objectives, instruments, and measurement program of the scientific instrumentation of the Kazachok stationary landing platform of the State Corporation Roscosmos and the European Space Agency (ESA) ExoMars-2022 project are presented. The scientific objectives of research on the landing platform included the long-term climate monitoring, the studies of the atmospheric composition, the mechanisms for dust lifting and related electrical phenomena, atmosphere–surface interactions, the subsurface water abundance, monitoring the radiation situation, and the study of Mars internal structure. To address these problems, 11 Russian and two European instruments with a total mass of 45 kg were built, tested and integrated into the spacecraft. These include a television camera system, meteorological complexes, a suite for studying dust and related electrical phenomena, optical spectrometers and an analytical complex for studying the atmospheric composition, a microwave radiometer, the neutron and gamma spectrometers for surface research, a seismometer, magnetometers and a Mars proper motion experiment to study its internal structure. Although the ExoMars-2022 project has been discontinued, the scientific objectives of the landing platform have not lost their relevance, and the technical solutions and developments implemented in scientific equipment are of interest and promising for further Mars exploration. [ABSTRACT FROM AUTHOR]

We propose a method for detecting earthquakes for high-speed trains based on unsupervised anomaly-detection techniques. In particular, we utilized autoencoder-based deep learning models for unsupervised learning using only normal training vibration data. Datasets were generated from South Korean high-speed train data, and seismic data were measured using seismometers nationwide. The proposed method is compared with the conventional Short Time Average over Long Time Average (STA/LTA) model, considering earthquake detection capabilities, focusing on a Peak Ground Acceleration (PGA) threshold of 0.07, a criterion for track derailment. The results show that the proposed model exhibit improved earthquake detection capabilities than STA/LTA for PGA of 0.07 or higher. Furthermore, the proposed model reduced false earthquake detections under normal operating conditions and accurately identified normal states. In contrast, the STA/LTA method demonstrated a high rate of false earthquake detection under normal operating conditions, underscoring its propensity for inaccurate detection in many instances. The proposed approach shows promising performance even in situations with limited seismic data and offers a viable solution for earthquake detection in regions with relatively few seismic events. [ABSTRACT FROM AUTHOR]

In this study, geodetic and seismic phenomena occurring in the subduction zone were observed with two tiltmeters that were installed in seafloor boreholes drilled in the Kumano Basin in the Nankai Trough, southwest Japan. We used one electrolytic-type tiltmeter and one pendulum-type tiltmeter installed 6 and 19 m below the seafloor, respectively. The two tiltmeters have been continuously operated since 2019 and 2021, respectively. The records of the two tiltmeters showed microseisms, seismic signals of normal earthquakes, and variations that correlated well with the M2 tide (tidal response). The noise environment relative to existing seafloor observatories was assessed by comparing the power spectral densities of the tiltmeters and broadband seismometers in the seismic frequency band. The two tiltmeters and the seismometers showed similar spectral peaks in the microseism band, and at lower frequencies below 10–2 Hz, the pendulum tiltmeter had a noise level that was up to 20 dB lower than that of the broadband seismometers. The tidal responses of the tiltmeters were analyzed to reveal corresponding mechanisms because a large amplitude of the tidal response makes it difficult to detect geodetic phenomena. Considerable azimuthal dependence was observed in the NW–SE direction for both tiltmeters. The principal direction was approximately the same as the down-slope direction of the local bathymetry. This tendency was thus interpreted to be caused by the differences in the thickness of the sedimentary layer along the direction of the slope. Furthermore, the validity of the tilt measurements for the two tiltmeters was demonstrated by in-situ loading experiments, where the theoretical response of an elastic half-space medium was computed and compared with the experimental results. The results of the loading experiments can be explained using theoretical values with a set of realistic elastic moduli. [ABSTRACT FROM AUTHOR]

A new, improved approach in the design of broadband seismometers is presented. The design results in the implementation of a high performance, low cost, and simple-to-operate instrument. The proposed seismometer is based on a modified accelerometer followed by a continuous time integrator for providing velocity voltage output. It has a broadband response, flat in velocity from 120 s to 75 Hz, high sensitivity 1200 V/(m/s), and 40 Vpp differential output range. The acceleration integration method provides high performance at low frequencies, with self-noise well below the New Low Noise Model at the range 80 s–16 Hz. The mechanical system provides a perfectly linear response of its displacement sensing system. Evaluation, classification, and noise determination of the presented instrument are performed in terms of direct experimental measurements, simulations, and calculations based on raw data from the proposed sensor and from a commercial product with approximately equivalent performance. Its technical features and performance specifications guarantee accurate sensing of local events, with maximum power at the frequency range of 5 to 10 Hz, but also make it ideal for the recording of distant tectonic activity, where extremely weak motions at long periods are expected. The whole design is robust, lightweight, and weatherproof, comprising in this way a useful tool to geoscientists. [ABSTRACT FROM AUTHOR]

A new era of lunar exploration has begun bringing immense opportunities for science as well. It has been proposed to deploy a new generation of observatories on the lunar surface for deep studies of our Universe. This includes radio antennas, which would be protected on the far side of the Moon from terrestrial radio interference, and gravitational-wave (GW) detectors, which would profit from the extremely low level of seismic disturbances on the Moon. In recent years, novel concepts have been proposed for lunar GW detectors based on long-baseline laser interferometry or on compact sensors measuring the lunar surface vibrations caused by GWs. In this article, we review the concepts and science opportunities for such instruments on the Moon. In addition to promising breakthrough discoveries in astrophysics and cosmology, lunar GW detectors would also be formidable probes of the lunar internal structure and improve our understanding of the lunar geophysical environment. [ABSTRACT FROM AUTHOR]

Between 2017 and 2019, the CSIR-NGRI, Hyderabad, Telangana, established a broad-band seismic-network with fifty-five 3-component broadband seismometers in the Himalayan region of Uttarakhand, India. Out of 55 three component broadband seismic (BBS) networks, we chose 17 for the present study. Using digital waveform data from twenty-one (21) regional Indian earthquakes of Mw 5.0–6.2 that were recorded in the 17 broadband seismometer, we compute fundamental mode group-velocity dispersion (FMGVD) characteristics of surface waves (Love and Rayleigh waves) and the average one-dimensional regional shear-wave velocity (Vs) structure of the Uttarakhand Himalayan region. First, we compute FMGVD curves for Love waves (6–73 s) and Rayleigh waves (at 6.55–73 s) period, and then, we finally invert these dispersion curves to compute the final average one-dimensional regional crustal & sub-crustal shear-wave velocity (Vs) structure below the Uttarakhand Himalaya. Our best model in Uttarakhand Himalayan region, India, reveals the 8-layered crust with a mid-crustal low velocity layer (MC-LVL) (approximately a drop of 1.5–2.3% in Vs) between 8 and 20 km depth in the proximity of MCT (Main Central Thrust). In the upper crustal part (0–20 km depths), our modelling suggests shear velocities (Vs) varies from 3.1 to 3.9 km/sec while shear velocities (Vs) in the lower crustal part (20–45 km depth) are modelled to be varying from 3.7 to 4.69 km per sec. The Moho-depth is calculated to be 45 km deep below the K-G Himalaya, and the shear-velocity (Vs) in the sub-crustal sector is 4.69 km/sec. Our estimated mid-crustal low-velocity layer (MC-LVL) could be linked to the presence of metamorphic fluids in the fractured Main Himalayan Thrust (MHT), resulting from the weakening of the crustal material at the interface between the overriding Eurasian plate and upper part of the underthrusting Indian plate. [ABSTRACT FROM AUTHOR]

The article presents the results of DSS seismic surveys in the Persian Gulf. Bottom analog-type seismographs and seismic airgun sources were used in the studies. The bottom seismographs were moored and shooting was carried out according to three regional profiles with a length from 100 to 250 km. The main result of these studies was a velocity–depth model of the sedimentary cover and Earth's crust up to the Moho boundary, which was revealed at a depth of about 43 km in the water area. Given that the thickness of the upper crust is only 4–5 km and based on the velocity characteristics of the remaining layers, the type of crust can be attributed to the subcontinental Archean type. This situation (complete absence or drowning of the upper layer of the Earth's crust) is typical of the waters closest to the Persian Gulf: the Black, Caspian, Mediterranean, and Red seas. A structure was found in the Earth's crust of the studied area, which may be a brachyanticline with an isometric dome-shaped shape, which corresponds to the platform-type folding in the areas of salt dome tectonics. No faults have been found in the crust of the water area of the Persian Gulf adjacent to the Bushehr Peninsula. [ABSTRACT FROM AUTHOR]

We computed the coda wave attenuation (QC) using reservoir-induced seismicity of the Koyna–Warna region in the western part of the Indian shield. We analyzed 484 local seismograms recorded by 16 broadband seismic stations at central frequencies 1.5, 3, 6, 12, and 24 Hz for lapse time window lengths 30s, 40s, 50s, and 60s using the single-backscattering method (Aki and Chouet 1975). The results show that spatial variation of QC, increases from west to east, and north to south in the study region. Our lapse time-dependent of QC estimates are (138.05±37.41)f(1.07±0.07), (191.18±45.24)f(1.00±0.07), (240.07±61.50)f(0.95±0.08), and (299.42±76.32)f(0.88±0.07) for 30s, 40s, 50s and 60s, respectively. The results show that low QC at low frequencies (<3 Hz), which indicates high attenuation at a shallow depth of the region. We compared QC with other regions of the world, as well as in India. Our results are high compared to the earlier studies in the Koyna–Warna region. The frequency-dependent parameter, η is greater than 1, which represents that the study region is seismically active. This study is important for evaluating the source parameters and seismic hazard assessment of the region. Research Highlights: These are the first scientific Qc results obtained using educational seismographs installed in schools in the western part of India. We have used 484 local earthquakes using 16 seismic stations in the Koyna-Warna region for this study. The region shows low QC to the west of compared to east of WGE and it is high for east of Koyna reservoir. The η values are high, varying between 0.76 and 1.19, which indicates the region is heterogeneous and seismically active. The seismic quality factor, QC values are little high and they agree with previous studies in the region. [ABSTRACT FROM AUTHOR]

The Western Desert of Egypt is an area of the Sahara which lies west of the River Nile, up to the Libyan border, and south from the Mediterranean Sea to the border with Sudan. It is mostly rocky desert; the desert covers 2/3 of the land of Egypt. Its highest elevation is 1000 m in the Gilf Kebir plateau to the far south‒west of Egypt, on the borders of Egypt, Sudan and Libya. We used to collect available data of the earthquakes, which occurred in or near the western part of Egypt during the period from 1900 to 2018, to investigate the seismic activity and its relationship to the geology and tectonics of Egypt. The frequency statistical analysis is used to calculate the frequency-magnitude relation for the data in this period. The results indicate that about 28 earthquakes are equal or exceed Richter magnitude M ≥ 4 every 100 years. The whole Western Desert is characterized by low to moderate levels of seismic activity. The southern part of Western Desert is considered to be seismically active. The source mechanism for some earthquakes was estimated using the waveform data recorded from the records of the Egyptian national seismograph stations and our results show the normal faulting mechanism with minor strike-slip component. The NNW trend has been chosen as a preferred rupture plane in consistence with surface and subsurface faults and microearthquake seismicity in the epicentral area as well. [ABSTRACT FROM AUTHOR]

In the latest geophysical survey crossing the Ninety East Ridge of the Indian Ocean, a new method was employed to perform proportional double seismic source excitation and synchronously receive signals from the sea surface and the seabed. The two seismic sources used for excitation were two sets of gun arrays with different energies and dominant frequencies, a G gun array and a Bolt gun array. The G gun array consisted of 3 G.II guns with a total capacity of 450 in3 and a dominant frequency of 20–100 Hz. The Bolt gun array consisted of 4 Bolt 1500LL air guns with a total capacity of 6000 in3 and a dominant frequency of 10–40 Hz. The seismic receiving system comprised a 480-channel seismic streamer towed from the sea surface and 21 ocean bottom seismometers (OBS). During offshore operations, the integrated navigation system produced equidistant trigger signals at an interval of 50 m. The trigger signals were distributed to the G gun array and Bolt gun array at a ratio of 3:1 after passing through a pulse signal proportional distributor. The two sets of gun arrays fired alternatingly at a given ratio. The receiving equipment on the sea surface and seabed simultaneously received the seismic signals excited by the two sets of gun arrays. After targeted data processing, in addition to the seismic profile generated by the conventional G gun seismic source, the deep seismic profile generated by the Bolt gun seismic source and the survey profile of the active-source OBS were obtained simultaneously. The penetration depths of the three sets of profiles reach 2 km, 6 km, and 30 km, respectively, greatly improving the efficiency of offshore deep-sea seismic surveys. [ABSTRACT FROM AUTHOR]

During 2017–2019, a temporary seismic network of fifty-five 3-component broadband seismometers was installed by CSIR-NGRI, Hyderabad, Telangana, in the Uttarakhand Himalayan region, India. We used digital waveform data of 21 local earthquakes of Mw 2.9–4.3 from nineteen 3-component broadband seismic stations of the above seismic network for computing source parameters of local earthquakes, through simultaneous inversion of shear wave (S-wave) spectra. Here, we used the ω square circular source spectral model of Brune (1970) to formulate the iterative inversion scheme to compute earthquake source parameters. The modelled corner-frequency (fc), source radius (r), seismic moment (Mo) and stress-drop (Δσ) vary from 2.4 to 4.0 Hz, 247–693 m, 2.4 × 1013 to 6.6 × 1015 N-m, 0.2–5.4 MPa, respectively. The maximum stress-drop (Δσmax) is computed to be 5.4 MPa for an event of Mw 4.3 while the minimum stress drop (Δσmin) is computed to be 0.2 MPa for an event of Mw 2.9. The scaling relations between Mo and fc is obtained as Mo = 3.2E + 17fc−5.5, while Mo and Δσ are modelled to be related as Mo = 4.59E + 13∆σ2.5. This relationship could be very useful for the future seismic-hazard assessment of the Uttarakhand Himalayan region. Research Highlights: Modelled source parameters of local earthquakes and scaling relationship in the Uttarakhand Himalayan region. Local earthquakes magnitude range of 2.9–4.3. Mo, r, fc, and ∆σ are 2.4×1013–6.6×1015 N-m. 247.0–693.0 m, 2.4–4.0 Hz, 0.2–5.4 MPa, respectively. Mofc5.5 ∝ constant & Mo∆σ2.5 ∝ constant. [ABSTRACT FROM AUTHOR]

Aswan area in South Egypt experiences continuous seismic activity due to seismogenic active faults, particularly Kalabsha and Seiyal active EW faults. The seismic site characterization is not properly identified, although the presence of high-density distribution of earthquake stations. The present study investigates fourteen earthquake stations of the Egyptian National Seismic Network, as well as six microtremor measurement sites. We analyzed ground motions due to seismogenic active faults recorded at surface from these fourteen earthquake stations. We measured microtremors for up to 120 min with portable seismometers at six sites in the vicinity of the High Dam area. The horizontal-to-vertical spectral ratios of earthquakes (EHVSR) and microtremors (MHVSR), their plots as a function of frequency and direction of motion, and diffuse field inversion are used in the study. Therefore, we could provide an obvious understanding of the site characterization including resonance frequencies, directional amplifications, and back-calculated subsurface velocity structures at these stations and sites. We found three predominant amplification directions of NS, EW, and NE–SW due to the horizontal components of the seismic waves. These amplification directions are near-transversal ~ transversal to the NS and EW strikes of the active fault system in the study area. In time–frequency analyses of the records, this directionality is observed clearly for S-wave and surface wave time windows. We validated the diffuse field inversion process not only using fitting between observed and inverted EHVSRs and MHVSRs, but also using available geological 2D cross sections and hydrological information in Aswan area. [ABSTRACT FROM AUTHOR]

Young geoscience students in developing countries, using Nigeria as a case study grow up with little or no idea on the working principle behind expensive and imported seismographs during fieldwork. To proffer solution to this problem, seismographs need to be designed locally using low-cost materials. The proposed seismograph is made up of a single channel. It uses Arduino microcontroller which controls the data acquisition system and trigger system to initiate the seismic data acquisition. A user-friendly application was designed using LabVIEW to monitor the seismic waveforms on a personal computer. The cost of the seismograph was as low as 70 US dollars. Its applications are to promote geoscience education among young geophysicist, and can even be extended to Physics laboratory experiments. The seismograph is not recommended for seismic exploration, but rather it is a template for understanding its working principle. [ABSTRACT FROM AUTHOR]

Abstract—In 2007–2015, Shirshov Institute of Oceanology of the Russian Academy of Sciences (IO RAS) carried out seismological observations in the water area and on the coast of the Baltic Sea using autonomous seismic stations. Here, in the area of the Sambia Peninsula, previously considered aseismic, a strong perceptible earthquake with a magnitude of about M = 4.6 occurred in 2004. The most interesting data were obtained by IO RAS in 2008–2009 from seismological monitoring using autonomous bottom and coastal seismic stations. The data obtained in 2010–2015 turned out to be unsuitable for full-scope processing due to several causes (losses of bottom seismographs, high noise level at coastal stations, etc.). Seismological monitoring in the west of the Kaliningrad region and in the adjoining area of the Baltic Sea detected weak earthquakes with magnitudes ML = 2.5–3 whose sources are confined to the development are of the Kravtsovskoe offshore oil field. Some of these earthquakes have been recorded by the stations of the Norwegian seismic array NORSAR and by the seismic stations of Sweden. The Kravtsovskoe oil field is located on the shelf northwest of the unique natural object, the Curonian Spit. The detected weak earthquakes are likely to be anthropogenic, induced by reservoir pressure disturbances as a result of intensive hydrocarbon production, and are probably precursors of a strong man-made earthquake. [ABSTRACT FROM AUTHOR]

A schematic diagram of a seismic receiver based on a GS-ONE LF geophone is proposed and implemented. An electronic circuit for summing seismic signals from geophones is developed, allowing the signal-to-noise ratio to be increased. Hardware correction is implemented to extend the geophone frequency range and sensitivity. The developed seismometer can be used when monitoring hydraulic fracturing operations as part of small-aperture seismic groups. [ABSTRACT FROM AUTHOR]

We develop an original algorithm for velocity estimation that incorporates the constraints of three seismic functionals: surface wave dispersion, P-receiver function, and S-waveform. Here, we use a parallelized reflectivity algorithm to generate synthetic seismograms and match the observed functionals by a global optimization scheme called very fast simulated annealing (VFSA). This method also allows us to assess the uniqueness and parameter independence of the resultant models. Synthetic tests using surface wave (SW) dispersion and receiver functions (RF), and then SW, RF, and waveforms windowed around the S arrival, establish that inclusion of the third functional results in the best recovery of the model. We employ the algorithm to model the Kachchh basin in Gujarat, India, because the area is of active interest for monitoring purposes, and prior results of RF and SW modeling are available for comparison. The waveform functionals used here are generated from broadband seismograms of teleseismic, deep (396–609 km), and moderate- to large-magnitude (6–6.8) earthquake events recorded at semipermanent seismograph stations in the Kachchh basin. Joint inversion of SW, RF, and S-waveform improves the velocity structure by revealing layers that were not identified in previous modeling that used SW and RF alone. Our model clearly shows low-velocity zones (LVZs), crustal and lithospheric thinning, and the lithosphere–asthenosphere boundary. We estimate the crustal thickness to be 41 km at all but one station, and the lithosphere to be in the range of 70–80 km. P- and S-velocities from the uppermost crust to the Moho vary from 4.7 to 7.0 km/s, and 2.7–4.1 km/s, respectively. [ABSTRACT FROM AUTHOR]

In this paper, the results of lidar monitoring of magmatic aerosol variation are presented. The lidar was installed in a hot dead-end tunnel at the end of the auxiliary adit of the Baksan Neutrino Observatory (BNO) of the Institute for Nuclear Research, Russian Academy of Sciences, which is located above the small magma chamber of the Elbrus Volcanic Center. A sharp (within ~30 min) threefold increase in the aerosol concentration in the hot tunnel was detected (October 28, 2019, 22:30 UTC). The analysis involved data on volumetric radon activity, air temperature, and humidity. A synchronous increase in temperature, humidity, and aerosol concentration in the tunnel, as well as their correlation with the deformation of the Earth's crust by tidal waves, which is monitored at the BNO with a laser strainmeter, was revealed. However, the threefold jump did not appear in these signals. The totality of data confirms the existence of a hot magma chamber at the base of Mount Andyrchi that was previously discovered by a small-aperture seismometer group. The assumption that the release of gases from the magma chamber could provide a jump in the concentration of aerosols without disturbing other parameters is discussed. [ABSTRACT FROM AUTHOR]

Submarine volcanism is widely developed in the South China Sea (SCS). However, the characteristics, distribution, and genesis of submarine volcanoes in the southern margin of the SCS remain obscure. In this study, we analyzed the characteristics of submarine volcanoes and identified a total of 43 submarine volcanoes in the southern margin of the SCS, based on a newly acquired 310-km seismic reflection profile, along with previous 45 multi-channel seismic (MCS) profiles, petrological results from volcanic rocks sampled by dredging and drilling, nearby ocean bottom seismometer (OBS) wide-angle seismic profiles, and gravity and magnetic data. The study ascertains that most of these volcanoes are located in fault-block belts and graben-horst zones with strong crustal stretching and thinning. These volcanoes exhibit positive high-amplitude external seismic reflections, weak and chaotic internal seismic reflections, and are accompanied by local deformation of the surrounding sedimentary strata. Meanwhile, they have higher positive gravity anomalies and higher magnetic anomalies than the background strata. The petrological dating results show that volcanic ages are primarily in the Pliocene-Pleistocene, with geochemical characteristics indicating dominance of oceanic island basalt (OIB) -type alkali-basalts. Extensional faults have obviously spatial correspondence with post-spreading volcanism, suggesting these faults may provide conduits for submarine volcanism. The high-velocity bodies (HVBs) in the lower crust and magma underplating exist in the southern SCS, which could provide a clue of genesis for submarine volcanism. The inference is that the intensity of post-spreading volcanism in the southern margin might be affected by stretching faults, crustal thinning and magma underplating. [ABSTRACT FROM AUTHOR]

The southwestern depression of the Qiongdongnan Basin (QDNB) hosts thick Cenozoic sediments and awaits major hydrocarbon discovery. Multichannel seismic (MCS) profile CFT2011 across the southwestern QDNB reveals a ∼60-km-wide fuzzy reflection zone (FRZ) within the sediments, but its origin and distribution remain unclear. Here ocean bottom seismometer (OBS) data of Line CFT2011 are processed with focus on the velocity structures by traveltime tomography inversion and analyzed together with the coincident and adjacent MCS profiles. The OBS velocity results show that the giant FRZ features lower velocity with difference up to 1.5 km/s and smaller vertical velocity gradient than the surrounding sedimentary sequences at the same depth, likely resulting from enhanced fluid infilling. The MCS profile exhibits that the giant FRZ is about 3–9-km thick and extends from the Paleogene strata rich in organic matters upward to the lower Pleistocene sediments. Within the shallow overlying sediments, multiple bright spots with reverse polarity are imaged and their reflection amplitudes increase with offset, consistent with the features of gas-charged sediments. They are probably shallow gas reservoirs with gases sourced from the deep FRZ. Therefore, the FRZ is proposed to be a giant gas-charged zone, which probably contains lots of hydrocarbon gases migrated vertically from the deep Paleogene source rocks through the boundary faults of the depressions and the minor fractures generated under overpressure. This FRZ is also imaged on the adjacent MCS profiles MCS-L1 and MCS-L2 with the width of about 40 km and 68 km, respectively. It is roughly estimated to cover an area of ∼1 900 km2 and host a volume of ∼11 400 km3 assuming an average thickness of 6 km, implying huge natural gas potential in the sedimentary depression of the southwestern QDNB of the South China Sea. [ABSTRACT FROM AUTHOR]

To clarify the characteristics of high-frequency (> 1 Hz) S and S-coda waves at ocean-bottom seismometers (OBSs), we analyzed seismograms observed at permanent OBSs and inland broadband seismometers around the Kii Peninsula in southwest Japan along the Nankai Trough. The coda amplitudes (both horizontal and vertical) at the OBSs were much larger than those at the inland rock-site stations. Because coda amplitudes relative to those at inland rock-site stations have been used as site-amplification factors, large site amplifications for both components can be expected due to the presence of thick oceanic sediments just below the OBSs; however, the observed maximum S-wave amplitudes in the vertical component exhibited similar attenuation trends against epicentral distances at both OBS and inland stations. To clarify the causes of this discrepancy, we conducted numerical simulations of seismic wave propagation using various three-dimensional seismic velocity structure models. The results demonstrated that coda waves at OBSs mostly comprise multiple scattered waves within a thick (> 2 km) sedimentary layer; consequently, coda amplitudes at OBSs become much larger than those at inland rock-site stations. Our numerical simulations also confirmed the generation of large coda amplitudes at regions with seawater depths ≥ 4 km, where no OBS was deployed. However, the thick sedimentary layer and seawater have limited effects on maximum S-wave amplitudes at the OBSs. Given that the effects of a thick sedimentary layer and seawater on S and S-coda waves differ, we concluded that the coda-normalization technique for site-amplification correction against a rock-site station could not be applied if stations are located within regions above the thick sedimentary layer or deeper sea depths. The site amplifications at the OBSs were corrected according to the horizontal-to-vertical ratios at each OBS; we adjusted the simulated horizontal envelopes at the OBSs using these ratios of the observed S-coda waves. As well as inland seismometers, the site-corrected simulation results practically reproduced the observed high-frequency envelopes at OBSs. [ABSTRACT FROM AUTHOR]

Cyclic behaviour is observed in volcanic phenomena ranging from caldera collapses to explosions, spattering or lava fountaining. The repeating processes can define irregular, regular or systematically changing patterns. These patterns yield information about the subsurface structure, which often is not considered in detail. We analyse the pattern of 7058 lava fountaining episodes that occur between 2 May and 14 June 2021 during the Geldingadalir eruption, Iceland. Our seismometer records the lava fountaining episodes as tremor episodes. We analyse the seismic tremor amplitude, the episode duration, the repose time and the sum of episode duration and repose time (cycle duration). We define six periods characterised by different patterns: Three periods feature long episodes that exponentially shorten with time. One period features coexisting long and short episodes in a haphazard sequence. One period shows a stable pulsing duration but increasing repose time, and one period has stable, short episodes and repose times. We conclude that the episodic fountaining starts because a shallow-conduit container forms on 2 May shifting the magma degassing from sustained continuous to an episodic state. This situation evolves until 11 May when a semi-stable state is reached. The length of the repose times is most likely influenced by the amount of outgassed magma present in the uppermost part of the shallow conduit. Finally, we suggest that the vent is mechanically eroded and widens with time causing increasing seismic tremor amplitudes. However, the trends are frequently punctuated by partial crater wall collapses that temporarily disrupt the system. [ABSTRACT FROM AUTHOR]

S-net is a large-scale ocean bottom (OB) network in the Japan Trench area, consisting of inline-type 150 observatories equipped with seismometers and pressure gauges. Among them, 41 observatories have been buried about one meter beneath the seafloors in the shallow water regions (water depth <1500 m). We analyzed the strong-motion data recorded at the S-net sites from earthquakes with magnitudes 3.5 < Mw ≤ 7 and focal depths < 70 km to understand the site amplification effect on the recorded motions. We used the spectral inversion technique and obtained some fundamental properties of the earthquake source spectra, path attenuation, and site factors from the horizontal-component S-wave portions of the recordings. We obtained that the source spectra followed the ω - 2 source model generally well, and the estimated magnitudes were mostly within ± 0.3 magnitude units of the catalog magnitudes. The stress drops increased systematically with the focal depths, and the values for the shallow earthquakes in the Pacific Plate were higher than those for the interplate earthquakes with comparable focal depths. The path-averaged quality factors were generally frequency-dependent and were somewhat larger than those in the past studies. The peak site frequencies ranged between about 0.2 and 10 Hz, while the peak amplification factors ranged between 10 and 50. Even though the peak frequencies and amplification factors differed from site to site, the peak frequencies were mostly higher than 2 Hz at the outer trench stations, while they were lower than 2 Hz at many inner trench stations. The amplification factors at a few OB sites in the shallow water regions were comparable with the theoretical ones computed from the 1-D subsea model. The amplification factors at intermediate frequencies (~ 0.3 to 2 Hz) generally increased with P-wave travel time in the sediments estimated from the multi-channel seismic survey. At about 20% of the sites (mainly at the unburied stations), spurious site spectra were recognized at frequencies higher than about 4 Hz. If the site spectra between 4 and 10 Hz are required, using only the X-component records is recommended. [ABSTRACT FROM AUTHOR]

The velocity structure of the mantle under the Baikal Rift Zone (BRZ) is investigated with the P‑wave receiver functions (PRFs) for a group of 10 seismograph stations. The BRZ presents one of the world's most active continental rift zones. The peculiarities of the BRZ include Cenozoic magmatism in the upper mantle of the southwestern part of the BRZ, which disappears in the central and northeastern parts. The analysis of seismic data reveals other indications of the significant lateral heterogeneity of the mantle beneath the BRZ. At half of the stations there is evidence of a sharp rise of the S velocity with depth at a depth of around 330 km, similar to the descriptions for the "X" or the "300-km" discontinuity. In the central and northeastern regions at depths from about 350 to 410 km there is a well pronounced low S velocity layer, which is practically missing in the southwestern part. The origin of this layer is apparently related to the upwelling and dehydration of wadsleyite in the transition zone. At depths from 500–600 to 660 km in the central and northeastern regions there is another low velocity layer that may be explained by the accumulation of garnetite in the process of subduction of the lithosphere of the Pacific. This layer is poorly pronounced in the southwestern region. The difference between the travel times of the P410s and P660s seismic phases (differential time) in the southwestern region (23.5 s) is close to the data for the standard model (Kennett, Engdahl, 1991). In the central and northeastern regions, the observed differential time is larger than the nominal time by 1.0 s. The rise of the differential time may be related to the cooling and/or hydration of the transition zone by the slabs of the subducted oceanic lithosphere. The obtained seismic data suggest a large role of processes of hydration and dehydration in the central and northeastern regions, however, this role is comparatively small in the southwestern region. [ABSTRACT FROM AUTHOR]

The results of new local seismic observations in the Laptev Sea obtained using ocean bottom seismographs were compared with actual data from global and regional earthquake catalogs. Most earthquake epicenters on the Laptev Sea shelf are grouped into several clusters located within the East Laptev province of horsts and grabens and enclosed between two extension detachments. The first one marks the eastern boundary of the Ust–Lena and Omoloi rift systems, and the second one is the eastern boundary of the Anisin, Zarya, and Belkovskii-Svyatoi Nos rift chains. At the same time, the linear zone of epicenters confined to the Gakkel Ridge, when moving to the shelf, shifts along the Khatanga–Lomonosov fault zone to the northeast in the direction of the second extension detachment, which probably remains the only active one. The spatial distribution of earthquakes in the Laptev Sea, taking into account the epicenters of micro- and weak seismic events on the shelf, does not have sufficiently clear boundaries that determine the contours of the previously assumed lithospheric microplate. [ABSTRACT FROM AUTHOR]

New heat flow data corrected for climate change over Northeastern Japan were obtained using the temperature profile of the borehole of the High Sensitivity Seismograph Network (Hi-net). The obtained spatial distribution of heat flow shows low heat flow on the forearc side, high heat flow along the Ou Backbone Range, and low heat flow in the plains on the back-arc side. However, the distribution is not clearly divided into high and low heat flow along the VF front; for example, the low heat flow extends from near the northern Kitakami Mountains on the forearc side to the Ou Backbone Range crossing the VF, while the high heat flow extends to the central Kitakami Mountains and Sendai plain on the forearc side. In addition, a crustal temperature structure model was developed that considers into account the presence of sedimentary layers, the temperature dependence of thermal conductivity, and differences in heat generation due to lithology. There is a good correlation between this temperature structure and the lower limit of the seismogenic layer, which is between 400 and 450 °C. Compared to previous studies, the crustal thermal structure calculation method assumed is a model whose estimated temperature distribution is sensitive to structural differences; however, a more accurate estimation of the temperature structure is possible if detailed structural information is available. On the other hand, it seems necessary to treat fluid behavior in more detail in areas of high heat flow. However, the estimation of crustal temperature structure, especially in regions with thick sedimentary layers, is considered an improvement over the previous study. [ABSTRACT FROM AUTHOR]

Tidal gravimeters can detect intermediate-period surface waves with high accuracy. Three gravimetric stations with estimated transfer functions and co-located with seismic stations were selected: two in Belgium (Membach and Rochefort, in Western Europe) and one in Germany (Black Forest, in Central Europe). The compatibility of gravimetric and seismic recordings of earthquakes in the period range of 10-180 s has been presented. The series of monochromatic signals separated from surface waves for selected events have been calculated using the multiple-filtering procedure, and averaged fundamental-mode Rayleigh wave group-velocity curves have been estimated on a regional scale. Next, averaged dispersion curves for three regions (Italy, Greece, and Western Turkey) were inverted by weighted linear inversion methods. Additionally, a quantitative analysis of resolution tests of inverted models was presented to show the capabilities of the gravimetric data to retrieve a shear-wave velocity distribution with a depth. In particular, a method for determining the depth range of the inverted model has been proposed. Results obtained based on gravimetric data were verified by results from seismic data obtained by applying the same procedure. This study shows the novel application of tidal gravimetric data in the determination of the regional Earth's structure based on intermediate-period surface waves recordings, as well as a comprehensive approach to the quantitative estimation of a final model resolution. [ABSTRACT FROM AUTHOR]

The sine calibration can be used for measuring the phase response of a seismometer. However, it has two shortcomings: (1) Transient responses that cannot be used for data processing appear multiple times. This increases the time consumption of the calibration test. (2) Testers have to manually select the steady-state response waveform from the sine response waveform of each frequency. This makes it difficult to realize automatic data processing. In this article, we introduce a new method of measuring the phase responses using superimposed sine signals and use this method to measure the phase responses of two seismometers in the frequency range of 0.002 to 1 Hz. In the test, the transient response appears only once. Therefore, the time span of the response waveform that cannot be used for data processing is greatly reduced. Moreover, when we select the steady-state response waveform, it is only necessary to avoid the transient response at the beginning of the response waveform. Therefore, it is easier to realize automatic data processing. We measure the phase responses of the seismometers 20 times with this new method and find that the largest standard deviation is only 0.0143°. We compare the mean value of the results of the multiple tests with the result of the traditional sine calibration and find that the largest error is only 0.019°. This work demonstrates that this new method is not only very accurate but also overcomes the two shortcomings of the traditional sine calibration. [ABSTRACT FROM AUTHOR]

The recording of seismic signals at the bottom of water areas is in most cases accompanied by intense noise, whose sources are near-bottom currents. An effective way has still not been found to combat this phenomenon, which significantly impedes the development of a marine seismological network. The main problems are that such interference is generated both on equipment elements, which, according to the operating conditions, should be on the bottom surface, and on the unevenness of the bottom surface. In this study, a number of mechanisms for exciting such interference are considered, including due to the direct impact of stationary and nonstationary flows on the sensitive elements of bottom seismographs and due to the generation of seismoacoustic fields in the aquatic environment during vortex and turbulent flows. The results of recording seismic signals on the bottom, in the sediment layer, and in the bottom borehole in the presence of intense noise from currents are compared. A number of practical recommendations are given to significantly reduce the impact of such noise. [ABSTRACT FROM AUTHOR]

We conducted temporary seismic observations at the Hitachi-Kashiwa Soccer Stadium on a J. LEAGUE game day to obtain unique seismic records due to the collective action (i.e., jumping) of supporters, which were also recorded in a permanent Metropolitan Seismic Observation network (MeSO-net) station. This study investigated seismic wave excitation as well as seismic wave propagation from the stadium to its surroundings. The rhythms of the jumps of the supporters were characterized by analyzing audio data recorded in the stadium, which were compared with the characteristic frequencies observed in the seismic records. The characteristic frequencies in the seismic records are integer multiples of the jumping rhythms, which is consistent with the loading model of jumping people proposed in earlier studies. This implies that seismometers could be useful for monitoring collective human activity. Travel times were studied using deconvolved waveforms because seismic waves generated by the supporters are sinusoidal with vague onset. Polarization analysis was performed to measure the amplitude and polarization azimuths. The observed seismic wave propagation was compared with synthetic waveforms calculated using one-dimensional physical properties based on the Japan Seismic Hazard Information Station (J-SHIS). The synthetic waveforms calculated with the shallow and deep layer combined model are more consistent with observations of travel times and amplitude decay than those calculated with the only deep layer model, although a part of the observations cannot be explained by both models. This result suggests that the subsurface structure of J-SHIS is good in this region, although a more detailed three-dimensional structure and topography must be considered to fully explain the observations. As human-induced seismic signals are expected to be generated in various situations, this study shows that such unique seismic waves can be used as an artificial seismic source for validating and improving local shallow subsurface structural models in urban environments. [ABSTRACT FROM AUTHOR]

A swarm of volcano-tectonic earthquakes (VTEs) started in May 2018 at the east of Mayotte. In addition, in the following month, long-period earthquakes (LPEs) also started occurring at this swarm zone, transmitting long-period (LP) waves whose records in broadband seismographs are analyzed. Since then, 251 LPEs of magnitude (Ms) between 3.2 and 4.9 have occurred. The LPE occurrence was maximum during October 2018; the occurrence decreased significantly from June 2019 and continued to January 2022. Prior to the VTE swarm, two more LPEs, each of magnitude 3.2, occurred in the same zone in May 2010 and January 2018. Records of many LPEs show that the amplitude of LP waves is small at the beginning, attains maximum by 3–5 min and decreases thereafter. However, there are other LPEs showing waves with a repeated increase and decrease in amplitude. The LP waves are of duration of about 26 min and consist of monochromatic Rayleigh waves with a period between 14.52 s and 15.63 s. The increase and decrease in the period happened simultaneously with the increase and decrease in the number of LPEs. An increase in period was noted in January 2018 prior to the volcano-tectonic activity that started in May 2018. Comparisons of synthetic and observed seismograms show that a LP waveform depicts amplitude-varying monochromatic oscillation of a volcanic fluid-filled resonator; varieties of amplitude variation pattern of an oscillation are noted for different LPEs. To explain such amplitude-varying oscillation, we need to refine the existing fluid mechanics in the resonator. [ABSTRACT FROM AUTHOR]

The occurrence of faulting in subducting plates is a major process that changes the mechanical properties of the subducting lithosphere and carries surface materials into mantle wedges. Two ocean-bottom seismometer networks deployed on the frontal accretionary wedge of the northern Manila trench in 2005 and on the outer slope of the trench in 2006 were used to detect earthquakes in the subducting plate. All available P and S manually picked phases and the waveforms of 16 short-period, three-component stations were used. Relocation was performed using the double-difference method with differential times derived from the phase-picked data. Two intraplate earthquake sequences of small-to-moderate magnitudes in the northern Manila subduction system were investigated in this study. The results revealed distinct fault planes, but a contrasting seismogeny over the northern Manila Trench. The seismicity in the frontal wedge (as measured in 2005) was mainly contributed by a fluid overpressure sequence, whereas that in the incoming plate (as measured in 2006) was contributed by the aftershocks of an extensional faulting sequence. The obtained seismic velocity models and Vp/Vs ratios revealed that the overpressure was likely caused by high pore-fluid pressure within the shallow subduction zone. By using the near-field waveform inversion algorithm, we determined focal mechanism solutions for a few relatively large earthquakes. Through the use of data obtained from global seismic observations, we determined that stress transfer may be responsible for the seismic activity in the study area during the period of 2005–2006. In late 2005, the plate interface in the frontal wedge area was unlocked by the overpressure effect due to a thrusting-dominant sequence. This event changed the stress regime across the Manila Trench and triggered a normal fault extension at the outer trench slope in mid-2006. However, in the present study, a hybrid focal mechanism solution indicating reverse and strike–slip mechanisms was implemented, and it revealed that the plate interface locked again in late 2006. [ABSTRACT FROM AUTHOR]

In this study, we probe the misalignment of 200 temporary broadband seismometers based on the polarization of P waves from regional and teleseismic earthquakes. The seismometers were deployed in the epicentral region of 2016 ML 5.8 Gyeongju earthquake, South Korea, and this unprecedented dense array provided a unique opportunity for investigating fault structures from microseismicity. For the full use of three-component seismic records, we estimate and provide time-dependent misorientation angles of the 200 seismometers from June 2018 to March 2021 with uncertainty assessments. Two methods based on the principal component analysis and the minimization of transverse P-wave energy are applied. Our estimates are characterized by small uncertainty (average median absolute deviation of 3.14°). Moreover, periods of suspected temporal changes in misorientation angles mostly coincide with periods of reported technical operations, which demonstrates reliability of our methods to precisely detect the temporal variation of misorientation angles. We expect our misorientation angles to serve as an essential metadata for further seismological researches utilizing the dense array data. [ABSTRACT FROM AUTHOR]

By using P and S wave receiver functions and P and S wave travel time residuals, we have found velocity models for 16 seismograph stations in Eastern Anatolia. Our study is focused mainly on the mantle lithosphere, asthenosphere and transition zone. The volcanism and uplift of the Eastern Anatolia Plateau are thought to be related to the Bitlis slab break off and delamination of the continental lithosphere. Sinking cold slab and lithospheric drips can reduce temperature in the mantle transition zone (MTZ) by up to a few hundred degrees C. However, our analysis of seismic data provides no robust evidence of significant cooling of the transition zone. In the mantle immediately above the 410-km discontinuity there is a pronounced low S wave velocity layer that may be a source of the volcanism in the study region. Another low velocity layer is present at the base of the MTZ. The obtained S wave velocity models of the upper mantle can be divided into three groups. In the first group, the lithosphere—asthenosphere boundary (LAB) is at a depth of ~ 60 km. In the second group, the LAB is at a depth from 90 to 100 km. In the third group, the mantle lithosphere is practically absent. On a scale of our analysis there is no clear correspondence between the obtained mantle velocity models and the volcanism (< 23 Ma) exposed at the surface. Only the models of the first group are well represented in the neighboring Central Anatolian Plateau. [ABSTRACT FROM AUTHOR]

Reliable information on the horizontal orientation of a seismometer is crucial to seismological research utilizing three-component seismograms. In this study, we provide misorientation angles of broadband seismometers in three permanent networks in South Korea from 2003 to 2021 by using two methods, denoted as PPCA and PminT, both utilizing P-wave polarization characteristics. Our estimates show that 36% of the sensors have been aligned within 5° from the geographic north during their operation periods, while 40% of the sensors have been rotated by more than 10° at least once. The estimates are highly consistent for both methods, with 95% of the total showing uncertainty less than 10°. Moreover, we identified a significant number of temporal changes in misorientation by taking automatic change point detection and visual inspection. The procedure described here and misorientation angles can be useful reference for seismic data preprocessing for research utilizing horizontal-component seismograms for earthquake sciences. [ABSTRACT FROM AUTHOR]

Debris flows are hazardous phenomena occurring at volcanoes, and monitoring them has proved as challenging as imperative in several cases. The use of seismic instruments to measure and study the physical properties of debris flows has witnessed significant progress in the last years, with the use of improved sensors, innovative methodologies and high-resolution analysis. However, the application of such studies to the practical task of providing early warnings remains limited by the significant amount of infrastructural and technological resources commonly required for their deployment. In Ecuador, debris flows at volcanoes are detected by means of seismic instruments which are usually part of broader monitoring networks, thus requiring calibration to provide quantitative information about the flows and feed early-warning systems. In the present work, a theoretical approach based on the Buckingham Π-theorem is used to determine an expression that linearly correlates the seismic signal produced by a transiting debris flow with its discharge rate, for instruments installed in different substrata and at variable distances from the drainage. The expression is experimentally tested with Acoustic Flow Monitors and Broad-band seismometers installed in the vicinity of drainages at Tungurahua and Cotopaxi volcanoes, where actual debris flows occurred in relation to eruptive activity. The experiments consist in comparing the measured peak amplitude values of the seismic signal envelopes with the estimated peak discharge rates of several events. The results confirm the validity of the theoretical expression with linear correlations observed between the seismic amplitudes and the discharge rates, thus defining calibration expressions that can be generally applied to varied environments and instruments. The seismic instruments calibrated through this methodology can provide instantaneous and reliable predictions of debris flow discharge rates within less than an order of magnitude and only requiring limited data processing and storage. Such level of prediction could help to improve early warning systems based on seismic instruments installed in locations where more developed instrumental arrays are unavailable or unpractical. [ABSTRACT FROM AUTHOR]

For tectonic earthquakes, slip rate spans a continuum from creep to supershear earthquakes, where slow slip events (SSEs) are important in releasing stress without radiating damaging seismic energy. Industrial-scale subsurface fluid injection has caused induced earthquakes, but the role of SSEs in fault activation is currently unclear. Ground-deformation observations, measured by satellite radar, show that SSEs up to magnitude 5.0 occurred during hydraulic fracturing (HF) operations in northwestern Canada, corroborated by reported deformation of the steel well casing. Although the magnitude 5.0 SSE exceeded the magnitude of the largest induced earthquake in this region (magnitude 4.55), it was undetected by seismograph networks. The observed SSEs occurred within a buried thrust belt and their magnitude and duration are consistent with scaling behavior of SSEs in unbounded natural systems, e.g. slab interfaces in subduction zones. [ABSTRACT FROM AUTHOR]

As seismologists continue to place more stringent demands on data quality, accurately described metadata are becoming increasingly important. In order to better constrain the orientation and sensitivities of seismometers deployed in U.S. Geological Survey networks, the Albuquerque Seismological Laboratory (ASL) has recently begun identifying true north with a fiber optic gyroscope (FOG) and has developed methodologies to constrain mid-band, vertical component sensitivity levels to less than 1% in a controlled environment. However, questions remain regarding the accuracy of this new alignment technique as well as if instrument sensitivities and background noise levels are stable when the seismometers are installed in different environmental settings. In this study, we examine the stability and repeatability of these parameters by reinstalling two high-quality broadband seismometers (Streckeisen STS-2.5 and Nanometrics T-360 Global Seismographic Network (GSN) version) at different locations around the ASL and comparing them to each other and a reference STS-6 seismometer that stayed stationary for the duration of the experiment. We find that even in different environmental conditions, the sensitivities of the two broadband seismometers stayed stable to within 0.1% and that orientations attained using the FOG are generally accurate to within a degree. However, one install was off by 5° due to a mistake made by the installation team. These results indicate that while technology and methodologies are now in place to calibrate and orient a seismometer to within 1°, human error both during the installation and while producing the metadata is often a limiting factor. Finally, we find that background noise levels at short periods (0.1–1 s) become noisier when the sensors are emplaced in unconsolidated materials, whereas the noise levels at long periods (30–100 s) are not sensitive to local geological structure on the vertical components. [ABSTRACT FROM AUTHOR]

Small-scale seismic velocity heterogeneity has been studied through the calculation of peak amplitude ratio as a means to quantify the strength of seismic wave scattering at volcanoes in Japan. This ratio is defined as the ratio of the maximum (peak) P wave energy in the transverse component seismogram envelope over that of the three-component sum seismogram envelope (transverse + radial + vertical). According to the previous study using Japan's Hi-net seismometer network, the peak ratio is observed to be larger near the (active) quaternary volcanoes. However, these Hi-net stations are not positioned on the volcanoes themselves. This study systematically examines the peak ratios at 47 active volcanoes across Japan, using seismometers operated by the Japan Meteorological Agency (JMA). Analyses were performed at four frequency bands: 0.5–1, 1–2, 2–4, and 4–8 Hz. We found that the JMA stations yield higher peak ratios than the Hi-net stations. Their differences are statistically significant at the 99.9% confidence level in all frequency bands. We also examined the differences between the ground surface and borehole stations of the JMA network. The former shows larger peak ratios, and for most frequency bands, the differences are also statistically significant at the 99.9% confidence level. This suggests an intensification of small-scale medium heterogeneities especially at shallow depths at active volcanoes, and that scattering might have been enhanced at the very shallow parts. [ABSTRACT FROM AUTHOR]

Site and path effects are always required to isolate the source term from a recorded seismograph for the estimation of source characteristics. In this paper, the site and path effects are assessed in terms of site amplification and anelastic attenuation (quality factor), respectively, to obtain an isolated source term for the estimation of earthquake source parameters in the Kinnaur Himalaya. These terms are then applied to calculate the source parameters of 75 local earthquakes in the range 1.5 ≤ moment magnitude (Mw) ≤ 3.6 recorded in the Kinnaur region. The scaling relations of earthquake source parameters are also established for this region. The site amplification curves and S-wave quality factor Qs(f) are determined and further utilized to correct the Fourier spectrum of earthquake records. The obtained source spectrum corrected for these two terms is compared with the theoretical source spectrum based on the Brune (J Geophys Res 75:4997–5009, 1970) source model. The root-mean-square error (RMSE) between the observed and theoretical spectrum is formulated by implementing iterative forward modeling. The obtained range of source parameters of 75 events reveals ranges of 2.73 × 1011–3.44 × 1014 N-m for seismic moment, 0.03–13 bar for stress drop, 0.3–0.9 km for source radius, and 5.64 × 1002–1.19 × 1008 J for radiated energy. [ABSTRACT FROM AUTHOR]

The ML 3.8 earthquake occurred in Rushan County On October 1, 2013, Shandong Province, eastern China. After this event, there are already more than 19 600 seismic events recorded by digital seismic seismographs, forming an earthquake sequence. Although three moderate and strong earthquakes occurred successively during this period, the surface rupture undiscovered, it is difficult to diagnose the seismogenic fault from the geological perspective. However, the 3-D velocity structure inversion can provide important information on the location of the moderate earthquakes of the source area, which can help to understand the formation of earthquakes. Compared with source locations, the focal mechanisms can visually reflect the geometric and kinematic characteristics of seismogenic fault rupture, being the basis for inferring the regional tectonic stress field. Moreover, changing the stress field will affect the geometric parameters of the crack and change the shear-wave splitting parameters. Therefore, it is of great significance to explore the processes of earthquakes and the seismogenic mechanisms utilizing the spatiotemporal variation of shear wave splitting parameters. In summary, we comprehensively analyzed the seismogenic fault of the Rushan earthquake sequence (RES) with the help of the above methods. The resluts showed that the seismogenic fault of RES is towards the NWW-SEE direction and dips towards the southwest, dominated by high-angle strike-slip movements with certain thrust dip-slip components. [ABSTRACT FROM AUTHOR]

Thos article paper the results of the development and creation of a three-component seismometer for measurements on Mars. In the low-frequency and quasi-static region of the spectrum, the instrument also performs the functions of a tiltmeter and gravimeter. A description of the sensing element, a single-axis sensor, is given. The instrument includes three such sensors, the sensitivity axes of which are perpendicular to each other. The test mass in each sensor has its mechanical and magnetic hardness maintained, and the natural frequency of the oscillator is about 5 Hz. Limiting sensitivity problems determined by eigen equilibrium thermal fluctuations are considered. Ways of solving the technical problems, i.e., delivering a unit of sensing elements under a protective casing to the surface of Mars and installing the instrument along the gravitational vertical, as well as the results of calibration by the tilt method, which showed that the instrument characteristics are close to the planned ones, are listed. The amplitude sensitivity of the instrument base vibrations in the vicinity of 1 Hz caused also by the noise of the capacitive transducer of small movements of the test mass into an electric signal was ~3 × 10–10 m. [ABSTRACT FROM AUTHOR]

The Hurst exponent was computed locally for three triaxial seismometers monitoring the KAGRA corner station and the two end stations of the interferometer allowing to estimate variability in the persistent behaviour of the seismometer time series. Results show that, in all the three cases considered, the vertical component of the seismometer has lower persistency compared to the horizontal degrees of freedom, as also confirmed by the low frequency region of the amplitude spectral density of the data. Furthermore, data of the horizontal components of the EXV end station, located in the direction of the X arm of the interferometer, exhibits lower values of seismic noise and of its Hurst exponent. This is possibly due to the lack of an exit point at this location, and hence to a reduced human activity, to the presence of water flow in the mine hosting the KAGRA detector or to differences in seismometers' self noise. [ABSTRACT FROM AUTHOR]

The evolution history of the Sea of Japan back-arc basin remains under debate, involving the opening of sub-basins such as the Japan and Yamato Basins. Detailed knowledge of the lithospheric structure will provide the key to understanding tectonic history. This study identifies the lithosphere–asthenosphere boundary (LAB) beneath the Sea of Japan back-arc basin using S-receiver functions (S-RFs). The study area, including the Japan and Yamato Basins, has been instrumented with broadband ocean-bottom seismometers (OBSs). S-RFs from these OBSs show negative Sp phases preceding the direct S arrivals, suggesting the LAB. The S-RFs also show abnormally reduced amplitudes. For further qualitative interpretation of these findings, we conduct transdimensional Bayesian inversion for S-wave velocity models. This less-subjective Bayesian approach clarifies that the low-velocity seafloor sediments and damped deconvolution contribute to the amplitude reduction, illuminating the necessity of such considerations for similar receiver function works. Inverted velocity structures show a sharp velocity decrease at the mantle depths, which we consider the LAB. The obtained LAB depths vary among sites: ~ 45 km beneath the Japan and Yamato Basins and ~ 70 km beneath the Yamato Rise, a bathymetric high between the two basins. The thick lithosphere beneath the Yamato Rise most likely reflects its continental origin. However, the thickness is still thin compared to that of eastern Asia, suggesting lithosphere extension by rifting. Notably, the Japan and Yamato Basins show a comparable lithospheric thickness, although the crustal thickness beneath the Yamato Basin is known to be anomalously thick. This consistency in the lithospheric thickness implies that both basins undergo similar back-arc opening processes. [ABSTRACT FROM AUTHOR]

The ice flow velocity is a basic feature of glaciers and ice sheets. Measuring ice flow velocities is very important for estimating the mass balance of ice sheets in the Arctic and Antarctic. Traditional methods for measuring ice flow velocity include the use of stakes, snow pits and on-site geodetic GPS and remote sensing measurement methods. Geodetic GPS measurements have high accuracy, but geodetic GPS monitoring points only sparsely cover the Antarctic ice sheets. Moreover, the resolution and accuracy of ice flow velocities based on remote sensing measurements are low. Although the accuracy of the location data recorded by the navigation-grade GPS receivers embedded in short-period seismographs is not as good as that of geodetic GPS, the ice flow velocity can be accurately measured by these navigation-grade GPS data collected over a sufficiently long period. In this paper, navigation-grade GPS location data obtained by passive seismic observations during the 36th Chinese National Antarctic Research Expedition were used to accurately track the movement characteristics of the ice sheet in the Larsemann Hills of East Antarctica and the Taishan Station area. The results showed that the ice sheet in the two study areas is basically moving northwestward with an average ice flow velocity of approximately 1 m mon−1. The results in the Taishan Station area are basically consistent with the geodetic GPS results, indicating that it is feasible to use the embedded GPS location data from short-period seismographs to track the movement characteristics of ice sheets. The ice flow characteristics in the Larsemann Hills are more complex. The measured ice flow velocities in the Larsemann Hills with a resolution of 200 m help to understand its characteristics. In summary, the ice flow velocities derived from GPS location data are of great significance for studying ice sheet dynamics and glacier mass balance and for evaluating the systematic errors caused by ice sheet movements in seismic imaging. [ABSTRACT FROM AUTHOR]

Separated attenuation values have not been used in post-seismic variation research, although the scattering attenuation (Qs−1) parameter that can be used to estimate crustal inhomogeneity due to cracks. In this study, three earthquakes that occurred in Kumamoto (M7.3), Tottori (M6.6), and Gyeongju (M5.8) in 2016 were investigated by applying a multiple lapse time window analysis to seismograms recorded before and after the events. At a low frequency, significantly greater variation of the Qs−1 value was observed than the intrinsic attenuation (Qi−1) for the Kumamoto earthquake, whereas similarly large variation was observed for the Gyeongju earthquake. For the surrounding Kumamoto earthquake area of increased attenuation, even higher decreases in Qs–1 and Qi–1 were also observed. The increases occurred within a two year-period after mainshock. The large increases in attenuation, corresponding to regions with high peak ground acceleration, were limited to the basin area with an elevation below 500 m. Furthermore, post-seismic increases in attenuation values were found to correlate with the magnitude and length of the quiet periods of the earthquakes. From this study, Qs–1 and Qi–1 were shown as new parameters that can quantitatively measure the post-seismic deformation due to crustal earthquake. [ABSTRACT FROM AUTHOR]

Adding newly processed old marine and land-based wide-angle seismic data to modern geological and geophysical information about the region provides the correct localization and understanding of interaction of the Sinai, Anatolian, and African plates in the Eastern Mediterranean. Wide-angle seismic profile data acquired by ocean bottom and land-based seismometers in 1989–1990 by USSR Academy of Sciences expeditions were reprocessed. Land-based shots and a marine airgun shooting were used as seismic signal sources. As a result, the northern and western boundaries of the Sinai Plate were determined; absence of subduction in the southwestern part of the Cyprus Arc was proved; the structure of the Earth's crust and upper mantle of oceanic crust of the African and Sinai plates and the continental crust of the island of Cyprus has been refined. [ABSTRACT FROM AUTHOR]

The 2011 magnitude (M) 9.0 Tohoku-oki earthquake was followed by seismicity activation in inland areas throughout Japan. An outstanding case is the M6.2 Northern Nagano earthquake, central Japan, occurred 13-h after the megathrust event, approximately 400 km away from its epicenter. The physical processes relating the occurrence of megathrust earthquakes and subsequent activation of relatively large inland earthquakes are not well understood. Here we use waveform data of a dense local seismic network to reveal with an unprecedented resolution the complex mechanisms leading to the occurrence of the M6.2 earthquake. We show that previously undetected small earthquakes initiated along the Nagano earthquake source fault at relatively short times after the Tohoku-oki megathrust earthquake, and the local seismicity continued intermittently until the occurrence of the M6.2 event, being likely 'modulated' by the arrival of surface waves from large, remote aftershocks off-shore Tohoku. About 1-h before the Nagano earthquake, there was an acceleration of micro-seismicity migrating towards its hypocenter. Migration speeds indicate potential localized slow-slip, culminating with the occurrence of the large inland earthquake, with fluids playing a seismicity-activation role at a regional scale. [ABSTRACT FROM AUTHOR]