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

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

The self-noise level of a seismometer can determine the performance of the seismic instrument and limit the ability to use seismic data to solve geoscience problems. Accurately measuring and simultaneously comparing the self-noise models from different types of seismometers has long been a challenging task due to the constraints of observation conditions. In this paper, the self-noise power spectral density (PSD) values of nine types of seismometers are calculated using four months of continuous seismic waveforms from Malingshan seismic station, China, and nine self-noise models are obtained based on the probability density function (PDF) representation. For the seismometer STS-2.5, the self-noise levels on the horizontal channels (E–W and N–S) are significantly higher than that on the vertical channel (U–D) in the microseism band (0.1 Hz to 1 Hz), which is a computing bias caused by the misalignment between the sensors in the horizontal direction, while the remarkably elevated noise on the horizontal channels at the low frequencies (<0.6 Hz) may originate from the local variation of atmospheric pressure. As for the very broadband seismometers Trillium-Horizon-120 and Trillium-120PA, and the ultra-broadband seismometers Trillium-Horizon-360 and CMG-3T-360, there is a trade-off between the microseism band range and low-frequency range in the PSD curves of the vertical channel. When the level of self-noise in the microseism band is high, the self-noise at low frequencies is relatively low. Although compared with the other very broadband seismometers, the self-noise level of the vertical component of the STS-2.5 is 3 dB to 4 dB lower at frequencies less than 1 Hz, the self-noise level of the STS-2.5 at high frequencies (>2 Hz) is slightly higher than others. From our observations, we conclude that the nine seismometers cannot reach the lowest noise level in all frequency bands within the working range. [ABSTRACT FROM AUTHOR]

Seismic noise suppression plays an important role in seismic data processing and interpretation. Aiming at remedying the problem of low quality of seismic data acquired by a seismometer, a novel denoising method based on wavelet maximum modulus and an adaptive threshold is designed. This adaptive wavelet maximum modulus (ATWMM) seismic signal noise reduction method is using the opposite polarity of the Lipschitz index with seismic signal and noise to extract the seismic signal of original data. Setting adaptive thresholding function related to wavelet decomposition scale to solve the problem of effective signal losing on a small decomposition scale. The experimental results show that the ATWMM method can extract more seismic signal from the noisy seismic data. Using RMSE and SNR evaluation, the synthetic Ricker seismic dataset experimental results show that the indexes are 0.0502 and 20.1617 dB. Compared with the wavelet modulus maximum (WMM) method, it has a 25.7% reduction and 14.6% increase. The real-field seismic data came from the JZW51 seismic monitoring station in China experimental results indicate that the proposed ATWMM method is effective in seismic signal denoising with SNR above 30.1855 dB of approximately 15.8% enhancement compared with the WMM method, that has improvement for quality of seismic data. [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]

The exploration of active faults provides a basis for earthquake disaster prevention and migration. Active seismic survey is generally considered to be one of the most effective active fault exploration techniques because of its high resolution and deep penetration. However, this technique always yields low-resolution seismic data of a near-surface active fault. This limitation can be improved with the increase of sensor density with conventional nodal seismometers, but this is an expensive approach for active fault exploration. Distributed Acoustic Sensing (DAS) seems to be an ideal technique to replace traditional nodal seismometer surveys because it has the advantages of high-density sensors, many channels, and low cost. However, the applicability of active seismic surveys via DAS in the exploration of active faults remains unknown. Here, we designed an active seismic survey test in the Datong Basin, North China, focusing on a graben marked by well-developed normal faults in and around the basin. We selected a buried active fault identified in our previous active seismic profile, and DAS and nodal seismometers were deployed over a possible site of the active fault to conduct a comparative active seismic survey using a tamping machine. The comparisons with data recorded by three-component nodal seismometers demonstrate that DAS can also provide effective near-surface active fault information, and the imaging results also showed that the combination of conventional telecommunication fibre-optic cable and active seismic data processing techniques can be effectively used for near-surface active fault exploration, which is helpful in determining the true location of an active fault. • DAS can be used in the exploration of near-surface active faults as a replacement of conventional nodal seismometers. • High-resolution near-surface active fault are derived from conventional reflection seismic surveys based on DAS. • The diffracted wave in DAS data with high spatial sampling rate can be used to locate the near-surface active fault. [ABSTRACT FROM AUTHOR]

We examine P-wave velocity structure at the base of the mantle beneath the western Pacific, near the western edge of the Pacific Large-Low Shear Velocity Province (LLSVP), using high-quality seismograms provided by a large-scale mobile broad-band seismic observation in northeastern China (the NECESSArray project). Forward modelling using the reflectivity method is conducted to explain the variation of P-wave traveltimes as a function of epicentral distance near the core shadow zone. Additionally, PcP-P traveltimes are examined to enlarge the survey area. As a result, a rapid variation of P-wave velocity is detected at the base of the mantle. Regions of thin (20-50 km thick) and low velocity (-2 to -5 per cent) layers at the base of the mantle are intersected by an 80-km-thick region with a high velocity (+2 per cent). Aslightly fast region exists at the northwest of the regionwith the thin low-velocity layer. These layers are typically separated by several hundred kilometres and would be difficult to explain by thermal effects alone. These observations suggest that very complicated thermochemical reactions occur near the edge of the Pacific LLSVP. [ABSTRACT FROM AUTHOR]

Continental China consists of a complex amalgamation of geotectonic units and has experienced strong and widespread tectonic deformation since the Mesozoic. To understand its geological structure better, we conducted a systematic receiver function analysis using a total of 83 509 teleseismic traces in the time period of 2009-2010 recorded by 798 broad-band stations, among which 749 stations are permanent digital seismic stations from China Earthquake Networks Center and 49 stations were temporarily deployed in northern Central Tibet. A standard H-κ stacking method is employed to determine Moho depth and Vp/Vs ratio underneath each station from teleseismic receiver function analysis. The obtained Moho depth variations are generally consistent with those determined from various deep seismic soundings profiles. We combine our results with those from previous receiver functions studies to produce a high-resolution map of Moho depth and Vp/Vs variation for continental China. Compared to previous studies, the new study concerns many more stations and the resulting Moho depth map has much higher lateral resolution, especially in the eastern China. Overall, the Moho depth variation has a remarkable correlation with major tectonic units in continental China. For example, across the well-known gravity gradient line in east China, there is a clear shift in Moho depths. In general, the map of Vp/Vs ratio shows relatively high anomalies underneath Tibetan Plateau, along the gravity gradient line, and under several volcanoes. [ABSTRACT FROM AUTHOR]

Continental China consists of a complex amalgamation of geotectonic units and has experienced strong and widespread tectonic deformation since the Mesozoic. To understand its geological structure better, we conducted a systematic receiver function analysis using a total of 83 509 teleseismic traces in the time period of 2009–2010 recorded by 798 broad-band stations, among which 749 stations are permanent digital seismic stations from China Earthquake Networks Center and 49 stations were temporarily deployed in northern Central Tibet. A standard H–κ stacking method is employed to determine Moho depth and Vp/Vs ratio underneath each station from teleseismic receiver function analysis. The obtained Moho depth variations are generally consistent with those determined from various deep seismic soundings profiles. We combine our results with those from previous receiver functions studies to produce a high-resolution map of Moho depth and Vp/Vs variation for continental China. Compared to previous studies, the new study concerns many more stations and the resulting Moho depth map has much higher lateral resolution, especially in the eastern China. Overall, the Moho depth variation has a remarkable correlation with major tectonic units in continental China. For example, across the well-known gravity gradient line in east China, there is a clear shift in Moho depths. In general, the map of Vp/Vs ratio shows relatively high anomalies underneath Tibetan Plateau, along the gravity gradient line, and under several volcanoes. [ABSTRACT FROM PUBLISHER]

The lateral expansion of the southeastern Tibetan Plateau causes devastating earthquakes, but is poorly understood. In particular, the links between regional variations in surface motion and the deeper structure of the plateau are unclear. The plateau may deform either by movement of rigid crustal blocks along large strike-slip faults, by continuous deformation, or by the eastward flow of a channel of viscous crustal rocks. However, the importance of crustal channel flow was questioned in the wake of the 2008 Wenchuan earthquake. Controversies about the style of deformation have persisted, in part because geophysical probes have insufficient resolution to link structures in the deep crust to the observed surface deformation. Here we use seismic data recorded with an array of about 300 seismographs in western Sichuan, China, to image the structure of the eastern Tibetan Plateau with unprecedented clarity. We identify zones of weak rocks in the deep crust that thicken eastwards towards the Yangtze Craton, which we interpret as crustal flow channels. We also identify stark contrasts in the structure and rheology of the crust across large faults. Combined with geodetic data, the inferred crustal heterogeneity indicates that plateau expansion is accommodated by a combination of local crustal flow and strain partitioning across deep faults. We conclude that rigid block motion and crustal flow are therefore not irreconcilable modes of crustal deformation. [ABSTRACT FROM AUTHOR]

SUMMARY We present S receiver functions and SKS splitting measurements from the China Seismograph Network located in the Qinghai and Gansu provinces. Teleseismic data are used to interpret the structure of the lithosphere-asthenosphere boundary (LAB) and upper-mantle deformation beneath the northeastern Tibetan Plateau (NETP) in regions north of the east Kunlun Fault. Based on our observations, the LAB lies at a depth of 125-135 km beneath the northeastern Songpan-Ganzi block and the west Qinling orogen, between 145 and 175 km beneath the Kunlun and Qilian orogen, and deepens below the Qaidam Basin (175-190 km), Ordos Craton (170 km) and Alashan platform (200 km). The NETP is characterized by a nearly uniform fast NW-SE S-wave direction. These observations are different from those to the south of the Kunlun Fault where fast S directions are rotating clockwise from the inner plateau. The change in fast directions across the Kunlun Fault implies a sudden variation of upper-mantle deformation. Shear wave splitting delay times vary from 0.8 to 1.9 s. Data from beneath regions north of the Kunlun-Ayimaqin suture showed that delay time was positively correlated with lithospheric thickness with an increase of 0.7 s per 100 km. This indicates that the anisotropy may develop in the uppermost mantle, such as the lithosphere, beneath the NETP. [ABSTRACT FROM AUTHOR]

The crustal structure of the northwestern sub-basin area of the South China Sea was modeled by inverting a wide-angle seismic survey line across the entire region and on both sides of its bounding continental margins. The survey line extended over 484 km. A total of 14 Ocean Bottom Seismometers (OBS) were deployed at intervals of 30 km to record air-gun array sources with a combined volume of 5160 in. The crustal velocity structure of the northwestern sub-basin area was acquired through the integration of multi-channel seismic data. OBS data were processed and modeled initially using ray tracing inversion techniques. Results indicate that crustal thickness under the continental slope decreases from 21 to 11 km, crustal thickness of the northwestern sub-basin is 7.7 km, and the depth to the Moho ascends from 21 km under the upper continental slope to 11 km under the middle basin. The crust of the northwestern sub-basin is similar to that of the eastern sub-basin in its oceanic crustal structure. This structure has a thicker layer 1 (sedimentary layer) and a thinner layer 2. These characteristics are especially clear in the eastern sub-basin, which differs somewhat from typical oceanic crust. The tectonic geometry and velocity structure of the northwestern sub-basin and its margins comprise a symmetrical conjugate and indicate a pure shear mode with regard to the continental margin rifting mechanism. We did not find clear seismic signals from high velocity layers under the lower crust of the continental margin in the northern part of the northwestern sub-basin, which provides new evidence for the idea that the western part of the northern continental margin of the South China Sea constitutes non-volcanic crust. Because the seafloor spreading period of the northwestern sub-basin was short, layer 2 might have experienced asymmetrical basalt magma flows, which may have blurred the magnetic anomaly lineations of the northwestern sub-basin. [ABSTRACT FROM AUTHOR]

SUMMARY This paper uses data from the Capital Area Seismograph Network in North China (NC), currently the largest regional seismograph network in China to investigate the compressional stress orientations from the polarizations of faster shear waves (PFS), a parameter of shear wave velocity anisotropy. Data are restricted to earthquakes deeper than 5 km to reduce the influence of the heterogeneous uppermost few kilometres of the crust. The results show that the regional compressional stress direction is ENE-WSW to nearly EW in NC, which agrees with seismic mechanisms and drilling and GPS data. Predominantly, PFS are also nearly EW in NC and are influenced by faults, geology and tectonics. PFS orientations at stations on seismically active faults are consistent with the strike of strike-slip faults and indicate large seismically active faults breaking, or nearly breaking, the free surface. PFS orientations are influenced by both regional compressional stress and by nearby faults. It is an effective way to study detailed spatial distribution of crustal compressional stress by shear wave anisotropy with dense seismograph network. Shear wave splitting may also be used to indicate hitherto unknown faults. [ABSTRACT FROM AUTHOR]

Using 1 Hz sampling records at one superconducting gravimeter (SG) station and 11 broadband seismometer stations, we found anomalous signals prior to the 2008 Wenchuan (汶川) earthquake event. The tides are removed from the original SG records to obtain the gravity residuals. Applying the Hilbert-Huang transform (HHT) and the wavelet analysis to the SG gravity residuals leads to time-frequency spectra, which suggests that there is an anomalous signal series around 39 h prior to the event. The period and the magnitude of the anomalous signal series are about 8 s and 3×10 m/s (3 μGal), respectively. In another aspect, applying HHT analysis technique to 11 records at broadband seismometer stations shows that most of them contain anomalous signals prior to the Wenchuan event, and the marginal spectra of 8 inland stations show an apparent characteristic of double peaks in anomalous days compared to the only one peak of the marginal spectra in quiet days. Preliminary investigations suggest that the anomalous signals prior to the earthquake are closely related to the low-frequency earthquake (LFE). We concluded that the SG data as well as the broadband seismometers records might be significant information sources in detecting the anomalous signals prior to large earthquakes. [ABSTRACT FROM AUTHOR]

• Integration of GPS and strong-motion data for more accurate coseismic displacement. • Novel model of fusing GPS and strong-motion data with baseline shift correction. • Better performance of the integrated model shown in 2017 Jiuzhaigou earthquake. Near-field coseismic displacement information is very important for seismological research. High-rate GPS and seismic sensors are mutually advantageous for capturing earthquake-induced coseismic displacements. The combination of them can help to improve the near-field observation results. In this paper, we propose a combined model of fusing GPS and strong-motion records by taking into consideration the corrections of baseline shift, and then apply a multi-rate Kalman filter to analyze the collocated GPS and strong-motion data. The performance of the proposed approach is validated using 1 Hz GPS data and 200 Hz strong-motion data collected within 150 km from the epicenter during the Ms 7.0 Jiuzhaigou earthquake occurred on August 8, 2017 in Sichuan Province, China (33.20°N, 103.82°E). It shows that the GPS displacements based on the Epoch-relative Positioning Approach well reflect the instantaneous motion state of stations during the earthquake, indicating that the maximum instantaneous displacement of SCJZ station is 3.2 cm in north and −2.0 cm in east, and the permanent displacement of SCSP station is −0.6 cm in north. More accurate and reliable broadband displacements less than 1 cm are obtained by the combination model, which provide the full spectrum of the seismic motion. The integrated displacements have more characteristics of strong-motion results in the early stage of the earthquake with less noise, and in the strong shaking period, tightly bound by the GPS displacements, the integrated displacements will not diverge, and thus accurately record the long-period information of seismic waves. [ABSTRACT FROM AUTHOR]

Abstract: As part of an extensive seismic research program, 33 portable broadband seismic stations were deployed along a line crossing the Bohai Bay Basin, northern China. Three teleseismic events were selected to constrain the seismic structure along a ∼280 km profile across the western edge of the basin. We determined the basin structure that described the observed shear horizontal (SH) wave field. The synthetic SH wave was calculated using a finite difference (FD) method with its computational domain localized in the basin area and input motions at the base of the model extrapolated from the displacement recorded at a nearby hard-rock station. Synthetic seismographs calculated for the models match the observations well in both waveform and travel time. Numerical tests indicate that the structural features of the preferred models are well resolved. The analysis of relations between structures and stratigraphic units along the cross sections allows multiple deformational events in the basin to be inferred. In conjunction with a profile across the southern edge that has been presented previously by Zhao et al. [14] [L. Zhao, T.Y. Zheng, W.W. Xu, Modeling the Jiyang depression, Northern China, using a wave field extrapolation FD method and waveform inversion, Bull. Seismol. Soc. Am. 94 (2004) 988–1001], the results reveal basin-wide extension with local inversion features in the Bohai Bay basin. [Copyright &y& Elsevier]

• We image how the crustal stucture nature vary in the SE Yellow Sea. • The Gunsan Basin is underlain by relatively thin upper and much thick lower crustal layers. • A transition in crustal structure is recognized from the South China Block to the Korean Peninsula. • The crustal structure is inherited from the growth of Rodinia. Two major tectonic units in NE Asia are the Sino-Korean (or North China) and South China Blocks that collided in the Permo-Triassic periods. The South China Block is suggested to extend eastward to the Korean Peninsula across the southern Yellow Sea. The Gunsan Basin is a well-defined, fault-bounded sedimentary basin in the SE Yellow Sea and is regarded as a structural link of the South China Block to the Korean Peninsula. We collected deep seismic sounding data recorded on ocean bottom seismometers to investigate how the crustal structure and nature of the South China Block vary in the Gunsan Basin toward the middle part of the Korean Peninsula. The main part of the Gunsan Basin is underlain by relatively thin (∼10 km thick) upper and much thicker (∼20 km thick) lower crustal layers, indicating a close affinity to the South China Block; whereas it shows distinction from the Korean Peninsula underlain by upper and lower crustal layers with equal (∼15 km) thickness. A change in crustal structure is recognized under the eastern margin of the Gunsan Basin toward the Korean Peninsula, that features a transition to the Korean Peninsula. The thick lower crustal layer in the Gunsan Basin appears to be inherited from the evolution of the South China Block including crustal shortening associated with the assembly of Rodinia in the Neoproterozoic before the collision between the Sino-Korean and South China Blocks. [ABSTRACT FROM AUTHOR]

The article provides information on several research initiatives in China that focus on geological hazards. It is said that the Greater North China Initiative (GNIC) includes a study of the lithospheric thinning. The Chinese Earthquake Administration has acquired 600 portable seismometers in parallel to the lightening project and the GNIC.

The article reports on the installation of a seismometer array by the Great North China Initiative of the Natural Sciences Foundation of China (NSFC) across the North China Plain. The said installation is part of the initiative's measure against the region's geology and seismic risk. Mian Liu, a geophysicist at the University of Missouri in Columbia stressed the importance of such technologies for quake prediction. Moreover, to help shed light on this tectonic anomaly, the Deep Earth Lightening project involving the Chinese Earthquake Administration, the Chinese Academy of Sciences and the NSFC is wiring north China with nearly 800 portable broadband seismometers that will image the crust and upper mantle to map faults and explore craton evolution and dynamics.