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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]

• 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]

• 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]