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On May 22nd, 2021, an MS7.4 earthquake occurred near the Maduo county of the Qinghai Province, China, within the Bayan Har Block. Seismic activities have been intense in this block, thus whether the Maduo Earthquake will bring subsequent seismic hazards to its surrounding regions raises wide concerns. In this paper, we first calculated the Coulomb failure stress changes caused by the Maduo Earthquake on nearby faults, and estimated how much these faults are brought closer or further from their next failures based on their stressing rates. Next, we combined the Coulomb failure stress changes with the rate-state fictional law to estimate the seismicity rate in the study region in the next decade. A declustered catalogue before the Maduo Earthquake was adopted to calculate background seismicity rate, and rate-state parameters are constrained by fault slip rates. Our results show that the Maduo Earthquake increases stress accumulations in the northwestern portion of the Qingshuihe fault (0.02 MPa at maximum), the two ends of the Kunlun Mountain Pass-Jiangcuo fault (0.01 MPa at maximum), and the northwestern portion of the Maduo-Gande fault (on average ∼0.09 MPa), and seismicity rates are expected to increase near these faults. What is especially worth noting is the seismic hazard in the region extending from the eastern end of the Kunlun Mountain Pass-Jiangcuo fault to the Maqin-Maqu seismic gap on the Eastern Kunlun fault, which is calculated to have experienced a maximum stress increase of 0.67 MPa after the Maduo Earthquake. On the other hand, stress accumulations are reduced in the southern end of the Elashan fault, the Eastern Kunlun fault segment to the west of Maduo, and the northwestern portion of the Dari fault. Seismic hazards are expected to be low in these regions. For the study region as a whole, the probability of an M≥6 earthquake taking place in the next decade is estimated to be 59%, about twice the value calculated for the time period before the Maduo Earthquake. [ABSTRACT FROM AUTHOR]

The analysis of seismic hazards relies on the statistical analysis of historical seismic data and the instrumental seismic catalog to obtain the regional earthquake recurrence interval and earthquake probability. The accuracy of analysis thus depends strongly on the completeness of the seismic data used. However, available seismic catalogs are too short or incomplete for the reliable analysis of the statistical characteristics of earthquakes. If a long-term synthetic seismic catalog can be generated using a physics-based numerical simulation, and the simulation results match the crustal deformation, seismicity, and other observations, then such a synthetic catalog helps us to further understand the characteristics of seismic activity and analyze the regional seismic hazard. In this paper, taking the northeastern Tibetan Plateau as a case study, we establish a three-dimensional visco-elastoplastic finite-element model to simulate earthquake cycles and the spatiotemporal evolution of earthquakes on the model fault system and obtain a seismic catalog on a time scale of tens of thousands of years. On the basis that the model satisfies the regional geodynamics of the northeastern Tibetan Plateau, we analyze seismicity on the northeastern Tibetan Plateau using the simulated synthetic earthquake catalog. The characteristics of earthquake recurrence at different locations and different magnitudes, and the long-term average probability of earthquake occurrence within the fault system on the northeastern Tibetan plateau are studied. The results are a reference for regional seismic hazard assessment and provide a basis for the physics-based numerical prediction of earthquakes. [ABSTRACT FROM AUTHOR]

Resolving rupture processes of great earthquakes has fundamental importance to the study of earthquake physics, rupture dynamics, fault zone structure, and evolving processes. It also plays an essential role in earthquake hazard estimation, emergency response and seismic hazard mitigation. This paper reviews the major progress of the earthquake rupture process studies in the last decades, with an emphasize on the research directions of the department geophysics of Peking University including real-time response, back-projection techniques, geodetic data analysis, joint inversion and inversion in complex earth medium. We discussed the advantages and limitations of tradition methods; proposed a systematic and integrated approach from fast-response to detailed study. We also raised perspectives of using source models for ground motion prediction and the possibility of full-dynamic inversion. [ABSTRACT FROM AUTHOR]

Plate subduction is the most magnificent process in the Earth. Subduction zones are important sites for proceeding matter- and energy- transports between the Earth's surface and the interior, continental crust growth, and crust-mantle interactions. Besides, a number of geological processes in subduction zones are closely related to human beings' daily life, such as volcanic eruptions and earthquakes, formation of mineral deposits. Subduction process thus has long been the centric topic of Earth sciences. The finding in 1980s that continental crust could be subducted to mantle depths is a revolutionary progress in plate tectonic theory. Compared to oceanic crust, continental crust is colder, drier, lighter, and much more geochemically/isotopically heterogeneous Hence, continental subduction process would affect the structure, compositions and evolutions of the overlying mantle wedge even more. During continental subduction and subsequent exhumation, fluids and melts can be generated in the (de)hydration process and partial melting process, respectively. These melts/fluids play important roles in crust-mantle interactions, elemental migrations, isotopic fractionations, and mantle metasomatism. By summarizing recent research works on subduction zones in this paper, we present a review on the types, physicochemical conditions and compositions of fluids/melts, as well as the migration behaviors of fluid-related characteristic elements (Nb-Ta-V) and the fractionation behaviors of non-traditional stable isotopes (Li-Mg) in subduction zones. The aim of this paper is to provide the readers an update comprehensive overview of the melt/fluid activities in subduction zones and of Li-Mg isotope systematics in subduction-related rocks and minerals. [ABSTRACT FROM AUTHOR]

The alternating electromagnetic (EM) field is one of the most sensitive physical fields related to earthquakes. There have been a number of publications reporting EM anomalies associated with earthquakes. With increasing applications and research of artificial-source extremely low frequency EM and satellite EM technologies in earthquake studies, the amount of observed data from the alternating EM method increases rapidly and exponentially, so it is imperative to develop suitable and effective methods for processing and analyzing the influx of big data. This paper presents research on the self-adaptive filter and wavelet techniques and their applications to analyzing EM data obtained from ground measurements and satellite observations, respectively. Analysis results show that the self-adaptive filter method can identify both natural- and artificial-source EM signals, and enhance the ratio between signal and noise of EM field spectra, apparent resistivity, and others. The wavelet analysis is capable of detecting possible correlation between EM anomalies and seismic events. These techniques are effective in processing and analyzing massive data obtained from EM observations. [ABSTRACT FROM AUTHOR]

In this paper, variations of shear wave splitting in the 2013 Lushan Ms7.0 earthquake sequence were studied. By analyzing shear wave particle motion of local events in the shear wave window, the fast polarization directions and the delay time between fast and slow shear waves were derived from seismic recordings at eight stations on the southern segment of the Longmenshan fault zone. In the study region, the fast polarization directions show partition characteristics from south to north. And the systematic changes of the time delays between two split shear waves were also observed. As for spatial distribution, the NE fast polarization directions are consistent with the Longmenshan fault strike in the south of focal region, whereas the NW fast direction is parallel to the direction of regional principal compressive stress in the north of focal region. Stations BAX and TQU are respectively located on the Central and Front-range faults, and because of the direct influence of these faults, the fast directions at both stations show particularity. In time domain, after the main shock, the delay times at stations increased rapidly, and decreased after a period of time. Shear-wave splitting was caused mostly by stress-aligned microcracks in rock below the stations. The results demonstrate changes of local stress field during the main shock and the aftershocks. The stress on the Lushan Ms7.0 earthquake region increased after the main shock, with the stress release caused by the aftershocks and the stress reduced in the late stage. [ABSTRACT FROM AUTHOR]

Composite borehole profiling combined with trenching is an effective way to acquire evidence of past ruptures of buried active faults. In this study, three composite borehole profiles and a large-scale trench excavation were carried out across the surface rupture zone of the 1976 M7.8 Tangshan earthquake. The following three major conclusions have been reached. (1) The surface rupture zone of the 1976 earthquake extends more than 47 km long to the south of Tangshan city, passing to the west of Sunjialou, to Daodi town in Fengnan County, to Xihe in Fengnan County. (2) The surface rupture zone is divided into south and north branches. The north branch has mainly right-lateral strike-slip motion, and the vertical displacement of the surface is up on the west and down on the east. On the other hand, the vertical displacement of the south branch is up on the east and down on the west, accompanied by some right-lateral slip. Such a faulting style cannot be explained by the movement of a single normal or reverse fault, but is consistent with the vertical displacement field induced by the right-lateral strike-slip of the fault belt. The drilling and trenching data from this study verify that such activity continued through the Late Quaternary on the Tangshan Fault. (3) The fault planes exhumed by trenching and the dislocations of strata revealed by the boreholes indicate that multiple faulting events occurred on the Tangshan Fault in the Late Quaternary. The timing of three ruptures prior to the 1976 earthquake was 7.61-8.13, >14.57, and 24.21-26.57 ka BP. Counting the earthquake of 1976, the recurrence interval of the four strong events is about 6.7 to 10.8 ka. On one of the three borehole profiles, the Niumaku profile, nine faulting events were detected since 75.18 ka BP with an average interval of 8.4 ka. In addition, this paper also discusses the difference between the Late Quaternary sedimentary environments to the north and south of Tangshan city based on stratum dating. [ABSTRACT FROM AUTHOR]

The N-S trending Xiaojiang fault zone and the NW-SE trending Qujiang-Shiping fault zone are adjacent active fault systems and seismogenic zones associated with strong and major earthquakes in Yunnan, China. To understand the interaction of the two fault systems, and its probable influence on earthquake occurrences, this paper conducts a synthetic study based on data of active tectonics, historical earthquakes, relocated small earthquakes, GPS station velocities and focal mechanism resolutions. The study makes several conclusions. (1) The active southward motion of the western side of the Xiaojiang fault zone (i.e. the side of the Sichuan-Yunnan block) has a persistent and intensive effect on the Qujiang-Shiping fault zone. The later fault zone has absorbed and transformed the southward motion of the western side of the former fault zone through dextral strike-slip/shearing as well as transverse shortening/thrusting. (2) Along the Xiaojiang fault zone, the present sinistral strike-slip/shearing rate decreases from 10 and 8 mm/a on the northern, central and central-southern segments to 4 mm/a on the southern segment. The decreased rate has been adjusted in the area along and surrounding the Qujiang-Shiping fault zone through reverse-dextral faulting and distributed shearing and shortening. (3) The tectonic-dynamic relation between the Xiaojiang fault zone and the Qujiang-Shiping fault zone is also manifested by a close correlation of earthquake occurrences on the two fault zones. From 1500 to 1850 a sequence of strong and major earthquakes occurred along the Xiaojiang fault zone and its northern neighbor, the Zemuhe fault zone, which was characterized by gradually accelerating strain release, gradually shortening intervals between M⩾7 events, and major releases occurring in the mid to later stages of the sequence. As a response to this sequence, after an 88-year delay, another sequence of 383 years (from 1588 to 1970) of strong and major earthquakes occurred on the Qujiang-Shiping fault zone, and had the same features in accelerating strain release and its temporal course. (4) Since there has been no M⩾7 event for 177 years on the Xiaojiang fault zone, the potential risk of a strong or major earthquake occurring on this fault zone in the future should be noticed and studied further. [ABSTRACT FROM AUTHOR]

In mountainous areas, landslides induced by destructive earthquakes are one of the main causes of human casualties, which is an important link in the chain of earthquake hazards. Earthquake-triggered landslides are mainly controlled by three factors, namely seismic property, topography, and geology. Many studies have been conducted on these controlling factors of earthquake-triggered landslides. However, little is known about the effect of coseismic displacement on the distribution of landslides under different slope aspects and slope angles, hindering our understanding of the mechanism of inducing landslides by the combination of surface displacement and slope geometry at the local scale and leading to controversial opinions about the abnormal number of earthquake-triggered landslides in several cases. Here, we took the 2008 Wenchuan Mw7.9 earthquake in China, the 2015 Gorkha Mw7.8 earthquake in Nepal, and the 2016 Kaikōura Mw7.8 earthquake in New Zealand as examples to investigate the relationship between the distribution of large earthquake-triggered landslides and the three-dimensional (3D) coseismic displacement field. We divided the landslide-prone area around the epicenter into regular grids and calculated the 3D coseismic displacement in each grid according to the radar satellite images and slip distribution model. Then, the 3D coseismic displacement was projected to two coordinate systems related to the slope where the landslides were located for statistical analysis. We determined that the surface uplift perpendicular to the slope is more likely to induce landslides, particularly when combined with large slope angles. Meanwhile, the number of landslides will be significantly reduced where the subsidence occurs. Regardless of uplift or subsidence, landslides are more likely to occur when the direction of coseismic horizontal displacement is far from the slope. The larger the slope angles are, the greater the effects of horizontal displacement and slope aspect. A dominant slope aspect also exists for earthquake-triggered landslides, which is different from the mean slope aspect calculated from the background topography. This dominant aspect angle is related to the focal mechanism and striking angle of surface rupture. These results indicate that we can simulate the 3D coseismic displacement field from known fault location and earthquake mechanism and combine the topographic data for landslide risk assessment in earthquake-prone mountainous areas to minimize the damage caused by possible earthquake-triggered landslides. [ABSTRACT FROM AUTHOR]

An M6.8 earthquake occurred in Luding, Sichuan Province, China, on September 5, 2022. Since towns and villages in the earthquake-stricken area are densely populated, the earthquake caused severe fatalities and economic losses. Rapid estimation of earthquake intensity and disaster losses is significantly important for post-earthquake emergency rescue, scientific anti-seismic deployment, and the reduction of casualties and economic losses. Therefore, we make a preliminary rapid estimation of the earthquake intensity and disaster losses in the aftermath of the Luding earthquake. The seismic intensity represents the distribution of earthquake disasters and the degree of ground damage and can be directly converted from the peak ground velocity (PGV) map. To obtain a reliable PGV distribution map of this earthquake, we combined the finite-fault model constrained by seismic observations, with the complex three-dimensional (3D) geological environment and topographical features to perform strong ground motion simulation. Then, we compared the consistency between the simulated ground motion waveforms and observations, indicating the plausibility and reliability of simulations. In addition, we transformed the PGV simulation results into intensity and obtained a physics-based map of the intensity distribution of the Luding earthquake. The maximum simulated intensity of this earthquake is IX, which is consistent with the maximum intensity determined from the post-earthquake field survey. Based on the simulated seismic intensity map of the Luding earthquake and the earthquake disaster loss estimation model, we rapidly estimated the death and economic losses caused by this earthquake. The estimated results show that the death toll caused by this earthquake is probably 50–300, with a mathematic expectation of 89. Thus the government should launch a Level II earthquake emergency response plan. The economic losses are likely to be 10–100 billion RMB, with a mathematical expectation of 23.205 billion RMB. Such seismic intensity simulations and rapid estimation of disaster losses are expected to provide a preliminary scientific reference for governments to carry out the targeted deployment of emergency rescue and post-disaster reconstruction. [ABSTRACT FROM AUTHOR]

The 2019 MS6.0 Changning earthquake occurred in the tectonically stable Sichuan Basin, where the epicenter and its adjacent areas are important shale gas and salt mine production fields, resulting in hot debates on whether the seismogenic mechanism of the 2019 Changning earthquake is related to human activities. As source characteristics and fluid pressure can provide important constraints on whether an earthquake is induced, we investigate the seismogenic mechanisms of the mainshock and 9 MW⩾4.0 aftershocks. In overall, the mainshock and the majority of the aftershocks are characterized by relatively shallow focal depths (1–4 km) and significant non-double-couple (non-DC) components. However, the mainshock and the aftershocks differ in two aspects: (1) the compensated-linear-vector-dipole components dominate the non-DC components of the mainshock, whereas the isotropic components dominate the most aftershocks; (2) the fluid overpressure of the mainshock is over 30 MPa, whereas the fluid overpressure of the most aftershocks is less than 10 MPa. Thus, we propose that the mainshock is triggered by weakened fault strength with long-term fluid injection, and that its large non-DC components are associated with complex rupture processes. Comparatively, the aftershocks may be triggered by postseismic stress transfer by combining the Coulomb failure stress changes in the poroelastic medium. Our results highlight the possible role of fluid in the occurrence of the Changning earthquake sequence. [ABSTRACT FROM AUTHOR]

We established a three-dimensional finite element model of the Anninghe-Zemuhe-Xiaojiang faults region using contact surfaces of different sizes to describe the spatial segmentation characteristics of the faults. Our model is based on constraints from GPS observations, models of the crust and upper mantle, precise earthquake locations, the tectonic stress field, the slip rate of the faults, and the rheology of the lithosphere in the Sichuan-Yunnan area. Considering the influence of strong earthquakes since A.D. 1327, we analyzed the main controlling factors of the characteristics of the strong earthquakes and also studied by numerical simulation the possible areas of future earthquake risk and their relationship with tectonic stress. The numerical results showed that the gravitational potential energy of the Qinghai-Tibet Plateau and the interaction of adjacent blocks are the main kinetic factors affecting the characteristics of the tectonic stress distribution. There appears to be some correspondence between the distribution of tectonic stress and the b value; however, we also found that some low b value locations correspond to regions of lower stress. This contradiction may be the result of some comprehensive factors, such as the release of strain energy caused by strong earthquakes. [ABSTRACT FROM AUTHOR]

The seismo-electromagnetic (EM) method is an important geophysical method that plays a major role in the observation of seismic anomalies related to earthquake precursors. It is the most promising method for a breakthrough in short-term earthquake prediction. The digital transformation and network upgrading implemented in the "Ninth five-year plan" and "Tenth five-year plan" have optimized the original observation system, improved the quality of observed data, enriched the seismic-electromagnetic information, and enhanced the analysis capability and timeliness of seismic-electromagnetic anomalies. These improvements are of major importance for the research on seismo-electromagnetics as well as for the development of new technologies. Since the beginning of the 21st century, China has launched a satellite named CSES that was designed for the generation and study of seismo-electromagnetic data and built a high-power transmitting source and a new CSELF observation network that is used specifically for earthquake monitoring. This platform has promoted the full-time three-dimensional EM monitoring and the identification of earthquake anomalies. Based on the study of anomalies related to earthquake precursors, the physical and numerical simulations, and the study on generation mechanism of anomalies in China, we summarize the characteristics of earthquake EM anomalies and discuss the advantages and disadvantages of different EM observation methods. Finally, considering the related questions of the seismo-electromagnetic prediction and implementing the recent developments both in China and abroad, we review the current status of seismo-electromagnetic research and propose strategies for future research. [ABSTRACT FROM AUTHOR]

On 22nd May 2021 (local time), an earthquake of Ms7.4 struck Maduo county in Qinghai Province, China. This was the largest earthquake in China since the 2008 Wenchuan earthquake. In this study, ascending/descending Sentinel-1 and advanced land observation satellite-2 (ALOS-2) synthetic aperture radar (SAR) images were used to derive the three-dimensional (3-D) coseismic displacements of this earthquake. We used the differential interferometric SAR (InSAR, DInSAR), pixel offset-tracking (POT), multiple aperture InSAR (MAI), and burst overlap interferometry (BOI) methods to derive the displacement observations along the line-of-sight (LOS) and azimuth directions. To accurately mitigate the effect of ionospheric delay on the ALOS-2 DInSAR observations, a polynomial fitting method was proposed to optimize range-spectrum-split-derived ionospheric phases. In addition, the 3-D displacement field was obtained by a strain model and variance component estimation (SM-VCE) method based on the high-quality SAR displacement observations. Results indicated that a left-lateral fault slip with the largest horizontal displacement of up to 2.4 m dominated this earthquake, and the small-magnitude vertical displacement with an alternating uplift/subsidence pattern along the fault trace was more concentrated in the near-fault regions. Comparison with the global navigation satellite system data indicated that the SM-VCE method can significantly improve the accuracy of the displacements compared to the classical weighted least squares method, and the incorporation of the BOI displacements can substantially benefit the accuracy of north-south displacement. In addition to the displacements, three coseismic strain invariants calculated based on the strain model parameters were also investigated. It was found that the eastern and western parts of the faults suffered more significant strains compared with the epicenter region. [ABSTRACT FROM AUTHOR]

Crustal thickness and composition are closely related to geology and tectonic evolution of the region. Studying the differences in the crustal thickness and composition of the South China Block (SCB) is important to gain a comprehensive understanding of multi-phase amalgamation, breakup, reworking, and regional geodynamic processes. In this study, teleseismic data from 135 high-density portable broadband stations from SinoProbe were processed using Common Conversion Point (CCP) stacking and H-κ stacking methods. The Moho depth and P-wave and S-wave velocity ratio (Vp/Vs) were studied. Our results revealed several insights about the tectonic processes in the SCB. First, the crustal structure and Vp/Vs ratios of the Cathaysia Block, Jiangnan Orogenic Belt, and Yangtze Block were significantly different. The average depth of the Moho in the Cathaysia Block was approximately 31 km, and the Vp/Vs ratios increased from the inland area (1.66) to the coastal area (1.78), indicating the oceanward increase of mafic proportion in the lower crust, which is related to the influence of the Paleo-Pacific Plate westward subduction. Second, the crustal thickness of Jiangnan Orogenic Belt deepens from east ca. 31 to the west ca. 42 km and the Vp/Vs ratios varied from 1.75 to 1.64, illustrating a relatively felsic crust, which could have been related to the Mesozoic upper crustal thickening under compression followed by the lower crust removal under the extensional background. Third, the average crustal thickness of the Yangtze Block was 42 km, and the Vp/Vs ratios ranged from 1.64 to 1.84, presenting a positive correlation between the Moho depth and the Vp/Vs ratio, which is explained by the relatively thick mafic lower crust. Based on the variations in the crustal structure and Vp/Vs ratios of the profile, we inferred that the central part of the Jiangnan Orogenic Belt was bounded by the Jiangshan-Shaoxing-Hengyang in the east and the Jiujiang-Shitai-Jishou in the west. The small-scale Moho depth undulations at the intersection of the Cathaysia Block and the Jiangnan Orogenic Belt could be related to the Mesozoic compression-extension geodynamic transformation. [ABSTRACT FROM AUTHOR]

Based on the existing materials of fault segmentation, characteristic earthquakes, and their empirical relationships, we calculated the parameters of the fault segments, such as length, width, magnitudes of characteristic earthquakes, etc. Constrained by GPS velocity field, the slip rates of these fault segments in depth were inversed using the 3-D half-space elastic dislocation model. As not all of the recurrence periods and co-seismic displacements of characteristic earthquakes are known, we selected the fault segments with these two parameters known and calculated the accumulation rate of average co-seismic displacement, which shows the faults' slip rate in seismogenic layer. Then, the slip rate in depth was compared with that in seismogenic layer, the relationship between them was obtained, and this relationship was used to get the recurrence periods and co-seismic displacements of all fault segments. After the studies above, we calculated the co-seismic deformation field of all the earthquakes larger than M6.8 from AD 1700 one by one and inversed the potential displacement in the co-seismic deformation field. Then, we divided the potential displacement by the slip rate from GPS inversion to get the influences of these fault segments, added the influences into the elapsed time of the characteristic earthquakes, and obtained the earthquake hazard degree of all the segments we studied in the form of the ratio of elapsed time to recurrence period; so, we name the ratio as the Impending Earthquake Risk (IER). Historical earthquake cases show that the fault segment is in safety when the IER is less than 1 but in danger after the IER becomes larger than 1. In 2009, the IER is larger than 1 on the following segments, 1.35 on the Tagong segment of Xianshuihe fault, 1 on the Menggu-Dongchuan segment, 1.04 on the Dongchuan-Xundian segment, and 1.09 on the Yiliang-Chengjiang segment of Xiaojiang fault. [ABSTRACT FROM AUTHOR]

Seismic and field observations indicate that the Mw7.4 Maduo earthquake ruptured the Jiangcuo fault, which is a secondary fault ∼85 km south of the northern boundary of the Bayan Hor block in western China. The kinematic characteristics of the Jiangcuo fault can shed lights on the seismogenic mechanism of this earthquake. Slip rate is one of the key parameters to describe the kinematic features of a fault, which can also provide quantitative evidences for regional seismic hazard assessments. However, due to lack of effective observations, the slip rate of the Jiangcuo fault has not been studied quantitatively. In this study, we consider the interaction between the Jiangcuo fault and the eastern Kunlun fault, and estimate the slip rates of the two faults using the interseismic GPS observations across the seismogenic region. The inferred results show that the slip rates of the Jiangcuo fault and the Tuosuo Lake segment of the Kunlun fault are 1.2±0.8 and 5.4±0.3 mm a−1, respectively. Combining the slip rate with the average slip inferred from the coseismic slip model, the earthquake recurrence interval of the Jiangcuo fault is estimated to be 1800−700+3700 years (1100–5500 years). Based on the results derived from previous studies, as well as calculations in this study, we infer that the slip rate of the Kunlun fault may decrease gradually from the Tuosuo Lake segment to the eastern tip. The Jiangcuo fault and its adjacent parallel secondary faults may have absorbed the relative motion of blocks together with the Kunlun fault. [ABSTRACT FROM AUTHOR]

The 2021 Qinghai Maduo MS7.4 earthquake was one of the strongest earthquakes that occurred in the Bayan Har block of the Tibetan Plateau during the past 30 years, which spatially filled in the gap of strong earthquake in the eastern section of the northern block boundary. In this study, the aftershock sequence within 8 days after the mainshock was relocated by double difference algorithm. The results show that the total length of the aftershock zone is approximately 170 km; the mainshock epicenter is located in the center of the aftershock zone, indicating a bilateral rupture. The aftershocks are mainly distributed along NWW direction with an overall strike of 285°. The focal depth profiles indicate that the seismogenic fault is nearly vertical and dips to southwest or northeast in different sections, indicating a complex geometry. There is an aftershock gap located to the southeast of the mainshock epicenter with a scale of approximately 20 km. At the eastern end of the aftershock zone, horsetail-like branch faults show the terminal effect of a large strike-slip fault. There is a NW-trending aftershock zone on the north side of the western section, which may be a branch fault triggered by the mainshock. The location of the aftershock sequence is close to the eastern section of the Kunlun Mountain Pass-Jiangcuo (KMPJ) fault. The sequence overlaps well with surface trace of the KMPJ fault. We speculate that the KMPJ fault is the main seismogenic fault of the MS7.4 Maduo earthquake. [ABSTRACT FROM AUTHOR]

Landslides are recurrent geological phenomena on Earth that cause heavy casualties and property losses annually. In this study, we use the Vp-k stacking and nonlinear waveform inversion methods of high-frequency receiver functions extracted from local earthquakes, to sequentially invert Poisson's ratios and S-wave velocities of the Quaternary Xishancun landslide, which is composed of three segments, i.e., h1, h2, and h3 from bottom to top. Our results show that Poisson's ratio values are generally higher than 0.33 and that the S-wave velocities vary from 0.1 to 0.9 km s−1. High Poisson's ratios (>0.44) are mainly distributed in the juncture regions between different segments, as well as the western edge of h2. These zones show significant variation in landslide thickness and are potentially hazardous areas. Low velocities of 0.05–0.2 km s−1 with thicknesses of 10–30 m are widely observed in the lower layer of the landslide. The high Poisson's ratios and low-velocity layer may be related to water-rich materials in these areas. Our study suggests that the high-frequency receiver functions from local earthquakes can be used to delineate geotechnical structures, which is valuable for landslide stability analysis and hazard mitigation. [ABSTRACT FROM AUTHOR]

However, paleoseismic studies elucidate that the WLSG has experienced several historical earthquakes with magnitude 6.5-7.0, therefore, strong earthquakes may also occur in lowFigure 1 T ectonic background of the seismic gap between the W enchuan and Lushan earthquakes. Backgr ound on the seismic gap between the W enchuan and the Lushan earthquakes The 2008 M w 7.9 W enchuan earthquake and the 2013 M w 6.9 Lushan earthquake, which occurred in the Longmenshan fault system (hereafter called LMSFS), caused dramatically casualties and huge economic losses. Moreover, the W enchuan and Lushan earthquakes greatly increase the Coulomb failure stresses in the WLSG, thus increasing the possibility of strong earthquakes (Guo et al., 2020). [Extracted from the article]

Large-scale detailed mapping plays a key role in revealing the rupture characteristics and mechanisms of strong earthquakes. Relatively few studies have been performed on the surface ruptures of large earthquakes in central and western Tibet due to its remote nature and high elevation. Based on high-resolution unmanned aerial vehicle (UAV) photography, we mapped the coseismic surface rupture of the 2014 Yutian Ms7.3 earthquake. Along the western Altyn Tagh fault system, the earthquake produced ≈37 km of surface rupture along the South Xor Kol fault (southern section S1), Xor Kol fault (central section S2) and Ashikule fault (northern section S3). Section S1 has a 16-km-long surface rupture with an average sinistral offset of 52±25 cm and a maximum offset of ~90 cm, while section S3 has a 14.2-km-long surface rupture with an average sinistral offset of 36±21 cm and a maximum offset of ~84 cm. A compilation of 5308 cracks yields an average crack width along the southern section of 85±71 cm and a maximum width of ~700 cm; the average width along the central section is 39±21 cm, and the maximum width is 243 cm; and the average width along the northern section is 61±44 cm with a maximum of ~340 cm. In addition, the average cumulative opening across rupture zone is 3.4±2.9 m along the southern section, with a maximum of ~17 m; 4.3±3.6 m along the central section, with a maximum of ~13 m; and 1.7±1.6 m along the northern section, with a maximum of ~6 m. Evidently, the average crack width and cumulative opening decrease towards bends and steps along the fault. A global synthesis of surface rupture distributions corresponding to strike-slip earthquakes indicates that the rupture zone is wider near the complex parts of fault geometries (such as bends, steps and fault bifurcations) than along straight sections, suggesting that the fault geometry has an obvious control on the surface rupture width. The widespread cracks at the intersection between the Xor Kol and South Xor Kol faults may indicate that an extensional regime is more likely to produce distributed offfault deformation, which provides an observational constraint for the numerical simulation of dynamic rupture on a fault. In addition to coseismic surface rupture, the Yutian earthquake also produced a large number of gravity-driven slides on alluvial fans with gentle slopes. The friction efficiency of the water-bearing salt layer beneath fans could decrease the sliding threshold and trigger instability under surface shaking. These distributed deformations and gravity-driven slides reflect the coupling between the rupture propagation and fault geometry and indicate that the rupture may have propagated in two directions along the Ashikule fault after passing through a step. Therefore, the investigation of coseismic surface rupture provides important observational constraints on the dynamic rupture process. [ABSTRACT FROM AUTHOR]

Songyuan is the most earthquake prone area in northeast China. Since 2006, earthquakes have occurred in the area in the form of swarms, with a maximum magnitude of ML5.8. There is much controversy about the cause of the Songyuan earthquakes. We attempted to determine the cause using a three-dimensional electrical conductivity structure inverted from a regional network of magnetotelluric data in the Songyuan area. The L-BFGS inversion method was applied, with a full-impedance tensor data set used as the inversion input. Combined with an evaluation of the earthquake locations, the resistivity model revealed a northeast-oriented hidden fault running through the Songyuan earthquake area (SEA), which was speculated to be the preexisting Fuyu-Zhaodong Fault (FZF). Our resistivity model also found an apparent lithospheric low-resistivity anomaly beneath the earthquake area, which breached the high-resistivity lithospheric mantle and stalled at the base of the crust. A petrophysical analysis showed that this lower crustal low-resistivity anomaly was most likely attributed to hydrated partial melting, which could release water into the lower crust during later magma emplacements. While weakening the strength of the FZF, these ascending fluids also increased the pore pressure in the fault, further reducing the shear strength of the fault. Shear stress action (a fault strike component of the east-west regional compress), together with possible near-surface disturbances, may drive the fault to slip and trigger the earthquakes in Songyuan. It is possible that the continuous replenishment of fluids from the deeper mantle forces the Songyuan earthquakes into the form of swarms. We infer that the Songyuan earthquakes could be attributed to a combination of preexisting faults, regional stress, and deep fluids associated with plate subduction, and near surface disturbances might induce the earthquakes in advance. The Songyuan earthquakes are inherently induced earthquakes, fed by deep fluids. [ABSTRACT FROM AUTHOR]

After the 2008 MW7.9 Wenchuan earthquake, the eastern Tibetan Plateau experienced a series of MW>6.0 earthquakes, including the 2013 MW6.6 Lushan, 2014 MW6.1 Kangding and 2017 MW6.5 Jiuzhaigou events. Based on available constraints, we build a three-dimensional viscoelastic finite element model to calculate Coulomb failure stress caused by these strong earthquakes. In this model, the geometry and slip vector of the initial rupture zone of each earthquake are used to better evaluate the earthquake-related stress projection. Considering reasonable ranges of viscosities for the crust and upper mantle in different tectonic units, numerical results show that after the Wenchuan earthquake, the coseismic Coulomb failure stress change at the hypocenters of the subsequent earthquakes increased to approximately +0.012–+0.040, +0.01–+0.03, and +0.008–+0.015 MPa, respectively. With viscoelastic relaxation of the lower crust and upper mantle, the Coulomb failure stress change at the hypocenters of these earthquakes accumulated to about +0.014–+0.042, +0.016–+0.036, and +0.003–+0.007 MPa just before their occurrence. This suggests that the Wenchuan earthquake indeed triggered or hastened the occurrence of the Lushan, Kangding and Jiuzhaigou events, supporting that strong earthquake clustering around the eastern Tibetan Plateau could be related to stress interaction between the seismogenic faults. Besides, ~94% and ~6% of the stress increase around (and before the occurrence of) the Kangding earthquake were contributed by the Wenchuan event and the Lushan event, respectively; the positive Coulomb failure stress change at the Jiuzhaigou earthquake hypocenter was related to coseismic slip partitioning of the Wenchuan earthquake. This means that stress interaction among the earthquakes could be controlled by the combined effect of stress of the previous events and by the complexity of earthquake ruptures. Thus, in researches on the earthquake-triggering mechanism, special attentions should be paid on both details of the rupture model and multiple factors of previous earthquakes. [ABSTRACT FROM AUTHOR]

Focal depths of the 2008 MS6.1 Panzhihua earthquake sequence and tectonic stress field in the source area are investigated. Source depths of 24 earthquakes in Panzhihua earthquake sequence with a magnitude M≥3.0 were determined using the seismic depth phase sPL; additionally, the focal depths of 232 earthquakes were measured by fitting the three-component waveforms of the P and S waves. The source depth of the main shock is ~12 km. The majority of the aftershocks with magnitude M≥3.0 occurred in the brittle upper crust at the depths range of 12–18 km. Further, the Source mechanisms of the 232 events around the Panzhihua earthquake source area were determined, and the results show that the earthquakes have predominantly strike-slip mechanisms in the Dianzhong Block, but display complexity of the focal mechanisms outside and near the boundary of the Dianzhong block. The 232 earthquake mechanisms from this study are combined with the solutions from the Global Centroid Moment Tensor (GCMT) catalog to derive 2D stress field. The inversion results show that the Dianzhong block is predominantly under a strike slip faulting regime and the direction of the maximum principal compression σ1 is northwest-southeast (NW-SE)-trending. The distribution is coincide with GPS velocity field. However, orientations of principal stress axes as well as the faulting types change outside and near the Dianzhong block. The results show that the tectonic stress field in the study area is predominantly controlled by the southeast (SE)-trending horizontal movement and clockwise rotation of the Dianzhong block as a result of the eastward movement of eastern Tibetan meeting the old and rigid South China block (SCB). The Panzhihua earthquake ruptured at ~12 km depth where the tectonic stress regime is under the SE-direction horizontal compression and the NE-direction horizontal extension. [ABSTRACT FROM AUTHOR]

Observation and research of rotational motions induced by earthquake has been ignored in the past decades due to the lack of understanding of important ground rotational motions and practical difficulty involved in directly measuring the rotational components. Currently, there is a paucity of methods to directly measure rotational motions in China. The present study attempts to discuss the possibility of obtaining seismic rotation from ground tilt data. The rotational motions generated by shear dislocation source are derived based on the displacement formula of elastic wavefields in the homogenous space. The characteristics of the three components of translational and rotational motions in near, intermediate, and far fields are compared and analyzed. Based on the corresponding relationship between the rotation and ground tilt on free surface boundary, a method to indirectly construct the rotational motions in the layered half space is obtained. Finally, the theoretical rotational motions generated by the Ms7.0 earthquake in Jiuzhaigou, Sichuan Province, on August 8, 2017 are calculated. The results indicate that the rotational motions generated by the Jiuzhaigou earthquake are concentrated in the vicinity of the epicenter (approximately 150 km) and mainly distributed in the area perpendicular to the fault strike. The existence of free surface boundary attenuates the total energy of the rotational field and especially in the direction of the dominant energy. The ground tilt data is compared with rotational motions in the layered half space, and the results indicate that theoretical rotations and actual ground tilt data are essentially consistent, especially in the far field region. Thus, it is possible to obtain horizontal components of rotation from ground tilt data, and this provides more information for a complete description of the motion characteristics of an underground medium. [ABSTRACT FROM AUTHOR]

An earthquake of Mw6.4 occurred in Pishan County in Xinjiang Province, northwestern Tibetan Plateau, on July 3, 2015. The epicenter was located on an active blind thrust system located at the northern margin of the Western Kunlun Mountain Orogenic Belt southwest of the Tarim Basin. We constructed a shovel-shaped fault model based on the layered-crust model with reference to the seismic reflection profile, and obtained the rupture process of the earthquake from the joint inversion of Interferometric Synthetic Aperture Radar (InSAR) measurements, far-field waveform data, and Global Positioning System (GPS) data. The results show that the seismic fault dips southward with a strike of 109°, and the rupture direction was essentially northward. The fault plane rupture distribution is concentrated, with a maximum recorded slip of 73 cm. The main features of the fault are as follows: low inclination angle (25°-10°), thrust slip at a depth of 9-13 km, rupture propagation time of about 12 s, no significant slip in soft or hard sedimentary layers at 0-4 km depth and propagation from the initial rupture point to the surrounding area with no obvious directionality. The InSAR time-series analysis method is used to determine the deformation rate in the source region within 2 years after the earthquake, and the maximum value is ~17 mm yr−1 in the radar line-of-sight direction. Obvious post-earthquake deformation is evident in the hanging wall, with a similar trend to the coseismic displacement field. These results suggest that the Pishan earthquake has not completely released the accumulated energy of the region, given that the multilayer fold structure above the blind fault is still in a process of slow uplift since the earthquake. Post-earthquake adjustment models and aftershock risk analysis require further study using more independent data. [ABSTRACT FROM AUTHOR]

At GMT time 13:19, August 8, 2017, an Ms7.0 earthquake struck the Jiuzhaigou region in Sichuan Province, China, causing severe damages and casualties. To investigate the source properties, seismogenic structures, and seismic hazards, we systematically analyzed the tectonic environment, crustal velocity structure in the source region, source parameters and rupture process, Coulomb failure stress changes, and 3-D features of the rupture plane of the Jiuzhaigou earthquake. Our results indicate the following: (1) The Jiuzhaigou earthquake occurred on an unmarked fault belonging to the transition zone of the east Kunlun fault system and is located northwest of the Huya fault. (2) Both the mainshock and aftershock rupture zones are located in a region where crustal seismic velocity changes dramatically. Southeast to the source region, shear wave velocity at the middle to lower crust is significantly low, but it rapidly increases northeastward and lies close to the background velocity across the rupture fault. (3) The aftershock zone is narrow and distributes along the northwest-southeast trend, and most aftershocks occur within a depth range of 5-20 km. (4) The focal mechanism of the Jiuzhaigou earthquake indicates a left-lateral strike-slip fault, with strike, dip, and rake angles of 152°, 74° and 8°, respectively. The hypocenter depth measures 20 km, whereas the centroid depth is about 6 km. The co-seismic rupture mainly concentrates at depths of 3-13 km, with a moment magnitude (Mw) of 6.5. (5) The co-seismic rupture also strengthens the Coulomb failure stress at the two ends of the rupture fault and the east segment of the Tazang fault. Aftershocks relocation results together with geological surveys indicate that the causative fault is a near vertical fault with notable spatial variations: dip angle varies within 66°-89° from northwest to southeast and the average dip angle measures ~84°. The results of this work are of fundamental importance for further studies on the source characteristics, tectonic environment, and seismic hazard evaluation of the Jiuzhaigou earthquake. [ABSTRACT FROM AUTHOR]

On August 8, 2017, a M7.0 earthquake occurred in Jiuzhaigou County, Sichuan Province, China, resulting in significant casualties and property damage. Therefore, it is critical to identify the areas of potential aftershocks before reconstruction and re-settling people to avoid future disasters. Based on the elastic dislocation theory and a multi-layered lithospheric model, we calculate the Coulomb failure stress changes caused by the Wenchuan and Jiuzhaigou earthquakes, discuss the relationship between the M 7.9 Wenchuan and M7.0 Jiuzhaigou earthquakes, and analyze the influence of the aftershock distribution and stress changes on the major faults in this region caused by the Jiuzhaigou earthquake. The co- and post-seismic stress changes caused by the Wenchuan earthquake significantly increased the stress accumulation at the hypocenter of the Jiuzhaigou earthquake. Therefore, the occurrence of the Jiuzhaigou earthquake was probably stimulated by the Wenchuan earthquake. The aftershock distribution is well explained by the co-seismic stress changes of the Jiuzhaigou earthquake. The stress accumulation and corresponding seismic hazard on the Maqu-Heye segment of the East Kunlun fault and the northern extremity of the Huya fault has been further increased by the Jiuzhaigou earthquake. [ABSTRACT FROM AUTHOR]

As one of the world's most active intracontinental mountain belts, Tien Shan has posed questions for researchers regarding the formation of different tectonic units and active shallow seismicity. Here, we used a huge data set comprising of 7094 earthquakes from local, regional and teleseismic seismic stations. We used waveform modeling and multi-scale double-difference earthquake relocation technique to better constrain the source parameters of the earthquakes. The new set of events provided us with better initial earthquake locations for further tomographic investigation. We found that reverse-faulting earthquakes dominate the whole study area while the fault plane solutions for earthquakes beneath the northwestern Tarim Basin and the Main Pamir Thrust are diverse. There is a low-velocity anomaly beneath Bashkaingdy at depth of 80 km, and high-velocity anomalies beneath central Tien Shan at shallower depths. These observations are the keys to understand the mechanism of Tien Shan's formation because of Tarim Basin northward and Kazakh Shield's southward subduction in the south and north respectively. Velocities beneath western Tien Shan are relatively high. We thus infer that the Western Tien Shan is relatively less deformed than the eastern Tien Shan primarily due to a relatively brittle mantle. [ABSTRACT FROM AUTHOR]

It has long been known that human activities such as waste fluid disposal and reservoir impoundment may cause earthquakes. Recently, anthropogenic activities to tackle the increasing energy demand and to address climate change issues are also reported to induce earthquakes. These activities have a common attribute in that fluids are injected and extracted underground and induce spatiotemporal changes of pore pressure and stress, which may cause slip on faults. Induced earthquakes not only pose significant impacts on seismic hazard assessment and preparation, but also raise the question to the society as how to balance the economic needs of resources development and the public's concerns about potential environmental impacts. Here we review the observations of fluid-injection/extraction induced earthquakes, ground deformation associated with these activities, and their physical mechanisms. Furthermore, we discuss the influences of induced earthquakes on seismic hazard models, regulatory policies on these anthropogenic activities, and current development of academic, industrial and government initiatives and collaborations in order to understand this intriguing phenomenon and address associated challenges. [ABSTRACT FROM AUTHOR]

From the 1960s to 1970s, North China has been hit by a series of large earthquakes. During the past half century, geophysicists have carried out numerous surveys of the crustal and upper mantle structure, and associated studies in North China. They have made significant progress on several key issues in the geosciences, such as the crustal and upper mantle structure and the seismogenic environment of strong earthquakes. Deep seismic profiling results indicate a complex tectonic setting in the strong earthquake areas of North China, where a listric normal fault and a low-angle detachment in the upper crust coexist with a high-angle deep fault passing through the lower crust to the Moho beneath the hypocenter. Seismic tomography images reveal that most of the large earthquakes occurred in the transition between the high- and low-velocity zones, and the Tangshan earthquake area is characterized by a low-velocity anomaly in the middle-lower crust. Comprehensive analysis of geophysical data identified that the deep seismogenic environment in the North China extensional tectonic region is generally characterized by a low-velocity anomalous belt beneath the hypocenter, inconsistency of the deep and shallow structures in the crust, a steep crustalal-scale fault, relative lower velocities in the uppermost mantle, and local Moho uplift, etc. This indicates that the lithospheric structure of North China has strong heterogeneities. Geologically, the North China region had been a stable craton named the North China Craton or in brief the NCC, containing crustal rocks as old as ~3.8 Ga. The present-day strong seismic activity and the lower velocity of the lower crust in the NCC are much different from typical stable cratons around the world. These findings provide significant evidence for the destruction of the NCC. Although deep seismic profiling and seismic tomography have greatly enhanced knowledge about the deep-seated structure and seismogenic environment, some fundamental issues still remain and require further work. [ABSTRACT FROM AUTHOR]

Detailed information about the crustal structure is essential for better understanding the occurrence and mechanisms of earthquakes and volcanoes. Here we present a study of the upper crustal P-wave velocity structure of two seismically and volcanically active areas in northern Iran using the two-dimensional Pg travel time tomography method. The imaging results suggest low velocities in the upper crust beneath the Damavand and Sahand-Sabalan volcanic areas in the central and western parts of northern Iran, respectively. The upper crustal low velocities in these two areas roughly coincide with previously imaged low Pn velocity anomalies, suggesting that the Late Cenozoic volcanic activity was probably caused by the upwelling of hot materials from the mantle. The image feature of the Pg velocity structure beneath the Sahand-Sabalan volcanic area further indicates that the hot materials stored in the upper crust beneath Sahand may be larger in size than those stored beneath Sabalan. Comparison of the Pg velocity images with the earthquake distribution in north Iran suggests that earthquakes mainly occur at moderately low velocity or low to high velocity boundary areas instead of significantly low or high velocity regions. The anisotropy results show that the Pg wave fast direction is consistent with the GPS direction at high Pg velocity areas and the fast direction is inconsistent with the GPS direction but consistent with the strike direction of faults at low velocity areas. Our new upper crustal structural images provide the basic observation for better understanding of the regional seismicity and volcanism, and link the surface geological phenomena to deep crustal and mantle processes associated with the active tectonics in northern Iran. [ABSTRACT FROM AUTHOR]

The Helan Mountains and Yinchuan Basin (HM-YB) are located at the northern end of the North-South tectonic belt, and form an intraplate tectonic deformation zone in the western margin of the North China Craton (NCC). The HM-YB has a complicated history of formation and evolution, and is tectonically active at the present day. It has played a dominant role in the complex geological structure and modern earthquake activities of the region. A 135-km-long deep seismic reflection profile across the HM-YB was acquired in early 2014, which provides detailed information of the lithospheric structure and faulting characteristics from near-surface to various depths in the region. The results show that the Moho gradually deepens from east to west in the depth range of 40-48 km along the profile. Significant differences are present in the crustal structure of different tectonic units, including in the distribution of seismic velocities, depths of intra-crustal discontinuities and undulation pattern of the Moho. The deep seismic reflection profile further reveals distinct structural characteristics on the opposite sides of the Helan Mountains. To the east, The Yellow River fault, the eastern piedmont fault of the Helan Mountains, as well as multiple buried faults within the Yinchuan Basin are all normal faults and still active since the Quaternary. These faults have controlled the Cenozoic sedimentation of the basin, and display a 'negative-flower' structure in the profile. To the west, the Bayanhaote fault and the western piedmont fault of the Helan Mountains are east-dipping thrust faults, which caused folding, thrusting, and structural deformation in the Mesozoic stratum of the Helan Mountains uplift zone. A deep-penetrating fault is identified in the western side of the Yinchuan Basin. It has a steep inclination cutting through the middle-lower crust and the Moho, and may be connected to the two groups of faults in the upper crust. This set of deep and shallow fault system consists of both strike-slip, thrust, and normal faults formed over different eras, and provides the key tectonic conditions for the basin-mountains coupling, crustal deformation and crust-mantle interactions in the region. The other important phenomenon revealed from the results of deep seismic reflection profiling is the presence of a strong upper mantle reflection (UMR) at a depth of 82-92 km beneath the HM-YB, indicating the existence of a rapid velocity variation or a velocity discontinuity in that depth range. This is possibly a sign of vertical structural inhomogeneity in the upper mantle of the region. The seismic results presented here provide new clues and observational bases for further study of the deep structure, structural differences among various blocks and the tectonic relationship between deep and shallow processes in the western NCC. [ABSTRACT FROM AUTHOR]

More than 80 layers of seismites were recognized from the Early Cretaceous Dasheng Group in the Mazhan and Tancheng graben basins in the Tanlu Fault Zone, eastern China. The responsible seismic events took place about 110-100 Ma in the Early Cretaceous. The fault zone was affected at the time by strong tectonics, due to tension-related stretching and scattered squeezing by strike-slip faults. These tectonic activities induced a series of strong earthquakes with Richter magnitudes (M) of 5-8.5. The earthquakes affected saturated or semi-consolidated flood and lake sediments, and produced intra-layer deformations by several processes, including liquefaction, thixotropy, drop, faulting, cracking, filling and folding, which resulted in the formation of various soft-sediment deformation structures, such as dikes and veins of liquefied sand, liquefied breccias, liquefied homogeneous layers, load structures, flame structures, ball-and-pillow structures, boudinage, diapirs, fissure infillings, a giant conglomerate wedge, and syn-sedimentary faults. The seismites are new evidence of tectonic and seismic activities in the Tanlu Fault Zone during the Early Cretaceous; the series of strong seismic events that can be deduced from them must be considered as a response to the destruction of the North China Craton. [ABSTRACT FROM AUTHOR]

Based on calculations of the tidal Coulomb failure stress and investigations of the correlation between the Earth tide and the Ning'er earthquake sequence, the processes of fault nucleation and failure were simulated. In these simulations we consider the influence of tidal stresses using the rate- and state-dependent friction laws. Furthermore, the effects on tidal triggering due to the stress amplitude and periodic oscillation properties were investigated, and the triggering effects between the tidal normal and tidal shear stresses were compared. The results showed that the Ning'er earthquake sequence was a physical consequence of tidal effects. A transition period T exists between the nucleation and failure processes of a seismic fault. When the period T of stress is equal to or becomes larger than T, the fault response becomes dependent on the periodic features of the loading stress; however, for T < T, the response of the fault is nearly independent of the period. Both the tidal normal and tidal shear stresses have similar effect in the nucleation and failure processes; the clock changes generally increase with the maximum amplitudes of the tidal stresses. Tidal normal and tidal shear stresses with positive amplitudes mainly induce earthquake triggering; however, the triggering effects induced by negative tidal stresses are smaller and faults are not sensitive to negative tidal stresses. Our results primarily reveal the physical mechanisms of tidal stress triggering. [ABSTRACT FROM AUTHOR]

In this study, we demonstrate an approach for inverting earthquake source parameters based on high-rate global positioning system (GPS) velocity seismograms. The velocity records obtained from single-station GPS velocity solutions with broadcast ephemeris are used directly for earthquake source parameter inversion using the Cut and Paste method, without requiring conversion of the velocity records into displacement records. Taking the El Mayor-Cucapah earthquake as an example, GPS velocity records from 10 stations with reasonable azimuthal coverage provide earthquake source parameters very close to those from the Global centroid moment tensor (Global CMT) solution. In sparse network tests, robust source parameters with acceptable bias can be achieved with as few as three stations. When the number of stations is reduced to two, the bias in rake angle becomes appreciable, but the magnitude and strike estimations are still robust. The results of this study demonstrate that rapid and reliable estimation of earthquake source parameters can be obtained from GPS velocity data. These parameters could be used for early earthquake warning and shake map construction, because such GPS velocity records can be obtained in real time. [ABSTRACT FROM AUTHOR]

A continuous GPS array across the southern segment of the Longmen Shan fault zone recorded the deformation during the process of the Lushan M7.0 earthquake that occurred on April 20, 2013. Such data can provide meaningful information regarding the dynamic evolution of crustal deformation in the seismogenic zone. Our studies have shown that the occurrence of the Wenchuan earthquake led to the loading of compressive and sinistral shearing strain on the southern segment of the Maoxian-Wenchuan fault, whereby the extrusion strain accumulated at a greater rate than before the Wenchuan earthquake. The strain time series in the seismogenic zone revealed that the principal compression strain rates decreased from west to east in the direction of N30°-45°W. Furthermore, the area to the east of Beichuan-Yingxiu fault behaved as a zone of compressive deformation with obvious sinistral shearing deformation. The surface strain and the first shearing strain time series decreased with time, while the area to the west of the Beichuan-Yingxiu fault behaved as a zone of dextral shear deformation that increased with time. Furthermore, the regional deformation field before the Lushan earthquake showed that the rate of extrusion strain accumulation in the southern segment of the Longmen Shan fault zone was obviously larger than before the Wenchuan earthquake. Moreover, the sinistral shearing strain accumulated in the area of the southern segment of the Maoxian-Wenchuan fault. Based on the above analysis, we consider that the eastward movement of the Bayan Har block increased considerably following the Wenchuan earthquake, which enhanced the accumulation of compression strain in the southern segment of the Longmen Shan fault zone. [ABSTRACT FROM AUTHOR]

Using broadband seismic records from regional networks, we determined the focal mechanisms and depths of 37 earthquakes in the 2013 M7.0 Lushan earthquake sequence (3.4⩽ M⩽5.1) by fitting the three-component waveform data. The results show that the earthquakes are predominantly thrust events, with occasional strike-slip mechanisms. Most earthquakes occurred at depths of 10-20 km. We derived the regional distribution of the average stress field in this area using the damped linear inversion method and the focal mechanisms obtained in this study. The inversion results suggest that the Lushan region and the adjacent area are mostly under compression. The orientations of the maximum principal axes trend NW-SE, with some local differences in the stress distribution at different depths. Compared with the distribution of the stress field in the Wenchuan earthquake area, the stress field in the southwest section of the Longmenshan Fault zone (LFZ) share similar characteristics, predominantly thrust faulting with a few strike-slip events and the maximum compression axes being perpendicular to the LFZ. [ABSTRACT FROM AUTHOR]

An M6.5 earthquake occurred on August 3rd, 2014 in Ludian of Yunnan Province in China, causing severe casualty and economic loss. Local broadband waveform inversion with the CAP method demonstrates that the earthquake is a strike-slip event, with the strike along 70° and 160° for the two nodal planes respectively. However, the geological structure in the epicentral region is complicated with abundant active faults, and it is challenging to identify the seismogenic fault with the focal plane solutions due to nodal-plane ambiguity. We resolved the rupture directivity by measuring the difference between centroid location and hypocenter of the Ludian earthquake with the time shift from CAP inversion, and found that the nodal plane with the strike of 160° is the ruptured fault plane. Moreover, the rupture is found to propagate from northwest to southeast. [ABSTRACT FROM AUTHOR]

To investigate the relationship between velocity structure and earthquake activity on the southeastern front of the Tibetan Plateau, we make use of continuous observations of seismic ambient noise data obtained at 55 broadband stations from the regional Yunnan Seismic Network. These data are used to compute Rayleigh wave Green's Functions by cross-correlating between two stations, extracting phase velocity dispersion curves, and finally inverting to image Rayleigh wave phase velocity with periods between 5 and 34 s by ambient noise tomography. The results show significant lateral variations in crustal and uppermost mantle structures in the studied region. Phase velocity anomalies at short periods (5-12 s) are closely related to regional tectonic features such as sediment thickness and the depth of the crystalline basement. The Sichuan-Yunnan rhombic block, enclosed by the Honghe, Xiaojiang and Jianchuan faults, emerges as a large range of low-velocity anomalies at periods of 16-26 s, that inverts to high-velocity anomalies at periods of 30-34 s. The phase velocity variation in the vicinity of the Sichuan-Yunnan rhombic block suggests that the low-velocity anomaly area in the middle-lower crust may correspond to lower crustal channelized flow of the Tibetan Plateau. The spatial distribution of strong earthquakes since 1970 reveals that the Yunnan region is inhomogeneous and shows prominent characteristics of block motion. However, earthquakes mostly occur in the upper crust, with the exception of the middle-Yunnan block where earthquakes occur at the interface zone between high and low velocity as well as in the low-velocity zones, with magnitudes being generally less than 7. There are few earthquakes of magnitude 5 at the depths of 15-30 km, where gather earthquakes of magnitude 7 or higher ones which mainly occur in the interface zone between high and low velocities with others extending to the high-velocity abnormal zone. [ABSTRACT FROM AUTHOR]

On April 20, 2013, the Lushan M7.0 earthquake struck at the southern part of the Longmenshan fault in the eastern Tibetan Plateau, China. The shear-wave splitting in the crust indicates a connection between the direction of the principal crustal compressive stress and the fault orientation in the Longmenshan fault zone. Our relocation analysis of the aftershocks of the Lushan earthquake shows a gap between the location of the rupture zone of the Lushan M7.0 earthquake and that of the rupture zone of the Wenchuan M8.0 earthquake. We believe that stress levels in the crust at the rupture gap and its vicinity should be monitored in the immediate future. We suggest using controlled source borehole measurements for this purpose. [ABSTRACT FROM AUTHOR]

The amplitudes of the Earth's free oscillations have a close relationship to earthquake focal mechanisms. Focal mechanisms of large earthquakes can be well analyzed and constrained with observations of long period free oscillations. Although the 2013 Lushan earthquake was only moderately sized, observable spherical normal modes were excited and clearly observed by a superconductive gravimeter and a broadband seismometer. We compare observed free oscillations with synthetic normal modes corresponding to four different focal mechanisms for the Lushan earthquake. The results show that source parameters can be analyzed and constrained by spherical normal modes in a 2.3-5 mHz frequency band. The scalar seismic moment M has a major influence on the amplitudes of free oscillations; additionally, the strike, dip, rake and depth of the hypocenter have minor influences. We found that the synthetic modes corresponding to the focal mechanism determined by the Global Centroid Moment Tensor show agreement to the observed modes, suggesting that earthquake magnitudes predicted in this way can readily reflect the total energy released by the earthquake. The scalar seismic moment obtained by far-field body wave inversion is significantly underestimated. Focal mechanism solutions can be improved by joint inversion of far- and near-field data. [ABSTRACT FROM AUTHOR]

Giant earthquakes generate rich signals that can be used to explore the characteristics of the hierarchical structure of the Earth's interior associated with the eigenfrequencies of the Earth. We employ the spectral element method, incorporated with large-scale parallel computing technology, to investigate the characteristics of global seismic wave propagation excited by the 2011 M9.0 Tohoku earthquake. The transversely isotropic PREM model is employed as a prototype of our numerical global Earth model. Topographic data and the effect of the oceans are taken into consideration. Wave propagation processes are simulated by solving three-dimensional elastic wave governing equations with the seismic moment tensor obtained from the Global Centroid Moment Tensor Catalog. Three-dimensional visualization of our computing results displays the nature of the global seismic wave propagation. Comparative analysis of our calculations with observations obtained from the Incorporated Research Institutions for Seismology demonstrates the reliability and feasibility of our numerical results. We compare synthetic seismograms with incorporated and unincorporated ocean models. First results show that the oceans have obvious effects on the characteristics of seismic wave propagation. The peak displacement and peak velocity of P waves become relatively small under the effect of the ocean. However, the effect of the ocean on S-waves is complex. The displacement and velocity of S waves decrease rapidly over time using an unincorporated ocean model. Therefore, the effects of the ocean should be incorporated when undertaking quantitative earthquake hazard assessments on coastal areas. In addition, we undertake comparative analysis on the characteristics of the Earth's oscillation excited by the 2004 Sumatra-Andaman, 2008 Wenchuan, and 2011 Tohoku earthquakes that incorporate the effect of the Earth's gravitational potential. A comparison of the amplitude spectra of the numerical records indicates that energy released by the three big earthquakes is different. Our comparative analysis realizes that the computing results can accurately reproduce some eigenfrequencies of the Earth, such as toroidal modes 0T2 to 0T13 and spheroidal modes 0S7 to 0S31. These results demonstrate that numerical simulations can be successfully used to investigate the Earth's oscillations. We propose that numerical simulations can be used as one of the major tools to further reveal how the Earth's lateral heterogeneities affect the Earth's oscillations. [ABSTRACT FROM AUTHOR]

Following the M7.9 Wenchuan earthquake, the M6.6 Lushan earthquake is another devastating earthquake that struck the Longmenshan Fault Zone (LFZ) and caused severe damages. In this study, we collected continuous broadband ambient noise seismic data and earthquake event data from Chinese provincial digital seismic network, and then utilized ambient noise tomography method and receiver function method to obtain high resolution shear wave velocity structure, crustal thickness, and Poisson ratio in the earthquake source region and its surroundings. Based on the tomography images and the receiver function results, we further analyzed the deep seismogenic environment of the LFZ and its neighborhood. We reveal three main findings: (1) There is big contrast of the shear wave velocities across the LFZ. (2) Both the Lushan earthquake and the Wenchuan earthquake occurred in the regions where crustal shear wave velocity and crustal thickness change dramatically. The rupture faults and the aftershock zones are also concentrated in the areas where the lateral gradients of crustal seismic wave speed and crustal thickness change significantly, and the focal depths of the earthquakes are concentrated in the transitional depths where shear wave velocities change dramatically from laterally uniform to laterally non-uniform. (3) The Wenchuan earthquake and its aftershocks occurred in low Poisson ratio region, while the Lushan earthquake sequences are located in high Poisson ratio zone. We proposed that the effect of the dramatic lateral variation of shear wave velocity, and the gravity potential energy differences caused by the big contrast in the topography and the crustal thickness across the LFZ may constitute the seismogenic environment for the strong earthquakes in the LFZ, and the Poisson ratio difference between the rocks in the south and north segments of the Longmenshan Fault zone may explain the 5 years delay of the occurrence of the Lushan earthquake than the Wenchuan earthquake. [ABSTRACT FROM AUTHOR]

On April 20, 2013 at 8:02 am, a magnitude 7.0 earthquake occurred in Lushan County, Sichuan Province, China, which induces massive landslides, causes great losses to life and property. Based on the locations of aftershocks provided by the China Earthquake Network Center and the characteristic of Longmenshan active faults system, combined with the current preliminary focal mechanism solution, the fault rupture direction is determined. With the finite fault inversion method, we invert the rupture process of the Lushan M7.0 earthquake by teleseismic waveforms data. The inversion results indicate that the main shock is dominated by thrust fault component and the rupture initiated at depth of 15 km, and most of slip ruptured around the hypocenter with the peak slip of about 1.5 m. Most of rupture slips released at the first 20 s and the main rupture occurred at the first 10 s after the onsets of the mainshock. Most of seismic energy released near the hypocenter with a length of 28 km, especially on both sides of the hypocenter with the range of 20 km, and the seismic energy released relatively smaller in other areas. There is a large area with weak slip between the main rupture and another two asperities on both sides of the hypocenter; it may imply that the accumulated strain on the rupture fault has not been completely released. Therefore, there is a significant possibility of having strong aftershocks in the areas where energy is not fully released. This is also the main reason why there are a lot of moderate to strong aftershocks in the Lushan aftershock sequence. In addition, there is an earthquake vacant zone with a length of about 50 km between the Wenchuan M7.9 earthquake and this event, which is of high earthquake risk and is deserved to be paid close attention to. [ABSTRACT FROM AUTHOR]

On April 20, 2013, a magnitude M7.0 earthquake occurred in Lushan, Sichuan Province, China, and caused heavy casualties and economic losses. Based on the local broadband waveforms in Sichuan and adjacent provinces regional networks and teleseismic broadband records from IRIS stations, the focal mechanism and the focal depth are determined by the CAP (Cut And Paste) and its upgraded methods, CAPtele and CAPjoint, respectively. The results show that the focal mechanisms and depth from different methods are steady, and the best double couple solution derived from the joint inversion is 210°, 44°, and 91° for strike, dip, and rake angles respectively for one nodal plane and 29°, 46°, and 89° for another with 16 km focal depth and M6.66 moment magnitude. In order to verify the reliability of the results, a number of tests are performed based on local seismograms with different velocity models. They indicate that there is about 10 degree's fluctuation in focal mechanisms and about 2 km variation in focal depth with a complex velocity structure. Furthermore, inverted by re-sampling the teleseismic waveforms on the basis of epicentral distance, the solutions are consistent with each other, which manifests that the teleseismic records are effective for constraining source parameters of the Lushan earthquake. [ABSTRACT FROM AUTHOR]

On April 20, 2013, an M7.0 earthquake occurred in Ya'an-Lushan region, Sichuan Province, China, killing and injuring more than one thousand people. Therefore, it is critical to outline the areas with potential aftershocks before reconstruction and re-settlement as to avoid future disasters. Based on the elastic dislocation theory and multi-layered lithospheric model, we calculate the co- and post-seismic stress changes caused by the Wenchuan and Lushan earthquakes to discuss the relationship between M7.9 Wenchuan earthquake and M7.0 Lushan earthquake, the influences on the distribution of aftershock caused by the Lushan earthquake, and the stress changes on major faults in this region. It is shown that the Coulomb failure stress increment on the hypocenter of Lushan earthquake caused by the Wenchuan earthquake is about 0.0037-0.0113 MPa. And the possible maximum value (0.0113 MPa) is larger than the threshold of stress triggering. Therefore, the occurrence of Lushan earthquake is probably effectively promoted by the Wenchuan earthquake. The aftershock distribution is well explained by the co-seismic stress changes of Lushan earthquake. By the two ends of the rupture of Lushan earthquake with increased Coulomb failure stress, a lack of aftershock recordings indicates the high seismic hazard. The stress accumulation and corresponding seismic hazard on the Kangding-Dafu segment of the Xinshuihe fault, the Beichuan-Yingxiu fault, the Pengxian-Guanxian fault, and the Ya'an fault are further increased by the Lushan earthquake and post-seismic process of Wenchuan earthquake. [ABSTRACT FROM AUTHOR]

Although there are many earthquake relics preserved in the southeast segment of the Ganzi-Yushu Fault in the central Tibetan Plateau, the recurrence regularity of paleoearthquakes is not yet clear. This work studies paleoearthquakes on this fault segment since the Holocene through geomorphic investigation and trench excavation. The results show that sinistral dislocation of the T/T terrace boundary is up to 80 m at the Cuoa Township. A 1.5 m-high fault scarp extends 3 km near the Renguo Township. A number of paleoearthquakes are exposed in trenches at two places, respectively. In combination with historical records, our work has identified 5 or 6 paleoearthquakes on this fault segment since last 5600 years. The occurrence times and recurrence intervals of these paleoearthquakes are estimated by C dating on strata in the trenches. Our analysis shows that these paleoearthquakes do not exhibit evident periodicity, but instead show a clustering characteristic. From 5600 a to present, seismicity of the southeastern segment of the Ganzi-Yushu Fault has two active periods and one quiet period, and the present-day time is just in the second active epoch. The recurrence intervals of each active epoch are different: 1000-1300 a in the first one, 534 a in the second one. [ABSTRACT FROM AUTHOR]

Identification of the meta-instable stress state and study of its mechanism and evolution of relevant physical fields would be of great significance for determination of potential seismic risks and estimation of critical times. In laboratory experiments, that the specimen enters the meta-instable is marked by accelerated stress release. Could we use the experimental result to identify the earthquake in natural conditions? Because the observational data from one station can only reflect the stress state beneath this station, the key problem for identification of the meta-instability is how to recognize regional stress state through observational data from many stations. In this work, we choose the evolution of the temperature field over varied deformation stages during a stick-slip event on a 5° bending fault as an example, and attempt to find the response features of the physical quantity when the fault enters the meta-instable state. We discuss the characteristics of stages for the stress build-up, stress-time process deviating from linearity before instability, meta-instability, instability, and post-instability, respectively. The result shows that the fault instability slide is a conversion process from independent activities of each fault segment to synergism activity. The instability implies completion of the synergism. The stage deviating from linearity is the onset of stress release, and it is also the onset of the synergism. At the meta-instability stages, stress release becomes dominant, while the synergism tends to finish. Therefore, the analysis of the regional overall stress state should not start from individual stations, and instead it should begin with the evolution of the whole deformation field. [ABSTRACT FROM AUTHOR]

Investigating source parameters of small and moderate earthquakes plays an important role in seismology research. For small and moderate earthquakes, the mechanisms are usually obtained by first motion of P-Wave, surface wave spectra method in frequency-domain or the waveform inversion in time-domain, based on the regional waveform records. We applied the wavelet domain inversion method to determine mechanism of regional earthquake. Using the wavelet coefficients of different scales can give more information to constrain the inversion. We determined the mechanisms of three earthquakes occurred in California, the United States. They are consistent with the previous results (Harvard Centroid Moment Tensor and United States Geological Service). This proves that the wavelet domain inversion method is an efficient method to determine the source parameters of small and moderate earthquakes, especially the strong aftershocks after a large, disastrous earthquake. [ABSTRACT FROM AUTHOR]