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20 February 2021, Volume 43 Issue 1

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THE SPATIO-TEMPORAL EVOLUTION OF THE FAULT DEFOR-MATION DURING THE META-INSTABILITY QUASI-DYNAMIC PHASE AND THE COSEISMIC STAGE: A VIEW FROM LABORATORY

LI Shi-nian, MA Jin, JI Yun-tao, GUO Yan-shuang, LIU Li-qiang

2021, 43(1):  1-19.  DOI: 10.3969/j.issn.0253-4967.2021.01.001

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A crucial question in earthquake science is how earthquakes start. Field and experimental observations show a short period exists between the fault reaching peak stress and the coseismic event. Therefore, it is of fundamental significance to capture the spatio-temporal evolution of a fault’s deformation during this premonitory stage. It can help us understand how the rupture of an earthquake initiates and also provide precursory information. Stick-slip events or lab quakes can be produced in controlled conditions to mimic earthquakes in nature. In previous studies, we proposed the fault meta-instability model focusing on depicting this stage(hereinafter referred to as the meta-instability stage)and interpreting the transition from energy/stress accumulation to energy/stress release. We further divided the meta-instability stage into two substages, i.e., the quasi-static phase and a quasi-dynamic phase, corresponding to slow energy release and irreversible energy release elevated rate.
However, how the meta-instability stage can facilitate the final failure remains puzzled. In contrast, the meta-instability stage exhibits slow and mild deformation, while the coseismic stage is fast and violent. In order to bridge these two processes, it is essential to record the complete dynamic process of stick-slip events, including the premonitory and coseismic stage. Thus, the data acquisition system required must feature a high signal-noise ratio, high frequency, continuous recording, and dense instrumentation. In 2016, we developed an ultra-high-speed, multi-channel and continuous recording data acquisition system for deformation measurement(UltraHiDAM). UltraHiDAM has 64 channels, 16-bit resolution, and 4MHz sampling frequency, and can perform parallel continuous data acquisition. It is able to record strain signals and acoustic emissions continuously and synchronously at a high sampling frequency up to 4MHz for as long as a few hours. To our best knowledge, it is the first system that is capable of doing so.
Based on this system, we conducted a series of stick-slip experiments. We recorded the entire deformation process of the laboratory earthquake cycles, including the relatively slow deformation in the quasi-static phase(several seconds before the stress drop), the relative fast deformation in the quasi-dynamic phase(a few microns before the stress drop), and the complete process of the transient coseismic slip. High frequency continuous synchronous sampling allows us to reveal as many details as possible of unstable sliding transient processes, and analyze mechanical problems related to the seismic source.
We report results of stick-slip experiments using saw-cut bare-surface granodiorite samples. The main findings of this paper are summarized as follows: First, the substages can be further recognized based on the local deformation characteristics(Table 2). Second, strain and stress start to localize before the quasi-static phase; such localization’s acceleration indicates the whole fault has entered the quasi-static phase. Third, the strain field during the quasi-dynamic phase is characterized by a wave-like acceleration and reciprocating propagation(Fig. 9). Fourth, there is a short preparation period for each sub-stage of the quasi-dynamic process(Fig. 6). The existence of such preparation periods may help the imminent earthquake prediction. Finally, even for the stick-slip events captured on a simplified plane laboratory fault, the coseismic process can be multiple rupture events, each event has its own AE waveform that is distinguishable in time(Fig. 8).
The implications are that there is indeed precursory information during the different substages before the coseismic event, most of which are associated with the localization and propagation of strain and stress. An earthquake source’s actual mechanical process can be complex in terms of multiple stress drops and ruptures.

AN EXPERIMENTAL STUDY ON THE TRANSIENT CREEP OF GRANITE

NIU Lu, ZHOU Yong-sheng, YAO Wen-ming, MA Xi, HE Chang-rong

2021, 43(1):  20-35.  DOI: 10.3969/j.issn.0253-4967.2021.01.003

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Many of the large earthquakes in the continental crust nucleate at the bottom of the seismogenic zone in depths between 10 and 20km which is related to the broad so-called ‘brittle-to-plastic or brittle-to-ductile’ transition region. From the field studies and seismic data, we could know that the dominant deformation mechanism at the base of seismogenic zone is likely to be semi-brittle flow of fault rocks. The physical and chemical processes acting in the ‘brittle-to-plastic’ transition are of great interest for a better understanding of fault rheology, tectonic deformation of the continental lithosphere and the generation of strong earthquakes. So it’s of great significance to know more about this transition. Despite the importance of semi-brittle flow, only few experimental studies are relevant to semi-brittle flow in natural rocks. In order to study the semi-brittle deformation and rheological characteristics of granite, we performed a series of transient creep experiments on fine-grained granite collected from the representative rock of Pengguan Complex in Wenchuan earthquake fault area using a solid-medium triaxial deformation apparatus(a modified Griggs rig). The conditions of the experiments are under the temperatures of 190~490℃and the confining pressures of 250~750MPa with a strain rate of 5×10^-4s^-1. The temperature and pressure simulate the in-situ conditions of the Wenchuan earthquake fault zone at the corresponding depths of 10~30km. We observe the microstructures of the experimentally deformed samples under the scanning electron microscope(SEM). The mechanical data, microstructures and deformation mechanism analysis demonstrate that deformation of the samples with experimental conditions could be covered by three regimes: 1)Brittle fracture to semi-brittle flow regime. We could see the strain and stress curves of the samples characterizing with strain hardening behavior and without definite yield point under low temperatures and pressures, which correspond to the depths of 10~15km; 2)Brittle-ductile transition regime. The strain and stress curves of the samples tend to be in a steady state with definite yield point under temperature and pressure at the depths of 15~20km. The main deformation mechanism is cataclasis, and dynamic recrystallization and dislocation creep are activated; and 3)Ductile flow regime which is at depths of 20~30km. The strength of granite increases with depth and reaches to the ultimate at the depth of 15~20km, and then decreases with depth at 20~30km. Based on the analysis of strength of granite, microstructures and deformation mechanism, we conclude that the granitic samples deformed with the characteristics of transient creep, and the strength of Longmenshan fault zone reaches maximum at the depths of 15~20km where it is in the brittle-to-plastic regime. Based on the Mohr circle analysis, the rupture limit at depths of 15~20km is close to the limit of friction, and at the same time, this depth range is also consistent with the focal depth of Wenchuan earthquake. Therefore, it implicates that the deformation and strength of Pengguan complex granitic rocks should control the nucleation and generation of the Wenchuan earthquake.

A POSSIBLE MECHANISM FOR REVERSE CROSS-BASIN FAULT IN GANYANCHI ASYMMETRIC PULL-APART BASIN ALONG THE HAIYUAN FAULT

LEI Sheng-xue, RAN Yong-kang, LI Yan-bao, LI Hai-ou, GAO Ye, GUO Wei

2021, 43(1):  36-52.  DOI: 10.3969/j.issn.0253-4967.2021.01.003

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Pull-apart basin and push-up structure are the two most common and important structures formed within a strike-slip fault system. The term of pull-apart basin was firstly introduced when researchers discussing the formation of Central Death Valley, California. A pull-apart basin typically forms and develops between en echelon surficial strands or along a releasing bend of the transform fault. The diagonal cross-basin fault, formed diagonally within a pull-apart basin, connects the two en echelon strands bounding the basin. This type of fault not only plays an important role in the extinction of pull-part basin, but also controls the occurrence of large earthquakes. Therefore, it is of great significance to study the formation and evolution of cross-basin faults. However, compared with extensively studied pull-apart basins, fewer studies have been conducted on cross-basin fault, which greatly hampers our understanding of the structural evolution process of pull-apart basin and strike-slip fault. In this study, we take the Ganyanchi(Salt Lake)basin, the largest pull-apart basin located in the central part of the Haiyuan Fault, northeastern corner of the Tibetan plateau, as an example to investigate the character and general mechanism of the cross-basin fault. Geomorphological investigation, shallow artificial seismic exploration, and composite drilling geological survey are carried out along the cross-basin fault in Ganyanchi Basin. The main conclusions are listed as below: 1)The cross-basin fault in Ganyanchi Basin is a reverse strike-slip fault dipping to SW, rather than the previously claimed a strike-slip fault with significant normal component; 2)Although with a classic rhombic shape, the Ganyanchi Basin is actually an asymmetric pull-apart basin, which is mainly controlled by the northern boundary fault, i.e., the Nan-Xi Hua Shan Fault. Under the control of the Nan-Xi Hua Shan Fault, more than 680m thick growth strata were accumulated in the basin, and a rollover anticline was formed by the growth strata; 3)The example of the Ganyanchi asymmetric pull-apart basin suggests that a possible mechanism for the reverse cross-basin fault is probably the “straightening of strike-slip fault”, that is, the earlier formed antithetical normal fault adopts reverse component of straightened basin boundary fault, and then undergoes rotation and finally becomes a synthetical reverse fault with great strike-slip component. The rollover anticline, mainly controlled by the master boundary fault of an asymmetric pull-apart basin, may also partly contribute to the rotation of the cross-basin fault. As more natural pull-part basins needed to be investigated, caution should be taken when this suggested model is applied elsewhere.

PALEOSEISMOLOGIC STUDY ON THE SHIMIAN FAULT IN THE NORTHERN SECTION OF THE DALIANGSHAN FAULT ZONE

FENG Jia-hui, CHEN Li-chun, WANG Hu, LIU Jiao, HAN Ming-ming, LI Yan-bao, GAO Shuai-po, LU Li-li

2021, 43(1):  53-71.  DOI: 10.3969/j.issn.0253-4967.2021.01.004

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The Daliangshan fault zone(DF)constitutes an important part of the large-scale strike-slip Xianshuihe-Xiaojiang fault system(XXFS). Affected by the channel flow of the middle-lower crust in the western Sichuan region, the XXFS is strongly active, and large earthquakes occur frequently. On average, there is an earthquake of magnitude 7 or more every 34 years. However, the DF, as an important part of the middle segment of the XXFS, has only recorded several earthquakes with magnitude 5-6, and no earthquakes with magnitude over 6 have been recorded. The reason for the lack of strong earthquake records may be related to the lack of historical records in remote mountainous areas, but the main reason may be attributed to the active behavior of the faults. He et al.(2008)hold that the DF is a new fault, resulting from straightening of the middle section of the XXFS, and its activity gradually changes from weak to strong, and will probably replace the Anninghe-Zemuhe Fault. However, this view lacks evidence of strong earthquakes. In recent years, some scholars have studied the paleoearthquakes on the DF, and found the signs of strong earthquake activity, and considered that the fault has the seismogenic capacity of earthquakes with magnitude more than 7. These studies are mainly concentrated in the middle and southern segments of the DF. Although there are scattered activity data and individual trench profiles, direct evidence of Holocene activity and paleoearthquake data are very scarce in the northern part of DF. On the basis of the previous studies, combined with our detailed field geomorphological surveys, we excavated a set of two trenches at Lianhe village in Shimian Fault to reveal the direct evidence of fault activity in Holocene. From paleoseismic analysis and radiocarbon samples accelerated mass spectrometry(AMS)dating, four paleoseismic events are identified, which are E1 between 20925—16850BC, E2 between 15265—1785BC, E3 between 360—1475AD, and E4 between 1655—1815AD. The results of the latest two events should be relatively reliable, and the latest event may be related to the Moxi earthquake of magnitude 7^3/4 on June 1, 1786 or the Dalu earthquake of magnitude ≥7 on June 10, 1786. Among the four events revealed, three are since the Holocene, and the recurrence interval of the latest two events is about 800 years. Compared with other active faults at the triple junction, the recurrence interval is slightly longer than that at the northern segment of the Anninghe fault zone, but close to that at the Moxi segment of the Xianshuihe fault zone. Compared with the western segment of Xianshuihe Fault and the northern segment of Anninghe Fault, the Shimian Fault also has a higher seismic risk, which needs further attention.

STUDIES ON NEW ACTIVITY OF LINTAN-DANGCHANG FAULT, WEST QINLING

ZHANG Bo, TIAN Qin-jian, WANG Ai-guo, LI Wen-qiao, XU Yue-ren, GAO Ze-min

2021, 43(1):  72-91.  DOI: 10.3969/j.issn.0253-4967.2021.01.005

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Located in the intervening zone between Tibetan plateau and surrounding blocks, the Lintan-Dangchang Fault(LDF)is characterized by north-protruding arc-shape, complex structures and intense fault activity. Quantitative studies on its new activity play a key role in searching the seismogenic mechanism, building regional tectonic model and understanding the tectonic interaction between Tibetan plateau and surrounding blocks. The LDF has strong neotectonic activities, and moderate-strong earthquakes occur frequently(three M6~7 earthquakes occurred in the past 500 years, including the July 22nd, 2013, Minxian-Zhangxian M_S6.6 earthquake), but the new activity of the fault is poorly known, the geological and geomorphological evidence of the Holocene activity has not been reported yet. Based on remote sensing interpretation and macro-landform analysis, this paper studies the long-term performance of LDF. Based on the study of fault activity, unmanned aircraft vehicle photogrammetry and differential GPS, radiocarbon dating, etc., the latest activity of LDF is quantitatively studied. Then the research results, historical strong earthquakes and small earthquake distribution are comprehensively analyzed for studying the seismogenic mechanism and constructing regional tectonic models. The results are as follows: Firstly, the fault geometry is complex and there are many branch faults. According to the convergence degree of the fault trace and the fault-controlled macroscopic topography, the LDF is divided into three segments: the west, the middle and the east. The west segment contains two fault branches(the south and the north)and the south Hezuo Fault. The south branch of the west segment mainly dominates the Jicang Neogene Basin, and the south Hezuo Fault controls the south boundary of Hezuo Basin. The middle segment has more convergent and stable trace, consisting of the main fault and south Hezuo Fault, and these faults separate the main planation surface of the Tibetan plateau and Lintan Basin surface geologically and geomorphologically. The fault traces in the east segment are sparsely distributed, and the terrain is characterized by hundreds of meters of uplifts. The branch faults include the main fault, Hetuo Fault, Muzhailing Fault and Bolinkou Fault, each controlling differential topography. Secondly, the motion property of the LDF is mainly left-lateral strike-slip, with a relative smaller portion of vertical slip. The left-lateral strike-slip offset the Taohe River and its tributaries, gullies and ridges synchronously, and the maximum left-lateral displacement of the tributary of Taohe River can reach 3km. Meanwhile, the pull-apart basins and push-up ridges associated with the left-lateral fault slip are also developed in the fault zone. The performance of vertical slip includes tilting of the main planation surface, vertical offsets of the boundary and interior of Neogene basin and hundred meter-scale differential topography. The vertical offset of the Neogene is 300~500m. Thirdly, one fault profile was newly discovered in Gongqia Village, revealing a complete sequence of pre-earthquake-coseismic-postseismic deposition, and this event was constrained by the radiocarbon ages of pre-earthquake and post-earthquake deposition. The event was constrained to be 2090~7745aBP(confidence 2σ), which for the first time confirmed the Holocene activity of the fault. Fourthly, a gully with two terraces at least on the west side of Zhuangzi Village in the east segment of the main fault retains a typical faulted landform. The T2/T1 terrace riser of the gully has a left-handed dislocation of 6.3~11.8m, and the scarp height on terrace T2 is 0.4~0.7m, the radiocarbon age of the terrace T2 is7170~7310aBP, so the derived left-lateral strike-slip rate since the early Holocene in the east segment of the main fault is 0.86~1.65mm/a, and the vertical slip rate is 0.05~0.10mm/a. The derived slip rates are in line with the regional tectonic model proposed by the predecessors, so the LDF plays an important role in the internal deformation of the West Qinling. The clockwise rotation of the middle to east segments of the LDF acts as an obstacle to the left-lateral strike-slip motion, which inevitably leads to the redistribution and rapid release of stress, so earthquakes in the middle-east segment of the LDF are unusually frequent.

STUDY ON THE DISTRIBUTION OF CO-SEISMIC LANDSLIDES AND TERRAIN FEATURES IN THE M_S6.5 LUDIAN EARTHQUAKE AFFECTED AREACHEN

Xiao-li, LIU Chun-guo, CHUAN Yi-jian, LAN Jian, WEI Yan-kun

2021, 43(1):  92-104.  DOI: 10.3969/j.issn.0253-4967.2021.01.003

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The 2014 Ludian, Yunnan M_S6.5 earthquake triggered an unusually large number of co-seismic landslides compared with other events of similar magnitudes in southwestern China. We use landslide-area ratios(LAR)based on a grid(cell size ≈1km²)to delineate the specific distribution of landslides rather than the averaging landslide density that is commonly employed. Results show that the distribution of co-seismic landslides triggered by the 2014 Ludian M_W6.5 earthquake contains LAR values ranging from less than 1.0% to 36.1% with the high values concentrating in river valleys of the study area. Then we examine the correlation between some topographic parameters and the co-seismic landslides in each grid cell and especially focus on the grids with larger LAR(>10%). In addition to examining the correlation between the elevation, local relief and average slope angle(S), we propose a notion of the expected slope degree(ES)to further analyze the correlation between these co-seismic landslides and local topographic features. The expected slope degree can be calculated by dividing the local relief(the difference between the maximum and minimum elevation)by cell length for each grid cell, which is a kind of smooth of the local terrain instead of the average slope. After this procedure, we make a regression analysis on the expected slope degrees and the average slope degrees for all grid cells. Finally, we choose those cells that deviate from the regression line as the unstable units and examine their relation with landslide distribution. Results show that the elevation has a negative relationship with both the average slope degrees and the local relief in the study area, which can be classified as a kind of negative topography implying strong river erosion on the landform. In general, there exists a good positive linear correlation between expected slope degrees and average slope degree for the study area. While larger LAR values(LAR≥10%)lie at the sites that deviate from the regression line, likely representing unstable slopes prone to landsliding. Moreover, it is found that most of these sites with high LAR are just located along river valleys, which permits to predict that mass wasting by future earthquakes or other factors will recur at these places. As a contrast, the analysis results of topographic parameters in the Jiuzhaigou earthquake affected area show different characteristics. Thus, the understanding of the relationship between the geomorphic features and the spatial distribution and scale of landslides is helpful to improve the accuracy of prediction on earthquake-induced landslides and to identify potential large-scale landslides in the early stage.landscape evolution, landslides, adaptive adjustment, expected slope, the Ludian earthquake

A SPECIAL MAGMATISM: PHREATOMAGMATIC ERUPTION

ZHANG Wen-qian, LI Ni

2021, 43(1):  105-122.  DOI: 10.3969/j.issn.0253-4967.2021.01.007

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Phreatomagmatic eruption is a kind of special eruption, which usually occurs when hot magma rises and contacts with the ground water. The water/melt interaction produces explosive eruption and base-surge deposits, which resulted in maars. Monogenetic maar is a common volcano type on continents and islands. This kind of volcanoes is widely distributed in many countries. Researchers have studied eruption process and products of phreatomagmatic eruptions by means of petrological, sedimentological, volcanic physical and geochemical methods and techniques. Additionally, they have also explored influence factors over eruption process through experimental and computer simulations.
Phreatomagmatic eruptions can be considered as the natural equivalent of a class of physical processes termed fuel-coolant interactions(FCI)by investigators of large industrial explosions. Initially, a small volume of water is vaporized due to contact with the melt, with pulsating increasing in the high-pressure steam volume within the aquifer, the dominant effect of the vaporization energy is to fragment the melt and country rocks. Subsequently, the steam, melt, and country rock mix and vapor explosion occurs after vaporization energy increases beyond the confinement strength in chamber. Finally, maar and base-surge deposits form when the overlying layer is broken by the impact of the explosion. Two contrasting environments exist with respect to groundwater availability for the phreatomagmatic explosions. 1)In hard rock environment, the wall rocks are cut by joints and faults, many of which are hydraulically active. Under such groundwater conditions, a maar-diatreme volcano would form. 2)When the magma rises into soft-rock environment rich in water or with high permeability, it will lead to the formation of tuff-rings.
Maar consists of the crater at the surface(which is cut into the pre-eruption land surface), the tephra ring surrounding the crater and the cone-shaped diatreme, root zone and feeder underlies the maar crater. This tephra ring is easily eroded in the late evolution process, and it usually contains base-surge deposits with obvious dune-like bedding, fallout deposits and individual blocks and bombs that were emplaced ballistically, in which the base-surge deposits are dominant. Besides, the base-surge deposits and individual blocks and bombs are deposited near the crater. Maar lake usually forms in the center of the maar crater. It may form in many years after the phreatomagmatic eruptions. After the eruptions, the maar craters may be filled with groundwater and surface water. Maar lake is different from other crater lakes. For example, its surface of crater is often lower than the pre-eruption surface. In addition, the hydrology and sedimentary environment of maar lake are relatively simple. Archives from sediments of maar lakes, especially annually laminated sediments, will provide high-resolution dataset and are conducive to the study of paleoenvironment and paleoclimate. Compared with the maar-diatreme volcano, the tuff-rings volcano is formed in water-rich and shallower environments and has a wider crater which is not cut into the pre-eruptive land surface. The tuff-rings ejecta usually contain less than 5% of country rock clasts only.
Base surge is a kind of pyroclastic density currents with great velocity, and it carries debris further than ballistic fragments. Base surge transports lapilli, magma fragments, broken country rocks and ash formed by the explosion. When the base surge flows move, it generates shear force to the lower ground. The base surge can be subdivided into two parts by the interface where the shear stress is zero. The density of lower base surge currents is relatively large and the particles are coarser. In contrast, the upper currents have less density, and the particles accumulate slowly with the decrease of energy. Some indicative sedimentary structures, such as climbing bedding, dune-like bedding, and accretionary lapilli, would form in the base-surge deposits due to their special genetic mechanism. The climbing bedding helps to determine the location of the crater during field investigations. Accretionary lapilli indicate the distant source facies.
The entire eruption process of phreatomagmatic eruption is relatively complicated. This process may be influenced by several factors, such as the characteristics of the magma, the location and topography of the explosion, the country rocks, and the amount of water involved in the explosion. Foreign scientists have carried out many quantitative studies on the dynamic process of phreatomagmatic eruption through field geology and simulation experiments, while domestic scientists mainly focus on the analysis of the structure, particle size, composition and morphology of base-surge deposit, and the study of the dynamic process is relatively rare. Quantitative studies of the process of phreatomagmatic eruption will be the key in the future research.
Volcanic hazard is one of the major disasters in the world. Base surge generated by phreatomagmatic eruptions owns great energy and velocity. It would generate great damage to people’s life and the environment due to its special transportation process. Further, volcanoes formed by phreatomagmatic eruptions are common in China and relevant research is important. This paper introduces the research progress concerning phreatomagmatic eruptions and their products, aiming to advance our understanding of this special eruption so as to improve our strategies for preventing future volcanic hazards and protect people’s lives and properties.

AN IMPROVED MINIMUM 1-D V_P VELOCITY MODEL IN THE ONSHORE-OFFSHORE AREA OF THE PEARL RIVER ESTUARY FROM 3-D ACTIVE-SOURCE SEISMIC EXPERIMENT

WANG Li-wei, WANG Bao-shan, YE Xiu-wei, ZHANG Yun-peng, WANG Xiao-na, LÜ Zuo-yong

2021, 43(1):  123-143.  DOI: 10.3969/j.issn.0253-4967.2021.01.008

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Layered 1-D velocity models are widely used in seismic network routine locations and in seismological studies, such as earthquake relocation, focal mechanism inversion, synthetic seismogram calculation and geodynamics simulation. It’s also used as a reference model for 2-D and 3-D tomographic inversions. Therefore, obtaining a more reliable 1-D velocity model is an extremely important work for the study of earthquake source parameters and seismic tomography. The onshore-offshore area of the Pearl River Estuary is located at the transition zone between South China block and South China Sea bock, the special ocean-land transitional crustal type and the littoral fault zone, which is the regional seismic control structure passing through it, makes it a potential seismic source. Meanwhile, the Pearl River Delta has the most developed economy and dense population in South China. However, the 1-D velocity model used in seismic network routine operations has not been updated since 1990. To investigate the seismic structure and potential strong earthquake risk in this area, we conducted a 3-D active source seismic experiment in 2015, which incorporated sea-based airgun sources and land-based dynamite sources, and seismic recorders both at the onshore and offshore area in the Pearl River Estuary. A high quality subset of the data was used to derive an improved 1-D seismic V_P model for seismological studies. The model is constructed using the VELEST program with first arrival P-wave travel time data, together with station corrections, which account for shallow velocity anomalies from the true velocity model. The reliability of our new model is assessed by good fitting of the travel time data of airguns and dynamites and better earthquake relocation results.
The final 1-D model provides a good fit for travel time data. After iterative inversion, the root-mean-square travel-time error is 0.07s in the onshore area and 0.21s in the offshore area. Within 6km top of the model, the P-wave velocity of onshore area is 5.22~5.99km/s, and the offshore area is obviously lower, which is 2.11~6.03km/s. The retrieved values are in agreement with the thick sedimentary basins in the Pearl River Estuary Basin whose velocity is obvious lower. Then the velocity smoothly increases with depth, within the depth range of 6~15km, the P-wave velocity of onshore area is slightly lower than the offshore area, which may be due to the wide-spread low velocity layer at the middle crust depth in South China Block. Below the depth of 15km, the P-wave velocity of offshore area is greater than that of the onshore area, which is consistent with the high velocity layer in the base of the thinned continental crust and the gradually uplifting of Moho depth seaward as reported in the previous studies.
The spatial distribution of station corrections correlates well with the near-surface structure and geological features. In the area onshore of the Pearl River Estuary, positive values of station corrections are mostly observed in correspondence with the Pearl River Delta sedimentary basins due to its lower velocity values, such as Sanshui Sag, Shunde Sag and Dongguan Sag, etc. While stations located in granite, limestone and metamorphic rocks outcropping area show early P-wave arrivals(negative station corrections). In the area offshore of the Pearl River Estuary, the spatial distribution of station corrections shows a significant lateral variation and 80%larger than the onshore area. It has a good spatial correlation with the buried depth of the sedimentary basement inverted by reflection seismic survey, where the deposits are thicker, the station corrections are positive, the underground medium presents a low velocity, and vice versa. Negative values of station corrections are observed northwest of the NE-trending littoral fault zone, while positive values correspond to the thick sedimentary basins in the Pearl River Estuary Basin southeast of the littoral fault zone.
At last, we relocated 425 earthquakes in the onshore area and 234 earthquakes in the offshore area with M_L≥0.0 using simul2000 algorithm. The result shows that our new model is better than the South China model, the seismic travel time residual after relocation is greatly reduced, the land P wave residual is reduced by 22.6%, and the S wave is reduced by 21.2%. The sea P wave residual is reduced by 25.7%, and the S wave is reduced by 15.6%. The new model is better for regional earthquake location.
We provide a more reliable V_P velocity model, which can be used to earthquake location, earthquake source parameter inversion and 3-D velocity model studies in the Pearl River Estuary.

RESEARCH ON THE CHARACTERISTIC OF QUATERNARY ACTIVITIES OF NW-TRENDING FAULTS IN ZHENJIANG AREA

ZHANG Peng, XU Kui, FAN Xiao-ping, ZHANG Yuan-yuan, WANG Yong, HAO Jing-run

2021, 43(1):  144-157.  DOI: 10.3969/j.issn.0253-4967.2021.01.009

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Running across the east of Zhenjiang city, the Wufengshan-Xilaiqiao Fault and Dantu-Jianshan Fault are two important NW-trending faults in Zhenjiang area. They controlled the Cretaceous stratigraphic deposition and Mesozoic volcanic activities, and also have obvious control effects on modern geomorphology and Quaternary stratigraphic distribution.
There have been many destructive earthquakes in Zhenjiang area, most of which occurred at the intersection of NW-trending faults and near EW faults. It is of great significance to study the nature, characteristics and the latest active age of the NW-trending faults in Zhenjiang area for the prevention and reduction of earthquake disaster in Zhenjiang City, but the past targeted research work and the knowledge of activity of the faults are very limited. Based on the project of “Urban active fault exploration and seismic risk assessment in Zhenjiang City”, a series of shallow seismic exploration work has been carried out on the two major NW-trending faults in Zhenjiang area, and representative points were selected to carry out drilling joint profiling to study the Quaternary activity characteristics of these two faults. The results are of great significance for urban earthquake disaster reduction, urban planning and land use.
The results of shallow seismic exploration show that the Wufengshan-Xilaiqiao Fault is dominated by normal faulting, dipping to the northeast, with a dip angle of about 60° and a displacement of 5~9m on the bedrock surface. The Dantu-Jianshan Fault is dominated by normal faulting, dipping to the southwest, with a dip angle of about 50°~55° and a displacement of 2~7m on the bedrock surface. All breakpoints of Wufengshan-Xilaiqiao Fault and Dantu-Jianshan Fault reveal that only the bedrock surface was dislocated, not the interior stratum of Quaternary.
On the Dalu site, there is no sign of dislocation in the stratum above the Middle Pleistocene, and the bottom boundary of the Middle Pleistocene has been dislocated, with a displacement of 2m. The dislocation of the bottom boundary of the lower Pleistocene is 3.2m on both sides of the fault, and the maximum displacement of the bedrock surface is 9.1m. The characteristics of the fault surface developed in the drill cores indicate that the latest activity of the fault is of sinistral normal faulting. According to the characteristics of dislocated stratum, the latest active age of Wufengshan-Xilaiqiao Fault is early Middle Pleistocene. On the Fangxian site, there is no sign of fault in the stratum above the Middle Pleistocene, and the bottom of the Middle Pleistocene may be affected by the fault. The displacement of the bottom boundary of Baishan Formation on both sides of the fault is 2m, and the maximum displacement of the bedrock surface is 6.7m. Due to the insufficient evidence of dislocation of Baishan Formation, the latest active age of Dantu-Jianshan Fault is estimated to be between early Pleistocene and early Middle Pleistocene.
The NW-trending Su-Xi-Chang Fault is an important regional fault in the Yangtze River Delta region. Its latest active age is the early Middle Pleistocene, and the displacement in the Quaternary is about 3m. The Wufengshan-Xilaiqiao Fault and the Dantu-Jianshan Fault can be regarded as spatial extension of the Su-Xi-Chang Fault to the northwest, and their activities are also consistent. This study shows that the two NW-trending faults in the Zhenjiang area have significant activity since the Quaternary, and are the main faults with relatively high earthquake risk in this area. Therefore, the intersection of these two faults with EW-trending faults and NE-trending faults should be the focus of attention for earthquake damage prevention in the Zhenjiang area.
The bedrock depth in the Zhenjiang area is relatively shallow, and the stratification difference within the cover layer is small, resulting in an unsatisfactory effect by the geophysical exploration methods. The Lower Pleistocene of the Quaternary system is basically missing, and the boundaries of the Middle and Upper Pleistocene are difficult to distinguish. Developed mainly in the bedrock and the bottom of the Quaternary, the stratum displacement is difficult to judge whether it was caused by sedimentary difference or fault activity. Therefore, the quantitative study of fault activity in this paper is still insufficient.

MULTI-SCALE DECOMPOSITION OF GRAVITY ANOMALY OF THE EASTERN DABIE OROGEN AND ITS TECTONIC IMPLICATIONS

LI Zhe-jun, YI Chong-zheng, ZHOU Dong-rui, ZHENG Hai-gang, WANG Jun, LI Jun-hui, NI Hong-yu

2021, 43(1):  158-176.  DOI: 10.3969/j.issn.0253-4967.2021.01.010

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Bouguer gravity anomaly is a comprehensive reflection of deep and shallow density disturbances of the Earth’s internal mass. Important tectonic information of internal structure at different depths can be obtained by source separation of Bouguer gravity anomaly. Bouguer gravity anomaly from ground-based observations and Bouguer gravity anomaly from EGM2008 of the eastern Dabie Orogen were merged based on least-squares collocation. By model construction of massive bodies and data experiments, optimal wavelet base(sym6)and the corresponding optimal level(6)for gravity anomaly decomposition of the study area were confirmed. Two-dimensional discrete wavelet transform method was applied to obtain low-frequency components and high-frequency components of merged Bouguer gravity anomaly of the study area, and average depths of disturbed surfaces of the wavelet decomposition results were determined by spectrum analyses. In combination with data of crustal structure, geologic structure, effective elastic thickness of lithosphere and seismic activities, the deep structure and shallow structure of the crust were analyzed, and the structural background of seismic activities was discussed. The result shows that steep gradient belts of low-frequency components of Bouguer gravity anomaly outline the density transfer zones of deep structure between the eastern Dabie Orogen and surrounding blocks. It is speculated that the suture zone between the eastern Dabie Orogen and the North China Block locates at the front edge of Qingshan-Xiaotian Fault(the eastern part)and Meishan-Longhekou Fault(the western part), the structure transfer zone between the Dabie Orogen and the Yangtze Block locates at the Tancheng-Lujiang fault zone(the eastern part)and 20km north of Xiangfan-Guangji Fault(the southern part). The interior of the eastern Dabie Orogen is characterized by significant low gravity anomaly, which means an obvious depression of Moho surface, and the steep gradient belts of gravity anomaly inside the eastern Dabie Orogen indicate the imperfection of deep structure. High-frequency components of Bouguer gravity anomaly reveal that density structure of the mid-upper crust was influenced by regional faults such as Feizhong Falut, Lu’an-Hefei Fault, Meishan-Longhekou Fault and Tancheng-Lujiang fault zone. The distribution of high-frequency Bouguer gravity anomaly shows that Luo’erling-Tudiling Fault has obvious effect on density structure of the mid-upper crust, and the range of influence breaks northward through the NWW-orientated Qingshan-Xiaotian Fault and Meishan-Longhekou Fault, and may extend to the front edge of Feixi-Hanbaidu Fault. Further analysis combined with seismic activities shows that plate contaction occurred along the suture zones(front edge of Qingshan-Xiaotian Fault)of deep structure between the eastern Dabie Orogen and the North China Block. Besides, deep and shallow structures of this area are both imperfect, and not strong enough for long-time stress accumulation. Therefore, rocks tend to break at weak points(locations where faults intersect and shallow structure transfers)and release stress frequently, which are main reasons why small earthquakes concentrated at Huoshan area.

A STUDY ON THE EARTHQUAKE SEQUENCE TYPE IN THE MIDDLE SECTION OF THE NORTH-SOUTH SEISMIC BELT AND ITS SURROUNDING REGIONS

QI Yu-ping, LONG Feng, LIN Sheng-jie, XIAO Ben-fu, ZHAO Xiao-yan, WANG Pei-ling, FENG Jian-gang

2021, 43(1):  177-196.  DOI: 10.3969/j.issn.0253-4967.2021.01.011

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Based on the statistical results of 86 earthquakes with magnitude≥5.0 in the middle section of the north-south seismic belt and its surrounding regions since 1973, the types of earthquake sequences and the spatial distribution characteristics have been studied. Main conclusions are drawn as follows: 1)The sequence types of moderate and strong earthquakes in the study area are dominated by mainshock-aftershock sequence type(MAT), followed by multiple main-shock type(MMT)and least the isolated earthquake type(IET)sequence. In the same sequence type, with the increase of earthquake magnitude, the proportion of the MAT sequence increased, while the number of MMT and IET gradually decreased, M≥7 earthquakes are mainly of MAT, and there are no IET earthquakes. Among the different rupture types, the MAT earthquakes are the most in the thrust-type, while the MMT earthquakes are more likely to occur in the strike-slip and the normal-fault earthquakes. 2)There is a relatively good linear relationship between the mainshock-aftershock sequence type earthquakes and the maximum aftershock magnitude of the MAT and MMT sequences; the largest aftershock of most earthquakes mostly occurred in 15 days after the mainshock, the largest aftershock of MAT mainly occurred within 3 days after the mainshock, the largest aftershock of MMT earthquakes mainly occurred within 12 days after the mainshock, and the largest aftershock of IET earthquakes mostly occurred on the day of the earthquake. 3)The spatial distribution of seismic sequence shows that the MAT earthquake distribution range is relatively wide, the MMT earthquakes are mainly concentrated in Batang-Litang, Mabian-Zhaotong, Songpan area in Sichuan Province and Yunlong, Yao 'an, Longling and nearby areas in northwest Yunnan Province. IET earthquakes are more likely to occur in Ganzi-Yushu fault zone, the northwestern segment of Xianshuihe fault zone and in Sichuan Basin. 4)The distribution of seismic sequence types in the middle section of the north-south seismic belt and its adjacent areas may be related to the geological structure, historical seismic activity and the crustal stress in this region. The distribution of seismic sequence types also reflects the tectonic movement and dynamic environment in this region.

RELOCATION OF THE 28 OCTOBER 2019 XIAHE M5.7 EARTHQUAKE SEQUENCE AND ANALYSIS OF SEISMOGENIC FAULT

LIU Xu-zhou, SHEN Xu-zhang, HE Xiao-hui, PU Ju

2021, 43(1):  197-208.  DOI: 10.3969/j.issn.0253-4967.2021.01.012

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The Gansu Xiahe M5.7 earthquake on 28 October 2019 is located between the Lintan-Tanchang Fault and the northern Xiqinling Fault. The earthquake sequence offered significant information for interpreting the focal mechanism solution and analyzing the seismotectonics, which is helpful for the estimation of the level of future seismic risk in this region.
There are 180 aftershocks recorded by Gansu seismic network within 650 hours after the Xiahe M5.7 main shock up to November 24, 2019. The waveforms recorded by 20 seismic stations that have a distance less than 200km to the Xiahe M5.7 main shock are collected. HZT station, the nearest station to the main shock has an epicenter distance of 19km. Locations of 94 earthquakes including the main shock are obtained after locating the earthquake sequence using HYPO2000 method, each earthquake is with more than 3 recognizable Pg phases during location. The result of location shows that the aftershock epicenters are concentrated and mainly distributed in an area of 20km×20km near the epicenter of the main shock, the hypocentral locations are distributed in the depth of 1~16km.
After locating the Xiahe M5.7 main shock and the aftershock sequence, we calculated the focal mechanism of the main shock using gCAP and P waveform polarity method, and relocated the main shock and aftershocks by the method of double difference algorithm.
The results show that the parameters of the focal mechanism solution are as follows: For the nodal plane Ⅰ, strike is 185°, dip is 56°, and rake is 127°; for the nodal plane Ⅱ, strike is 312°, dip is 48°, and rake is 48°.The relocation results show the occurrence features of the seismogenic fault that dips northeast with dip angles about 47°~54°, which is near to nodal plane Ⅱ of the focal mechanism solution.
The epicenter of Xiahe M5.7 earthquake is closer to the western Lintan-Tanchang Fault, which has a complex geometry structure composed of several parallel and oblique secondary faults. Before Xiahe M5.7 earthquake, the activity of the western segment of the fault was weak. The Lintan-Tanchang main fault and its secondary faults are part of the transition zone of structural transformation between the Eastern Kunlun Fault and the north margin of West Qinling Fault, which caused several moderate earthquakes occurring near the Lintan-Tanchang Fault since 2003. The Minxian M_S6.6 earthquake 2013, which is the largest earthquake of these earthquakes, has a seismogenic fault which is a secondary fault of Lintan-Tanchang Fault, this fault merges into the Lintan-Tanchang main fault in the deep part.
The result of the relocation shows that the hypocentral location of Xiahe M5.7 is not on Lintan-Tanchang main fault but on a branch fault, since the epicenter of the earthquake has a certain distance from the western segment of Lintan-Tanchang main fault. Previous studies are lacking about seismogenic fault of Xiahe M5.7 earthquake. In this paper, based on the focal mechanism of Xiahe M5.7 earthquake, which is same to the mechanisms of the moderate strong earthquakes occurring near the Lintan-Tanchang Fault in the past twenty years and the feature of Lintan-Tanchang main fault with its secondary faults, we speculated that the seismogenic fault of Xiahe M5.7 earthquake is one of the secondary faults of Lintan-Tanchang Fault, and it has the same structure as other secondary faults that the fault merges into the Lintan-Tanchang main fault in the deep part. The seismogenic fault of Xiahe M5.7 earthquake has a strike angle of about 312° and a dip angle of about 48°, the strike angle is consistent with the intensity distribution from the post-earthquake investigations.

THE RELOCATION, FOCAL MECHANISMS OF THE DINGQING EARTHQUAKES AND A PRELIMINARY STUDY OF ITS SEISMOGENIC STRUCTURE

LI Qi-lei, LI Yu-li, TU Hong-wei, LIU Wen-bang

2021, 43(1):  209-231.  DOI: 10.3969/j.issn.0253-4967.2021.01.013

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Based on the broadband records of the digital seismic networks of Qinghai, the focal mechanisms of the Dingqing, Xizang earthquakes(M_S≥3.0) are of the obtained with Cut-and-Paste(CAP)inversion method and from USGS, seven of them are normal fault type with a little strike-slip component. The dominant direction of the fault strikes is near SN, the dominant distribution of dip angles is 58°~69°, and the dominant distribution of rake angle is -81°~-103°. The dominant direction of P axis is SWW, and that of T axis is SEE. The best double couple solution of the M_S5.5 earthquake in 2016 is 12°, 58° and -103° for strike, dip and rake angles, respectively, the second nodal plane solution is 216°, 34° and -70°, the centroid depth is 7.3km, and its moment magnitude is 5.3. For the M_S5.1 earthquake in 2020, the solution is 9°, 57°, -101° for strike, dip and rake angles, respectively, the second nodal plane solution is 209°, 35° and -74°, the centroid depth is 6.8km, and its moment magnitude is 4.9.
The double difference relative positioning method(HypoDD)is used to relocate the Dingqing earthquakes from February 1, 2015 to March 5, 2020. Broadband data of 9 seismic stations of Qinghai seismic network, Tibet seismic network and scientific array within about 400km around the epicenter are used, and the relocation of 217 earthquakes is obtained. After relocation, the Dingqing earthquake sequence is more clustered than before, with zonal distribution along NE-SW direction, which is in agreement with the fault strike of focal mechanism solutions, but not consistent with the major strike-slip faults in the region. The focal depths of the Dingqing earthquakes are close to the normal distribution, 75 percents of them range from 8 to 12km. The focal depths of earthquakes in 2015-2018 are confined in the range of 5~15km, and that in 2018—2020 are mainly from 7km to 12km, the range of focal depths is significantly reduced after 2018. After the occurrence of M_S5.5 earthquake in 2016, the earthquakes ruptured rapidly to the west and south, and most of the aftershocks were of magnitude 3 or below, and the sequence attenuation was fast, which may be because that the mainshock released most of the energy in the sequence. The aftershocks of the M_S5.1 earthquake in 2020 mostly ruptured along the horizontal direction or to the deep. The earthquakes occurring from 2019 to March 2020 are located in the middle of the sequence in spatial distribution, and there are two dominant directions of NE-SW and SSE in the spatial distribution of epicenters, showing an L-shape distribution. The reason may be that the earthquake encountered obstacles in the rupture along the NE-SW direction, the strain energy was not fully released, and then turned to the SSE faults after stress adjustment to induce subsequent aftershocks. In the NE direction of the “L-shape”, in addition to the M_S5.1 earthquake on January 25, 2020, there were also earthquakes with M_S5.5 on May 11, 2016 and M_S4.5 on October 12, 2017, while only a few earthquakes with M_S3.4 and below occurred in SSE direction, indicating that the NE-trending faults are the dominant area of Dingqing earthquakes activity in recent years.
Since the focal mechanism solutions of M_S5.5 earthquake in 2016 and M_S5.1 earthquake in 2020 are both of normal fault type, the dominant distribution direction of aftershocks is NE, according to the analysis of relocation, focal mechanism and geological structure background, it is inferred that the seismogenic structure of M_S5.5 earthquakes in 2016 and M_S5.1 in 2020 may be of a same normal fault type with NE direction. The fault plane may be nodal plane Ⅰ, i.e. the nodal plane with strike of 12°, dip angle of 58°, rate angle of -103° and strike of 9°, dip angle of 57° and rate angle of -101°. Because the Dingqing earthquakes occurred in the hinterland of the Qinghai-Tibet Plateau, the related research data on the distribution and attitude of small-scale faults is very scarce, so it is difficult to determine the seismogenic faults of the Dingqing earthquakes.

STUDY ON THE 3D CRUSTAL VELOCITY STRUCTURE OF BODY-WAVE IN GONGHE AREA

LUO Ren-yu, CHEN Ji-feng, YIN Xin-xin, LI Shao-hua

2021, 43(1):  232-248.  DOI: 10.3969/j.issn.0253-4967.2021.01.014

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A M_W6.4(M_S7.0)earthquake occurred in Gonghe, Qinghai on 26 April 1990. The Gonghe area is located on the northeastern margin of the Qinghai-Tibet Plateau. The geological tectonic movement in this area is mainly affected by the uplift of the Qinghai-Tibet Plateau. There has been no earthquakes larger than moderate strength in the Gonghe Basin since the historical records, and there are no large-scale active faults on the surface of the epicenter area, so the earthquake has aroused great concern. No major earthquakes have occurred in the Gonghe area since 1995, but the data of small earthquakes is very rich, which ensures the completion of the research. The TomoDD method combines the double-difference relocation method with seismic tomography, and solves two problems at the same time, one is the problem of fine positioning of the earthquake, and the other is the calculation of the 3D velocity structure of the earth’s crust. In this paper, we collected 63872 P and S wave arrival time data in Gonghe and surrounding area recorded by Qinghai, Gansu seismic networks and temporary seismic array from January 2009 to January 2019. The 3D crustal velocity structure and source position parameters of the region are inversed. The relationship between the geological structure setting of the main shock and the velocity structure and seismicity of the region was analyzed. The results show that the crustal velocity structure in the Gonghe area shows lateral inhomogeneity. The Gonghe mainshock is located in the low-velocity anomaly directly below the Gonghe Basin, close to the high-low-velocity anomaly boundary. There is an obvious high-speed anomaly in the southwest of the mainshock, which thrusts from underground to near-surface in the northeast direction. It is estimated that the Wayuxiang-Lagan concealed fault is located at 35.95°N, the dip of the fault is about 45° at the deep part. It is inferred that the occurrence of the Gonghe main shock is caused by the sliding of the Wayuxiangka-Lagan Fault whose strike is NWW and dip is SSW under the action of horizontal tectonic stress. The high-velocity anomaly is about 5~40km deep underground in the northeast direction of the Riyueshan Fault, and a large number of small earthquakes occurred around the high- and low-velocity transition zone. It is presumed that under the action of the near-horizontal NE-directed tectonic stress, the high- and low-velocity zones were further interacted to generate faults and ground folds, and a large number of small earthquakes occurred during the fusion process.

SLIP RATES AND SEISMIC MOMENT DEFICITS ON MAJOR FAULTS IN THE TIANSHAN REGION

ZHU Shuang, LIANG Hong-bao, WEI Wen-xin, LI Jing-wei

2021, 43(1):  249-261.  DOI: 10.3969/j.issn.0253-4967.2021.01.015

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Late Cenozoic and modern tectonic deformation in mainland China is mainly characterized by active block movement, and the average slip rate of faults in the fault zone at the block boundary is an important indicator for quantitatively measuring the intensity of fault activity. The Tianshan Mountains, as the largest revival orogenic belt within Eurasia, with crustal movement basically manifesting as near north-south deformation and a large number of strong seismic surface ruptures, is one of the regions with strong tectonic movement and one of the key seismic hazard zones in China. Many experts have conducted relevant studies on the Tianshan region using GPS technology and have obtained some useful conclusions. These studies have not divided and analyzed the fault zone in detail, but only divided the Tianshan seismic zone into several major fault zones, such as the eastern and western sections of the northern Tianshan, and the eastern and western sections of the southern Tianshan. In order to analyze the activity characteristics of the major faults in the Tianshan region more clearly, this paper refines the major faults and selects 14 major active faults in combination with the distribution of active faults in China proposed by Xu Xi-wei et al. 18 blocks are divided into secondary blocks in Tianshan region, with the major active blocks in the Tianshan region taken as the boundary; The GNSS data of the surrounding areas of 1999—2015 in the Tianshan seismic zone are collected in this paper and used to calculate the velocity field results, and the block locking depth and the slip rate of major faults are calculated using the elastic block model to quantify the seismogenic capacity of major faults. Because the fault closure will produce obvious elastic deformation gradient around the fault, the greater the depth of fault closure is, the greater the influence will be. The fault locking depth can be constrained by the method of GPS data fitting of this model, and the influence of fault locking depth is verified by the method of GPS minimum residual RMS in this paper. According to the optimal locking depth obtained in this paper, the velocity field in Tianshan area is simulated and calculated. The residual mean value of the velocity field simulated by the elastic block model is small, and the average velocity error in the east-west direction is 1.57mm/a, the average velocity error in the north-south direction is 1.72mm/a. At the same time, the slip rate of major faults is obtained. The results show that: the horizontal shortening of the whole Tianshan region is significant, which is consistent with the tectonic background of the region, and the shortening value in the southern Tianshan region is higher than that in the northern Tianshan region; the shortening tensile rate is significantly larger than the slip rate, which shows that the fault zone at basin mountain junction in the Xinjiang Tianshan region is dominated by backwash activity; the extrusion rate in the western section of the southern Tianshan fault zone is in a high value state, reaching(-6.3±1.9)mm/a, which is higher than that in the eastern part of the southern Tianshan; the extrusion rate in the western part of the northern Tianshan is also higher than that in the eastern part. All the strong earthquakes of magnitude 8 and more than 80% of the strong earthquakes of magnitude 7 and above in China occurred in the boundary zones of active blocks according to the historical records, the motion characteristics of the boundary zone of active blocks play an important role in controlling the generation and occurrence of earthquakes, and the seismicity of faults may be quantitatively calculated by the loss of seismic moment. In this paper, we collected a list of strong earthquakes of magnitude 6 and above in the Tianshan area since 1900, estimated the seismic moment release of the main faults in the Tianshan seismic zone based on the above list, and compared it with the calculated seismic moment accumulation to obtain the seismic moment loss of the corresponding fault. Among them, the maximum release of seismic moment of the Beiluntai Fault reached 8.69×10¹⁹N·m; due to the release of several moderate and strong earthquakes, the seismic moment of middle of Bo-A Fault and Keping Fault have not reached the deficit state at present, the surplus is -1.85×10¹⁹N·m and -3.06×10¹⁹N·m, respectively; The smallest area of earthquake release is the northern Tianshan mountain front fault, which is only 0.11×10¹⁹N·m, because there was only one earthquake with a magnitude of 6 in 1907, and the earthquake accumulation reached 11.53×10¹⁹N·m, generating an earthquake deficit of 11.42×10¹⁹N·m, which could produce a magnitude of 7.3 earthquake. The results show that front margins of the northern Tianshan Fault, the Maidan Fault, the north section of Ertix Fault and the west of Kashihe Fault have a large seismic moment loss and have the potential to generate earthquakes of magnitude 7 and above, while Beiluntai Fault and the middle section of the Keping Fault show a surplus state, and there is no possibility of a strong earthquake in a certain period of time in the future.