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20 April 2021, Volume 43 Issue 2

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Research paper

PRESENT-DAY STRIKE-SLIP RATE AND ITS SEGMENTAL VARIATION OF THE TALAS-FERGHANA FAULT IN CENTRAL ASIA: INSIGHT FROM GPS GEODETIC OBSERVATIONS

DAI Cheng-long, ZHANG Ling, LIANG Shi-ming, ZHANG Ke-liang, XIONG Xiao-hui, GAN Wei-jun

2021, 43(2):  263-279.  DOI: 10.3969/j.issn.0253-4967.2021.02.001

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The Talas-Fergana Fault(TFF)with a total length of more than 1 000km is a large dextral strike-slip fault across the West Tianshan Mountains in the northwest direction. The fault plays an important role in accommodating deformation in Central Asia and has attracted much attention by geologists due to the huge controversy in its strike-slip rate and kinematic pattern. Previous studies indicated that its average dextral strike-slip rate is 8~20mm/a since Late Holocene based on offset ephemeral stream valleys and ¹⁴C dating method. Some researchers recently updated the strike-slip to 2.2~6.3mm/a by the application of multiple dating methods(¹⁰Be, ²⁶Al, ³⁶Cl, luminescence, and radiocarbon)and satellite images with higher precision. But the strike-slip rates derived from modern GPS velocity field are only~2mm/a or even as low as 0.8mm/a. Thus, there is a substantial divergence between geological results and geodetic results in the strike-slip rate of the TFF. Some scholars believe that the huge difference between the geological rate and the rate obtained by geodetic measurements is caused by fault locking. In this study, the updated GPS data was used to establish velocity field of the West Tianshan Mountains relative to the stable Eurasian framework and the velocity field without self-rotation. The velocity field shows that the Tianshan Mountains are under intense crustal shortening and deformation. Moreover, for the TFF, as an important boundary fault in the western Tianshan Mountains, whether the far velocity field or the near velocity field, the differential movement of the crust is not obvious. And far-field velocity vectors away from the TFF show that there is minor difference of crustal movement along the fault. The TFF does not have the typical characteristic of locked fault that there is a big difference in velocities of far-field vectors, but a small difference in that of near/mid-field vectors. Thus, the activity of the fault is weak actually.
To further illustrate the overall low slip rate of the TFF, we compare the maximum shear strain rate and its distribution characteristics along the Altyn Fault and the Haiyuan Fault with large slip rates with the results of the TFF. The maximum shear strain rates along the Altyn Fault and the Haiyuan Fault are concentrated along the fault, and are as high as~60nano strain/a and~40nano strain/a, which are much larger than the overall maximum shear along the TFF. This shows that the sliding rate of the TFF is much lower than the strike-slip rate of the Altyn Fault of 9~15mm/a, and even slightly lower than the sliding rate of the Haiyuan Fault of 4~8mm/a. Therefore, we are more certain that the current activity rate of the TFF is far less than 8~20mm/a estimated by some geological methods.
The half-space elastic dislocation model is used to more rigorously re-constrain the current strike-slip rate of the TFF. The results show that the fault is divided into three segments. The TFF dextral strike-slip rate increases from the northwest section to the middle section and decreases from the middle section to the southeast section. And the strike-slip rates of the northwestern, middle and southeastern segments are(2.1±0.7)mm/a, (3.3±0.4)mm/a and(2.4±0.7)mm/a, respectively. The TFF is dominated by strike-slip motion, but there is also a weak dip-slip motion in the middle section of the TFF, with a magnitude of about 1mm/a.
The above results confirm the current low strike-slip rate of the TFF obtained by GPS which is much less than the strike-slip rate of 8~20mm/a estimated by geological methods. And through the GPS results, it is certain that the TFF presently has a low fault activity rather than a locked fault. To reconcile the high geological strike-slip rates and the geodetic results, a new deformation pattern of the West Tianshan Mountains may be needed. And more detailed GPS observations are required to explore whether the TFF has penetrated into the southern foreland basin of the West Tianshan Mountains.

RELATIONSHIP BETWEEN STRESS INTERACTION AND STRONG EARTHQUAKE ACTIVITY OF JIASHI EARTHQUAKES (M_W≥6.0)SINCE 1998

ZHOU Yun, PAN Zheng-yang, WANG Wei-min, HE Jian-kun, WANG Xun, LI Guo-hui

2021, 43(2):  280-296.  DOI: 10.3969/j.issn.0253-4967.2021.02.002

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Jiashi seismic region of Xinjiang is located at the junction of South Tianshan, Tarim Basin and Pamir. From January 1997 to March 2003, a series of strong earthquakes occurred in Jiashi area, Xinjiang. In particular, since January 21, 1997, 7 earthquakes with M_S>6 have occurred in the area in just 4 months, which is extremely rare in China and in the world. According to the theory of seismic stress triggering, earthquakes are interrelated with each other. When an earthquake occurs, it will regulate the stress state of its surrounding active faults, thus triggering or inhibiting the seismic risk of surrounding potential faults. Also, the viscoelastic relaxation theory believes that the rheological action of the hot lower crust and mantle will transfer the coseismic stress field changes of the lower crust and upper mantle to the upper crust seismogenic layer in a period ranging from years to hundreds of years after the earthquake. This in turn affects the mechanical properties of the fault.
We invert the focal characteristics of the two Jiashi earthquakes in 1998 and 2003, reconstruct the fault rupture model, and calculate the interaction stress between the two earthquakes and the M_W6.0 earthquake in 2020 based on the viscoelastic relaxation theory to study the triggering relationship among the three earthquakes. We use a viscoelastic layered semi-infinite space model(PSGRN/PSCMP program)to estimate the changes of fault stress caused by coseismic and post seismic deformation. At the same time, we study the influence of different model parameters, including viscosity coefficient, receiving fault parameters and earth medium model parameters, on the calculation results.
The results show that: 1)The 1998 earthquake is a typical sinistral strike-slip earthquake, with fault strike of 57°, dip angle of 81° and the focal depth of 11.5km. The 2003 earthquake is a thrust strike-slip type earthquake with focal depth of 15.2km, the seismogenic fault is a northward low dip angle reverse fault and the main rupture direction of the earthquake is SE, the strike angle is 293° and dip angle is 20°. According to the calculation of fault dip angle, the deepest rupture caused by two seismogenic faults is less than 20km; 2)The 1998 earthquake led to the increase of stress in the western section of the 2003 earthquake fault and the unloading of stress in the eastern section, which increased the stress in the vicinity of the epicenter of the 2003 earthquake by 0.01~1MPa, showing an obvious triggering effect. The first two earthquakes resulted in the increase of stress in the eastern section of the 2020 earthquake fault, but the increase of stress at the epicenter was not more than 0.006MPa, which did not have an obvious triggering effect. This earthquake was mainly caused by other factors, possibly including the tectonic loading or viscoelastic triggering of previous big earthquakes; 3)Considering that there are many factors affecting the distribution of Coulomb stress on the fault, we calculated the stress distribution with different parameters, and the results show that the change of parameters has no obvious effect on the results. In the mutual triggering relationship between the earthquakes, the effect of viscoelastic relaxation after earthquake is not obvious, and the effect of coseismic stress step change is dominant. This may be due to the fact that the fractures caused by the two earthquakes are concentrated in the shallow part, and the coseismic stress in the deep part is relatively small, so the viscoelastic relaxation effect is not obvious. Our stress results are in good agreement with the aftershock distribution obtained by previous relocation methods. The results show that after a long-term stress evolution, the stress loading of the eastern segment of the 2003 earthquake fault is still more than 0.01MPa, so the seismic risk monitoring of this segment should be strengthened. Our results have certain significance for understanding the mechanism and regular pattern of earthquake occurrence in Jiashi area.

ANALYSIS ON BACKGROUND AND TRIGGERED SEISMICITY OF JIASHI, XINJIANG, CHINA BASED ON SPATIAL-TEMPORAL ETAS MODEL

ZHANG Sheng-feng, ZHANG Yong-xian, FAN Xiao-yi

2021, 43(2):  297-310.  DOI: 10.3969/j.issn.0253-4967.2021.02.003

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Several earthquakes above M_S6.0 occurred in Jiashi, Xinjiang region in China in history. A new M_S6.5 event occurred in this area on Jan. 19, 2020, for which a ‘virtual scientific investigation’ was carried out by China Earthquake Administration in a short time after the earthquake. In this ‘virtual investigation’, a vital important question is that which fault controls the occurrence process of this event, and what is the correlation between this event with other previous earthquakes. To understand the solutions of these questions well, the seismologists analyzed different types of monitoring data, the source parameters, focal mechanisms, seismic waves, InSAR data, regional tectonics, seismic activity including the aftershock sequence, abnormalities of CH4 and GPS TEC, etc. Some conclusions can be drawn on the features of this earthquake and the potential of large aftershocks based on analysis of these kinds of data.
Statistical seismology tools may provide a significant constraint in the case that some earthquakes cannot be described very well through traditional approaches. Concerning the seismicity in the Jiashi area, whether this earthquake is independent background seismicity, or has a certain triggering relationship to the other previous events, is a main question to be well answered during this research. So, to explore the features of the background events and the triggering ability of the events in this area, we used the spatial-temporal epidemic type aftershock sequence(ETAS)model to fit the seismicity using the earthquake events from Jan. 1, 1970 to Jun. 1, 2020 to obtain the spatial and temporal distribution of total seismicity rate, background seismicity rate and clustering seismicity rate. Then the stochastical declustering method based on ETAS model was used to separate all the events into background events and clustering events. The result shows that the clustering seismicity has a main contribution to the total seismicity in this region. The north and south part of the study area show different features of background and triggering seismicity. The north part shows a more homogenous spatial distribution of background seismicity, while the south part shows a high level of triggering or clustering seismicity. Through the calculation of the ETAS algorithm, this M_S6.5 event has a 99%probability of being a triggered event, in which the main contribution is from an M_S5.3 event that occurred 1 day before this event. On the other hand, we find that among all of the events which have contribution to others, an M_L4.1 event that occurred on Apr. 21, 2020 has the highest ability to ‘disturb’ the other events, the probability reaches 0.505, but this needs to be confirmed by other methods. As generally recognized by seismologists, this M_S6.5 event and other previous large earthquakes are mainly influenced by the western Himalayan syntaxis in this region.
As everyone knows, to find the statistical solutions for these questions based on the stochastic or statistical theory, we need focus on the analysis of large group events, rather than a single one. Some algorithms and methods of statistical seismology seem to provide us an opportunity to analyze the group features of seismicity in the study region. Through the analysis using spatial-temporal ETAS model, we can use the statistical methods to describe the spatial and temporal behavior of the background and triggered events, and obtain some special information which cannot be obtained effectively with other traditional tools in some cases. In addition, the traditional ETAS models have been rapidly expanded and developed in recent years, such as 3D-ETAS model incorporating focal depth information, and the finite element ETAS model incorporating the element of spatial morphology of the seismogenic faults. In this view, we can suggest that statistical seismology approaches may have a chance to supply a significant balance point between the pure scientific research and the earthquake consultation work in the future, especially between the conventional seismic research and operational work in China.

THE APPARENT DENSITY VARIATION OF THE FOCAL AREA BEFORE AND AFTER JIASHI M_S6.4 EARTHQUAKE AND ITS TECTONIC SIGNIFICANCE

LIU Dai-qin, CHEN Shi, WANG Xiao-qiang, ZHANG Bei, LI Jie, WU Chuan-yong, LU Hong-yan

2021, 43(2):  311-328.  DOI: 10.3969/j.issn.0253-4967.2021.02.004

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On Jan. 19, 2020, a magnitude 6.4 thrust earthquake occurred in Jiashi County, Xinjiang Uygur Autonomous Region, and the seismogenic structure is the Keping Fault. The epicenter of the earthquake is located inside the gravity monitoring network, which covers more than 90 gravity monitoring points from Aksu and Kuche to Kashgar and Wucha in the west, Taxkorgan in the south and Hetian area in the southeast. In this paper, the high-precision gravity measurement data in the western margin of the Tarim Basin from 2013 to 2020 before and after the earthquake are used and three absolute gravity measurement points at Kuche, Taxkorgan and Wushi are taken to provide space-time gravity reference constraint. Then, the test board method is used to carry out the field source resolution test, and combined with the theoretical gravity anomaly value of actual measurement points obtained by the gravity forward modeling method, the field source model parameters are obtained by the inversion method. Then, in light of “seeking the source by field and combining the field and source”, using the surface repeated gravity observation data, the point value sequence obtained based on the Bayesian gravity adjustment method, and the field source inversion method for the time-varying gravity signal, the paper evaluates the basic principle of equivalent source inversion method and the field source monitoring capability of the research area. The actual repeated gravity observation data are tested and inversed to obtain spatial and temporal variation characteristics of gravity field sources, as well as the dynamic variation of regional gravity field sources and the structure characteristics of apparent density of multi-period field sources before and after the earthquake in the study area in the last 10 years. Finally, the variation process of gravity field in the seismogenic tectonic area of the 2020 Jiashi magnitude 6.4 earthquake is analyzed and discussed in combination. The study concluded that the gravity survey network on the western margin of the Tarim Basin has a good ability to distinguish field source parameters around the epicenter of the Jiashi M_S6.4 earthquake, but its ability to monitor the interior Tarim Basin between the tectonic system on the west side of Hetian and Aksu is relatively weak. The significant gravity change before the Jiashi M_S6.4 earthquake started in 2017. The apparent density change showed a regional increasing trend as a whole, and the morphology first showed the EW orientation and gradually turned to the NEE orientation, which is consistent with the structural direction of the Keping fault system. The apparent density change trend weakened in 2019. After the earthquake, the apparent density demonstrated a NEE-directed decrease. Before and after the earthquake, the apparent density of the field source increased from positive to negative, and after the earthquake, this apparent density change was more consistent with the tectonic trend and extended to the entire Keping fault system, indicating that the field source change signal obtained from gravity monitoring is closely related to the seismic event and the structure-controlled field source environment change. After the Jiashi M_S6.4 earthquake, the apparent density of the field source decreased, which was consistent with Keping tectonic system. The lower apparent density appeared in the area from the epicenter of the earthquake to Atushi, which may be related to the redistribution of fluid material in the earth’s crust caused by the rapid isostatic adjustment of crustal material near the fault after the earthquake. However, the gravity data observed in April 2020 may still contain the coseismic effect information of the earthquake. The research methods and results of this paper can provide valuable reference for the study of source characteristics of time-varying gravity field and the analysis and interpretation of seismic gravity precursor signals, and also have important indicative significance for understanding the crustal tectonic activity patterns around the seismogenic zone and fault zone.

SEISMICITY FEATURE AND SEISMOGENIC FAULT OF THE M_S6.4 EARTHQUAKE SEQUENCE ON JANUARY 19, 2020 IN JIASHI, XINJIANG

CUI Ren-sheng, ZHAO Cui-ping, ZHOU Lian-qing, CHEN Yang

2021, 43(2):  329-344.  DOI: 10.3969/j.issn.0253-4967.2021.02.005

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The 2020 M_S6.4 Jiashi earthquake occurred on January 19, preceded by an M_S5.7 foreshock on January 18. These two earthquakes occurred close in space and time raising the question of the relationship between the two events. Using the observation data recorded by fixed stations and temporal stations of Xinjiang seismic network, the seismicity feature and the seismogenic fault of the Jiashi M_S6.4 earthquake sequence are studied in this paper. We relocated the Jiashi earthquake sequence from January 18 to August 31, 2020, and obtained the relocations of 1 460 earthquakes by the double-difference algorithm. The high-precision earthquake catalog reveals detailed spatial and temporal evolution of the earthquake sequence. The relocations show that the M_S6.4 earthquake is located at 39.835°N, 77.148°E, and the focal depth is 14.9km. The earthquake sequence is distributed in two dominant directions, one is NNW direction, the other is near EW direction. The length of the NNW earthquake belt is about 20km, and the length of the near EW earthquake belt is about 40km. The dip angle of the seismogenic fault of the NNW earthquake belt is steep, dipping to the west. The dip angle of the seismogenic fault of the near EW earthquake belt is steeper in the west, and gradually becomes more gentle from west to east, dipping to the south slightly. The main shock(M_S6.4) and the foreshocks including the M_S5.7 event occurred along the NNW earthquake belt. A large number of aftershocks occurred along the near EW earthquake belt, and two aftershocks above M5 occurred at the eastern side of the EW earthquake belt. The aftershocks on the south side of the main shock are rare, perhaps affected by the hard blocks of the Tarim Basin. The aftershocks distribution clearly illuminates a near EW-striking structure, likely the extension of the NNW-striking fault activated during the initial sequence. The dominant depth of the earthquake sequence is between 10km and 20km, the focal depth of aftershocks along the near EW direction is gradually shallower from west to east. We determined the focal mechanism solutions of the M_S≥5.0 earthquakes by the CAP method. The results of focal mechanism inversion show that the focal mechanism of the main shock and two aftershocks above M_S5 are mainly thrusting, and the M_S5.7 foreshock is mainly strike-slip. We also determined the moment tensor solution of the main shock using ISOLA method as a single-source. The focal mechanism solutions of the main shock obtained by the two methods are consistent. The moment tensor solution of the main shock has a large non-double couple component, which proves that the rupture process is very complex. By inversion of the main shock using ISOLA method as a multi-source, the main shock, which was reported as a single event, is instead composed of two sub-events, a strike-slip rupture and the second thrust rupture. Within 4s, a strike-slip earthquake triggered a second large rupture on a thrust fault. The first rupture is consistent with the mechanism of the M_S5.7 foreshock, and the second rupture is consistent with thrust-faulting mechanisms in the ensuing aftershock sequence. By analyzing the data of spatial distribution and focal mechanism of the earthquake sequence, it is speculated that the Jiashi M_S6.4 earthquake occurred in the middle and lower crust below the detachment layer of the Kalpin thrust tectonic zone. The occurrence of the main shock is caused by the joint action of the two faults, the NNW-striking fault with a high dip angle and the near EW-striking fault dipping south. The M_S6.4 rupture initiated the adjacent previous NNW-striking rupture of the M_S5.7 event, extending the earlier rupture both to the NNW and EW directions. The M_S6.4 earthquake is the result of the interaction between the two blocks, the south Tianshan Mountains and the Tarim Block.

RELOCATION AND FOCAL MECHANISM FOR THE XINJIANG JIASHI EARTHQUAKE ON 19 JANUARY, 2020

GUO Zhi, GAO Xing, LU Zhen

2021, 43(2):  345-356.  DOI: 10.3969/j.issn.0253-4967.2021.02.006

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As the most active and spectacular intracontinental mountain ranges in Central Asia, Tian Shan is a natural laboratory to explore and understand the geodynamic processes involved in intracontinental mountain building. The origin of Tian Shan can be traced back to the collision and accretion of several micro-continents, island arcs, and accretionary prisms initiated in the Paleozoic. This tectonic activity continued into the Mesozoic. From the Cretaceous to the early Tertiary, the mountain ranges were eroded and reduced to a flat plain. Then, in the later Tertiary, uplift was rejuvenated as a far-field consequence of the India-Eurasia collision, and continues to present day, characterized by the active seismicity in Tienshan in modern times. The geology of present-day central Tian Shan is mainly composed of intermontane basins and subparallel ranges, separated by the east-west striking Cenozoic active thrust faults stretching approximately 2 500km in length. On the southern and northern margins of the Tian Shan Range, the Tarim Basin and the Kazakh Shield act as stable blocks. Situated in the southwest foreland of Tien Shan, the Kashgar region is a seismic active area since the 20^th century, several strong earthquakes stroke the region and the surround areas, causing severe damage to the local residents. In this study, we apply the double-difference relocation technique and W-phase method to relocate the 19 January, 2020 Xinjiang Jiashi earthquake, and to determine the focal mechanisms using data provided by China Earthquake Networks Center. The relocated epicenters of the 2020 Jiashi earthquake sequence show two dominant spatial distribution directions. The major NWW-trending spatial distribution shows a Kepingtage Fault-paralleled narrow belt stretching about 34km, with most of aftershocks distributed in the northern side of the fault. The secondary spatial distribution shows a NNW-striking belt stretching about 8km. The depth profiles show a predominant epicentral depth at the range of 10~20km. The focal parameters for the 19 January, 2020 Xinjiang Jiashi M6.4 earthquake are: strike 76°, dip 81°, rake 109° for the nodal plane Ⅰ, and strike 190°, dip 21°, rake 26° for nodal plane Ⅱ, and the moment magnitude is M_W5.87. The focal parameters indicate that the earthquake event is characterized by dominant thrust with minor strike movement. Combined with the analysis of the relocated epicentral locations, focal mechanisms and geological settings, it is inferred that the seismogenic fault of the 19 January 2020 Jiashi M6.4 earthquake is the west segment of the near E-W trending Kepingtage thrust fault.

PRELIMINARY STUDY FOR SEISMOGENIC STRUCTURE OF THE M_S6.4 JIASHI EARTHQUAKE ON JANUARY 19, 2020

LI Jin, JIANG Hai-kun, WEI Yun-yun, SUN Zhao-jie

2021, 43(2):  357-376.  DOI: 10.3969/j.issn.0253-4967.2021.02.007

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On January 19, 2020, an M_S6.4 earthquake occurred in Jiashi county. This earthquake located in the intersection of the three tectonic systems of South Tianshan, Tarim Basin and West Kunlun-Pamir. From 1997~2003 a group of strong earthquake swarms with M_S≥6.0 occurred in this area, which constitute an extremely rare Jiashi strong earthquake swarm in mainland China. Based on the digital waveforms of Xinjiang Seismic Network, the best double-couple focal mechanisms of the main shock, foreshock and some aftershocks with M_S≥3.6 were determined by CAP method, the Jiashi M_S6.4 earthquake sequence was relocated by multi-step locating method. We analyzed the characteristics of focal depth, focal mechanisms and source rupture to determine the seismogenic structure. The nodal plane parameters of the best double-couple focal mechanism by CAP method are: strike 190°, dip 32° and rake 31° for nodal plane Ⅰ, and strike 74°, dip 73° and rake 118° for nodal plane Ⅱ; the centroid depth is 12.1km. The focal mechanism of main shock is thrust type, but the M_S5.4 foreshock is a strike-slip event with a focal depth of 17.1km, and the focal mechanism parameters are: strike 83°, dip 78°, rake 173° for nodal plane I and strike 174°, dip 83°, rake 12° for nodal plane Ⅱ. The foreshock and mainshock are very close in space, but the rupture types are quite different, which shows the complexity of the seismogenic structure. The relocated sequence shows two dominant distribution directions, namely, the near EW direction and the near SN direction. Most of the aftershocks in the sequence are distributed in the EW direction, parallel to the strike of the Kepingtage nappe structure. The M_S5.4 foreshock and the M_S6.4 mainshock are both located near the dominant distribution in the near NS direction, and have a certain spatial distance from the distribution of aftershocks in the near EW direction. This feature may reflect that the mainshock and subsequent aftershocks are located on different fault zones. Combined with the geological structural background near the source area, it is inferred that the seismogenic structure of the M_S5.4 foreshock is a strike-slip fault L₀ with a high dip angle in NNW direction, and the basic information of the seismogenic fault L₀ may be: strike NNW(about 175°), the fault plane is nearly upright, and the fault depth can reach about 15km. L₀ may be a branch fault of the NNW-directed seismogenic structural system of the Jiashi earthquake swarm from 1997 to 1998. Since most of the aftershocks distributed on the east side of the Fault L₀, we judge that L₀ and related faults may have a certain control effect on the distribution of aftershocks. According to the location of the main shock, the spatial distribution of aftershocks and the occurrence characteristics of the fault in the source area, it is inferred that the seismogenic structure of the Jiashi M_S6.4 mainshock is a NS-directed gentle-dipping fracture. The main shock caused the simultaneous activity of the Kepingtage nappe structure, resulting in a dense distribution of aftershocks with a certain distance from itself.

THE DEFORMATION OF 2020 M_W6.0 KALPINTAGE EARTHQUAKE AND ITS IMPLICATION FOR THE REGIONAL RISK ESTIMATES

ZHANG Ying-feng, SHAN Xin-jian, ZHANG Guo-hong, LI Cheng-long, WEN Shao-yan, XIE Quan-cai

2021, 43(2):  377-393.  DOI: 10.3969/j.issn.0253-4967.2021.02.008

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The continuous collision between Tian Shan and Tarim Basin causes not only the uplift of mountains, but also the earthquakes across the entire Tian Shan, particularly in the transient zone from mountains to the adjacent basins, where the critical infrastructures and residents are seriously under threat from these earthquake hazards. On 19^th January, 2020, an earthquake occurred in the Kalpintage fold thrust belt in the southwest Tian Shan foreland. We call this event the 2020 M_W6.0 Kalpintage earthquake, which is the first moderate earthquake captured by modern geodetic measurement techniques. This event therefore provides a rare opportunity to look into the local tectonics and seismic risk in southwest Tian Shan. In this study, we obtained the coseismic deformation of 2020 M_W6.0 Kalpintage earthquake from Sentinel-1A SAR and strong motion data, and then inverted its kinematic slip model. We derived the InSAR interferograms from both ascending and descending tracks. Both of them present similar deformation patterns, two deformation peaks over the Kalpintage anticline. That means: 1)The surface deformation is dominated by vertical displacement, and 2)the coseismic rupture plane is highly suspected to be the shallowly dipping decollement at the base of the sediment cover. We got the 3-D displacements of 6 strong motion stations by double integrating the strong motion acceleration signals. The result shows tiny displacement on the strong motion stations, except for the Xikeer station, which locates at the front of the Kalpintage anticline, where the InSAR interferograms are seriously incoherent. Two slip models can equally fit to the ascending and descending InSAR interferograms: One is a strike slip model with strike of N-S, the other is a thrust model with strike of E-W. This ambiguity in the slip models for the M_W6.0 Kalpintage earthquake is caused by 1)the extremely small dip angles of the causative fault, 2)the inherent shortcomings of the InSAR measurements i.e. the 1-D measurements along the line of sight, the polar orbiting direction of the SAR satellite, and 3)the serious atmospheric delay due to contrasting topography in southwest Tian Shan. We did not distinguish the two ambiguous models with InSAR data due to the weak constraints of InSAR for this event. However, the two quite different slip models show the same spatial dimension and position beneath the Kalpintage anticline, also the same seismic slip vector moving toward the Tarim Basin. We then presumed the two slip models refer to the same fault plane, the weak decollement at the base of the sediment cover, and its rupture released the compressive strain in this fold and thrust belt in the southwest Tian Shan front. The confusing problem is neither the strike slip model nor the thrust model can explain the displacement derived from strong motion. The simple error estimates show small uncertainty in the strong-motion-derived displacement, but we cannot really know the real errors without the comparison to the collocated continuous GNSS observation. Because of the discrepancy between the strong motion displacement and InSAR-derived slip model, we speculate the inelastic deformation occurred in front of the Kalpintage anticline where thick weak sediments exist. We think this earthquake ruptured the decollements in the lower sediments bounded by the adjacent anticlines, which are uplifted in this event. The M_W6.0 Kalpintage earthquake balanced the stress accommodated during the convergence of the Tian Shan and Tarim Basin. We managed to explain all of the ruptures in the southwest Tian Shan by combining the regional tectonic, geophysical data and the available earthquake catalogues with good quality and then estimated the earthquake hazards. The earthquakes, including 1902 M_W7.7 karshigar, 1996 M_W6.3 Jiashi, 1997—2003 Jiashi sequence and 2020 M_W6.0 Kalpintage earthquake, can be explained in one frame, the underthrusting of the Tarim Basin toward the southwest Tian Shan. Our calculation suggests that a M_W7.0+ event could be generated around Kalpintage anticline belt if without barriers on the decollements.

A TYPICAL THRUST RUPTURE EVENT OCCURRING IN THE FORELAND BASIN OF THE SOUTHERN TIANSHAN: THE 2020 XINJIANG JIASHI M_S6.4 EARTHQUAKE

ZHANG Wen-ting, JI Ling-yun, ZHU Liang-yu, JIANG Feng-yun, XU Xiao-xue

2021, 43(2):  394-409.  DOI: 10.3969/j.issn.0253-4967.2021.02.009

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A M_S6.4 earthquake occurred on January 19^th, 2020 at Jiashi, Xinjiang, this earthquake is another strong earthquake since the Jiashi M_S6~7 earthquake swarm events from 1997 to 2003, and the epicenter was located near the Kalpin nappe in the western part of southern Tianshan. The Kaplin nappe is located in front of southern Tianshan Mountains, which is a thin skinned thrust belt composed of a series of nearly NE-SW thrust nappes under the strong and sustained regenerative orogeny in the Tianshan area. There are some differences in focal positions and fault parameters given by different institutions, therefore in this paper, high resolution InSAR coseismic deformation fields were obtained based on the ascending and descending tracks of Sentinel-1 SAR images to obtain the focal mechanism. The 30m resolution SRTM DEM data is chosen as the external DEM to eliminate the phases caused by topography, the robust Goldstein filtering is applied for phase smoothing, and the Delaunay minimum cost flow method is used for phase unwrapping. The variation range of interference fringes shows that the east-west span of the earthquake deformation field is about 40km, and that of the north-south direction is about 20km, the displacement results show that the maximum uplift displacement is 5.9cm and the maximum subsidence is 3.7cm along the LOS direction of the ascending data, the maximum uplift displacement is 6.4cm and the maximum subsidence is 2cm along the LOS direction of the descending data. And then the InSAR-derived deformation fields are used to obtain the seismogenic mechanism of this earthquake, and to improve the computational efficiency, the quadtree segmentation method is used to desample the original high-resolution InSAR observations before inversion. The coseismic slip distribution of the causative fault was inversed using a uniform sliding inversion method based on a Bayesian approach, and then the fine slip distribution of the fault plane of Jiashi earthquake was inversed using the distributed slip inversion method based on the constrained least squares. It should be noted that the fault plane is set as the shovel shape according to the geometric relationship between the seismogenic fault parameters inverted by uniform sliding and the exposed position of the Kapling Fault on the surface during the distributed slip inversion. According to the difference between the observed and simulated values, it can be seen that the residual error of the inversion model is small, indicating the reliability of the inversion result. The final result shows that the epicenter is located at 39.9°N, 77.28°E and the strike and dip angle of the seismogenic fault is 276° and 10.7°, respectively, the maximum dip slip and strike slip of fault plane is about 0.29m and 0.03m, respectively, which are located at the depth of about 5km underground. The cumulative coseismic moment is 1.73×10¹⁸N·m from InSAR inversion, which is equal to the moment magnitude of M_W6.1 and the Kalpin Fault is supposed to be the causative fault. Then, regional GPS-derived surface strain rate, tectonic dynamic background, and regional deep and shallow structures were comprehensively analyzed. The results show that the Jiashi M_S6.4 earthquake is a typical thrust event that occurred in the thrust nappe of the southern Tianshan. The 2020 Jiashi event and the 1997—2003 Jiashi M6~7 earthquakes swarm are the results of rupture of many faults with different scales and properties. And these events are all controlled by the thrust nappe of southern Tianshan.

CHARACTERISTICS OF GEOLOGICAL HAZARDS IN THE EPICENTER OF THE JIASHI M_W6.0 EARTHQUAKE ON JANUARY 19, 2020

YAO Yuan, LI Tao, LIU Qi, DI Ning

2021, 43(2):  410-429.  DOI: 10.3969/j.issn.0253-4967.2021.02.010

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On January 19, 2020, an M_W6.0 earthquake occurred in Xikeer town, Xinjiang, northwest China. This earthquake was another strong earthquake event that occurred on the Kepingtage fold-and-thrust belt(FTB)after the 2003 Bachu-Jiashi M_W6.3 earthquake. The Kepingtage FTB is bounded by the southern Tian Shan area to the north and the Tarim Basin to the south. The Kepingtage FTB is ~300km long from east to west and 60~140km wide from north to south. It is composed of a series of monoclinal or anticlinal mountains(fold-and-thrust)with a near east-west direction and parallel distribution. Combined with the focal mechanism, seismic reflection profiles, and interferometric synthetic aperture radar coseismic deformation, we can reveal the seismogenic structure of this earthquake. The Jiashi event was mainly of a horizontal compression movement; the slip distribution was concentrated at a depth of 4~6km, and the fault-slip angle was~15°. Our results show that the seismogenic structure of the Jiashi event is the Keping thrust fault at the leading edge of the Kepingtage FTB. We carried out detailed field surveys, measurements, and drone aerial photography of the earthquake area after the earthquake. In the magistoseismic area(Ⅷ degrees), a lot of seismic geological disasters were found, including ground fissures, sand liquefaction, and collapse. This paper summarizes and describes the characteristics of geological hazards from four observation points. In observation point 1, a large area of ground fissures were developed in the area of Xikeer overpass. According to the statistics of ground fissures of this area, the dominant direction of the ground fissures is NEE, the south-north extrusion uplift is 0.1~0.15m, and the horizontal displacement is 0.05~0.1m. In observation point 2, the earthquake caused serious damage to the Xikeer dam, creating the tensile fissures at the dam crest, with a maximum depth of 4m and a maximum length of 900m. In observation point 3, a series of ground fissures were observed parallel to the road in the west of Xikeer town, and the length of ground fissures is~500m. A large area of sand liquefaction developed along the ground fissures, and the liquefied material was gray brown argillaceous silty. In observation point 4, a series of large and huge fresh rock collapses developed in the Shankou gully north of the epicenter. The largest single collapse is 50~100m³, and the largest collapse range is about 200~300m². According to the field investigation and dynamic calculation results, the maximum horizontal deformation is 29.8cm, located downstream of the dam crest. The horizontal deformation upstream of the dam crest is 22.35cm. Because of the sand liquefaction that occurred behind the dam, local settling of the foundation behind the dam also occurred. The horizontal deformation upstream and downstream of the dam crest are inconsistent, which produced the longitudinal fissures on the dam crest. We collected a large amount of strong-motion earthquakes data from the 2020 Jiashi earthquake. By combining the fault strike and upper and lower wall effects, the PGAs of the foreshock, main shock, and aftershocks were fitted, and isoseismal lines were generated. The Xikeer dam is located at the region where the vibration intensity of the Jiashi event was the highest. The effects of the aftershocks were also superimposed mainly in this area. Notably, sand liquefaction and most of the fissures were caused by the main shock, while the aftershocks(M_S>4.0)exacerbated this damage. However, in this study, we could only determine the extent of the damage caused by the main shock, because our detailed field investigation and drilling were conducted in April 2020, after the main shock and aftershocks.

SEISMIC GROUND MOTION SIMULATION CONSIDERING REGIONAL CHARACTERISTICS: A CASE STUDY OF THE JIASHI M_S6.4 EARTHQUAKE IN 2020

WANG Hong-wei, WEN Rui-zhi, REN Ye-fei

2021, 43(2):  430-446.  DOI: 10.3969/j.issn.0253-4967.2021.02.011

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The regional characteristics of seismic ground motion are of importance for its reliable prediction, and the resulting seismic hazard mitigation and seismic risk controllability. The Jiashi region in Northwest China, which is located in the boundary between the western segment of the South Tianshan and the northwest Tarim Basin, is subject to high seismic hazard due to the active seismicity. This region has frequently suffered from the moderate to strong earthquakes. For the purpose of investigating the regional characteristics of the ground motion in Jiashi region, we separated simultaneously the earthquake source spectra, propagation path attenuation, and site responses from the horizontal S-wave spectra in the 366 three-component strong-motion recordings with high quality after manual detection and selection based on the nonparametric spectral inversion technique. The datasets were recorded at 25 strong-motion stations in 46 earthquakes of M3.0~6.4 from July 2007 to February 2020 occurring in the Jiashi region, mainly including both Jiashi seismic sequences in 2018 and 2020.
We thus developed the region-specific empirical models for the Jiashi region, including the path attenuation, the path duration, and the linear site response models. At the same time, the source parameters theoretically depicting the omega-square source spectral model, including seismic moment M_W, corner frequency f_c, and stress drop Δσ were retrieved from the inverted source spectra for the 46 earthquakes considered using the grid-searching method. Their reliability was further verified from the normalized fitting residuals between the inverted and theoretical source spectra varied with f_c, and the f_c-M_W plots. We determined that M_W=5.893, f_c=0.362Hz, and Δσ=6.684MPa for the M_S6.4 Jiashi earthquake in 2020. According to the inverted path attenuation curves against hypocentral distance less than 120km, the ground motions generally show the slower path attenuation at local distances, while the faster path attenuation at regional distances was clearly found, which could be ascribed to the significant anelastic attenuation in the Jiashi region. The path attenuation in the Jiashi region was approximately modeled by the empirical expression, simultaneously including the hinged bilinear geometrical spreading model and the anelastic attenuation term in the function of quality factor Q. The yield Q values were regressed as 60.066f^0.988 at frequencies of 0.254~30Hz. The strong anelastic attenuation may be attributed to the prominent interaction of the seismic wave propagating within the high inhomogeneous crust. The transition distance R₁=60km, and the geometrical spreading exponents n₁=0.30 and n₂=0.59 before and after R₁ well defined the preferred hinged bilinear geometrical model. The site responses show the dependence on the V_S30 values, and the higher amplifications are generally predominant at sites with the lower V_S30 values, and vice versa. Compared with the linear site amplification model proposed by Syehan and Stewart(2014) for the NGA-West2 project, the local sites in the Jiashi region show the very weak amplification effects on the high-frequency ground motions at 10~30Hz, while the good consistencies at low-to-intermediate frequency bands occur. On the basis of the SS14 model, the modified model applicable for the Jiashi region was proposed in this study. The path durations, defined by an effective 95%~5%significant duration, were calculated for the strong-motion recordings(a total of 502)in the Jiashi region, and were applied for the development of the empirical model. The prolonged path durations were observed in those ground motions recorded in the Jiashi region, significantly higher than those observed in the Sichuan region.
Furthermore, the yield regional characteristics were preliminary examined by the ground motion simulations. The stochastic finite-fault method was further applied to reproduce ground motions in the M_S6.4 Jiashi earthquake with these regional characteristics under consideration. The kinematic source models were first generated stochastically by the filtering method in the two-dimensional wavenumber domain. These stochastic source models can well represent the source rupture process in physical and its sufficient variability. We then verified the reliability of these regional characteristics from the good agreements of the ground motion intensity measures between the observations and the simulations, including the peak ground accelerations and velocities, and the spectra accelerations. Moreover, the key role of the regional characteristics in ground motion simulation was clearly showed by the significant deviations of the long-period ground motions at large distances between the observations and the simulations based on the path attenuation models in the Sichuan-Yunnan region. Finally, the ground motion maps were developed for the M_S6.4 Jiashi earthquake according to the regional characteristics and a best-performing source kinematic rupture model.
The region-specific ground motion empirical models are very helpful for reliably predicting ground motions, and mitigating seismic hazards in the Jiashi region with the active seismicity.

CHANGE IN BEDROCK TEMPERATURE BEFORE AND AFTER JIASHI M_S6.4 EARTHQUAKE IN XINJIANG ON JANUARY 19, 2020

CHEN Shun-yun, SONG Chun-yan, YAN Wei, LIU Qiong-ying, LIU Pei-xun, ZHUO Yan-qun, ZHANG Zhi-he

2021, 43(2):  447-458.  DOI: 10.3969/j.issn.0253-4967.2021.02.012

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Recent studies have confirmed that the bedrock temperature changes when the crustal stress changes, and the information of dynamic change in crustal stress can be obtained through the observation of bedrock temperature. Moreover, there are abundant fluids in the shallow crust, and the deformation of the crust will inevitably lead to the migration of fluids, which will change the bedrock temperature. The temperature change of bedrock is equivalent to the secondary fluid thermal effect caused by crustal stress change and may be an another indirect sensitive index of crustal stress dynamic change. The bedrock temperature data of Xianshuihe fault zones show that the variation of groundwater flow rate after the Kangding M_S6.4 earthquake is consistent with the zoning characteristics of co-seismic volumetric strain in the strike-slip earthquake, indicating that the variation of near-field fluid migration characteristics is probably related to the variation of co-seismic static stress change. Moreover, the response form of bedrock temperature to the dynamic change of crustal stress and its secondary fluid effect is not consistent, as the former shows step-rise characteristics, while the latter shows exponential variation. The observation of bedrock temperature itself can obtain the dynamic change information of crustal stress and the information of shallow crustal fluid migration. Compared with crustal stress change, the variation range of fluid secondary heat effect caused by stress change may be significantly magnified(approximately an order of magnitude), which is more conducive to capturing signals, and thus may even obtain precursory fluid change information.
On January 19, 2020, an M_S6.4 earthquake occurred in Jiashi, which happened in the bedrock temperature observation network. In particular, the Xike’er observation station is about 1.3 kilometers away from the epicenter, providing an opportunity to analyze bedrock temperature changes before and after the earthquake. The results showed that: 1)Obvious changes in bedrock temperature were found before and during the M_S6.4 earthquake. The appearance of co-seismic response means that these changes before the earthquake are related to the earthquake and may be precursory signals. 2)In terms of time, the bedrock temperature before the Jiashi earthquake first changed abnormally on the stable background, and the change reached the peak, and then fell back. When the earthquake was impending, there was a significant acceleration of the change, and the earthquake occurred after some time. The acceleration characteristics of change impending earthquake may be related to the meta-instability process of earthquakes. 3)Spatially, changes in temperature before the earthquake occurred in or near the seismogenic fault, and no obvious abnormal information was observed at the measurement points far away from the seismogenic fault, indicating that short-term and impending precursors are more likely the “near field” information; From the perspective of depth, the change in temperature before the earthquake was observed only at the local depth range. This implies that there is obvious uncertainty in the depth in precursor observation. Upon this, the ideal situation should be to carry out multi-depth joint observation, so as not to miss important precursor information. 4)Combining with the Kangding M_S6.3 earthquake on November 22, 2014, a comparative analysis is made. Similar to the Jiashi earthquake, the temperature at measurement points located in or nearby seismogenic fault of Kangding M_S6.3 earthquake shows significant changes. This means that change in the temperature before the Jiashi M_S6.4 earthquake is not an isolated case, and is a representative of universal phenomenon that occurs before strong earthquake. In a word, the change of bedrock temperature before and after the earthquake shows that the precursor information has the characteristics of near field, structural correlation and sensitive to stress change.

Application of new technique

A HIGH-PRECISION TEMPERATURE MEASUREMENT SYSTEM BASED ON BRIDGE-TYPE CONSTANT CURRENT SOURCE AND ITS SIGNIFICANCE FOR DETECTING DYNAMIC CHANGE IN CRUSTAL STRESS THROUGH BEDROCK TEMPERATURE

ZHANG Zhi-he, CHEN Shun-yun, LIU Pei-xun, LIU Qiong-ying

2021, 43(2):  459-469.  DOI: 10.3969/j.issn.0253-4967.2021.02.013

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The theoretical and in situ investigations in recent years indicated that the dynamic change information of crustal stress can be obtained through the observation of bedrock temperature. In particular, the magnitude and spatial distribution characteristics of the co-seismic stress variation obtained based on the co-seismic temperature response of the Kangding M_S6.3 earthquake were consistent with the results obtained by seismology, which confirms the validity of the field analysis of the co-seismic stress variation by temperature observation. In the future, with the further improvement of temperature measurement technology, this method for detecting dynamic change in crustal stress through bedrock temperature is expected to bring new opportunities for earthquake science.
However, the stress changes caused by earthquakes are related to the distance from the measurement point to the source and decay rapidly with the increase of the distance, and they are also related to magnitude and decrease exponentially with magnitude. For example, the co-seismic temperature response observed in the Kangding M_S6.3 earthquake did not appear in the subsequent M_S5.8 earthquake. One key reason is that the detection of co-seismic stress changes in M_S5.8 earthquake requires a precision of temperature measurement system to be up to 0.01mK. But, the precision of the instruments used at that time was about 0.2mK, which could not detect temperature changes in the order of magnitude of 0.01mK. So, in order to make the above-mentioned method play a greater role in earthquake science, especially in detecting the stress variation information before a strong earthquake, it is urgent to develop a more accurate temperature measurement system.
In this paper, a new version of high-precision temperature measurement system is developed successfully, after considering a series of technical improvements such as constant current commutation driving and multiple Kalman digital filtering, based on the low temperature drift fixed value resistor and the temperature measuring resistor with a balanced bridge of four-wire temperature sensors. The new system has a designed temperature resolution of 0.003mK. According to in situ tests, the precision of temperature measurement reaches 0.03mK. In addition, field observations have confirmed the feasibility of the temperature measurement system. Based on the relation between stress change and temperature response, a dynamic stress change of the magnitude of 0.03MPa can be obtained, which implies that the observable range of thermal stress can be greatly improved with comparison to the previous version of 0.2MPa. It should be emphasized for earthquake research that co-seismic Coulomb stress is an importance parameter, whose magnitude focuses on a range of about 0.01~0.1MPa. Up to now, co-seismic Coulomb stress change cannot be measured directly(at least partially)because of limitation of detecting method. Since the ability of measuring dynamic stress by our system developed in this paper has reached to 0.03MPa, reaching the magnitude of the co-seismic Coulomb stress change, the potential application of the system is to explore the change of the co-seismic Coulomb stress. This is of great benefit to promote the development of ways of observing the dynamic changes in crustal stress through bedrock temperature.