SEISMICITY FEATURE AND SEISMOGENIC FAULT OF THE MS6.4 EARTHQUAKE SEQUENCE ON JANUARY 19, 2020 IN JIASHI, XINJIANG

doi:10.3969/j.issn.0253-4967.2021.02.005

Abstract

Abstract: 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.

Key words: Jiashi M_S6.4 earthquake, earthquake relocation, focal mechanism, seismogenic fault

摘要： 文中利用新疆地震台网固定台站和流动台站的观测资料研究了2020年1月19日新疆伽师M_S6.4地震序列的活动特征和发震构造。重定位结果显示, 地震序列呈NNW和近EW 2个优势方向分布, 其中NNW向余震条带长约20km, 近EW向余震条带长约40km。 NNW向条带的发震断层倾角较陡, 倾向W; 近EW向条带的发震断层西侧倾角较陡, 向E逐渐变缓, 略S倾。 M_S6.4主震深度为14.9km, 地震序列优势分布于10~20km深度, 近EW向的余震深度呈现由西向东逐渐变浅的特征。震源机制解结果表明, M_S6.4主震和2次5级以上余震的震源机制以逆冲为主, 2020年1月18日M_S5.7前震以走滑为主。利用ISOLA方法对M_S6.4主震进行多源模型反演, 结果显示主震由走滑和逆冲性质的2个子事件组成。推测M_S6.4地震发生在柯坪逆冲推覆构造带滑脱面以下的中下地壳, 主震由NNW向W倾的高角度隐伏断裂和近EW向S倾构造先后破裂组成。

关键词: 伽师M_S6.4地震, 重定位, 震源机制解, 发震构造

CLC Number:

P315.2

CUI Ren-sheng, ZHAO Cui-ping, ZHOU Lian-qing, CHEN Yang. SEISMICITY FEATURE AND SEISMOGENIC FAULT OF THE M_S6.4 EARTHQUAKE SEQUENCE ON JANUARY 19, 2020 IN JIASHI, XINJIANG[J]. SEISMOLOGY AND GEOLOGY, 2021, 43(2): 329-344.

崔仁胜, 赵翠萍, 周连庆, 陈阳. 2020年1月19日新疆伽师M_S6.4地震序列的活动特征和发震构造[J]. 地震地质, 2021, 43(2): 329-344.

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