2020年新疆伽师M_(S)5.4、M_(S)6.4两次中强地震发震构造探讨

Seismogenic structure of the 2020 Jiashi,Xinjiang M_(S)5.4 and M_(S)6.4 moderate earthquakes

2020年1月,新疆伽师连续发生M_(S)5.4、M_(S)6.4两次中强地震,震中分别位于天山南麓和塔里木盆地交界的推覆构造前缘。基于地震精定位和震源机制解揭示褶皱和逆冲带的深部几何结构对于理解这两次中强震发生机理具有重要的作用。本文利用18个区域固定地震台站资料,对2009年1月1日至2021年7月31日期间的地震展开重定位研究,并对伽师M_(S)5.4前震、M_(S)6.4主震以及7次4级以上余震开展震源机制求解,进一步反演得到震源区构造应力场。地震定位结果显示,整个地震序列呈NNW和EW2个优势方向分布,前震序列和余震序列在时空分布上存在明显差异,前震序列主要沿着NNW向展布,而余震主要在近EW向的奥兹格尔他乌断裂上展布,并表现出双层分布特征;震源机制反演结果表明,2020年1月18日伽师M_(S)5.4前震为一次走滑型地震事件,而伽师M_(S)S6.4主震和7次余震均为逆冲型事件;另外,应力场反演显示主震震源区为近NS向挤压特征,与该区域地表应力状态基本一致。结合上述结果以及周边地质构造背景,我们认为M_(S)5.4前震和M_(S)6.4主震的发震构造不同:1月18日M_(S)5.4前震发震构造为NNW向具有走滑性质略向W倾的隐伏断层,为“类转换断层”;而M_(S)6.4主震的发震构造为柯坪塔格推覆体中的EW向隐伏推覆盲逆断层,且触发了不同深度的发震断层面。M_(S)5.4前震和M_(S)6.4主震发震断层面走向和倾角的差异,反映了天山地区近NS向不均匀的构造挤压作用。。

In January 2020,two moderate earthquakes with magnitudes of M_(S)5.4 and M_(S)6.4 occurred in the nappe structure of the boundary between the Southern Tianshan Mountains and Tarim Basin.To investigate the seismogenic structure of these two events,we systematically analyzed the earthquake sequence locations,focal mechanisms,and stress field in the source region.Using the P and S arrival-time data from January 1,2009,to July 31,2021,recorded by 18 seismic stations of the Xinjiang network,we obtained precise seismic event locations.Results show that the temporal and spatial distribution of the foreshock and aftershock sequences displays obvious differences in migration behaviors.The former is mainly distributed along the NNW direction,whereas the latter is distributed along the Ozgertau fault in the EW direction and characterized by a double-layer feature.Furthermore,we derived the focal mechanism solutions of the M_(S)≥4.0 events,including the M_(S)5.4 foreshock and the M_(S)6.4 main shock.The inversion results illustrate that the M_(S)5.4 foreshock is a strike-slip event,whereas the main shock and seven aftershocks are thrust events.Based on our results and the regional geological background,we conclude that the seismogenic structures of the M_(S)5.4 foreshock and M_(S)6.4 main shock are related to different faults.The M_(S)5.4 event is located in a blind“quasi-transformation”fault with the NNW direction,and the M_(S)6.4 earthquake occurs on a blind thrust nappe fault in the EW direction on the Kepingtage(Kalpintag)nappe.Our results indicate that the Tienshan orogenic belt has a complex tectonic environment.The difference in the strikes and dips of the M_(S)5.4 foreshock and M_(S)6.4 main shock reflects the stress compression in the near-NS direction in the source region.