2022年6月1日芦山M_(S)6.1主震震源机制及余震研究

Study of focal mechanism and aftershocks of the M_(S)6.1 earthquake in Lushan on June 1th,2022

据中国地震台网中心测定,北京时间2022年6月1日17时00分08秒四川省芦山县发生了M_(S)6.1级地震,3 min后又发生了一次M_(S)4.5级余震(为此次地震序列的最大余震).从中国地震台网中心发布的震中位置来看,这次主震与2013年M_(S)7.0级芦山地震的震中位置非常接近(约10 km),目前关于2013年芦山地震的发震断层仍有争议,对此次主震震源机制解及余震序列的研究可以为该区域深部孕震环境提供更多约束.本研究中,首先基于四川地震台网记录的事件波形数据,采用ISOLA软件进行波形拟合,获得2022年芦山主震的双力偶震源机制解为:222°/43°/104°(节面Ⅰ:走向/倾角/滑动角)及23°/48°/76°(节面Ⅱ),矩心深度16 km,矩震级MW5.9;其最大余震的震源机制解为237°/36°/109°(节面Ⅰ)、34°/56°/76°(节面Ⅱ),矩心深度17 km,矩震级MW5.0;两次地震均以逆冲为主.其次,基于震中距150 km范围内宽频带台站在2022-06-01—2022-06-10时间段记录的连续波形数据,采用LOC-FLOW流程进行了余震识别和重定位研究,共计获得838个余震事件,在平面上大部分展布于主震的西北侧,在深度剖面上则主要分布于主震的上倾方向且指示断层面倾向为SE.另一方面,根据地震波传播理论,台站记录波形可以由震源时间函数、传播路径效应及仪器响应的卷积表示,假设两次地震的震中位置相距很近,由同一台站记录的两个事件波形应该具备相同的仪器响应和近似的传播路径效应,所以记录的波形差异反映了震源破裂过程的差异;本文对比分析了31个不同方位台站分别记录的2013年和2022年芦山主震的事件波形,发现两者在10~20 s周期范围内具备很好的相似性,说明两次地震破裂过程有一定相似性.该区域位于龙门山断裂带南段,地表主要以NE-SW走向、NW倾向的断裂为主,而根据本文震源机制和余震定位的结果,我们推测2022年芦山地震应该发生在一条倾向SE的隐伏逆断层上,与该区域长期以来遭受青藏高原侧向挤压的NW-SE向构造应力场具备一致性.

According to the measurement of the China Earthquake Networks Center,at 17:00:08 on June 1,2022,Beijing time,a M_(S)6.1 earthquake occurred in Lushan County,Sichuan Province.Three minutes later,another M_(S)4.5 aftershock(the largest aftershock of this earthquake sequence)occurred.From the epicenter announced by the China Earthquake Networks Center,the epicenter of M_(S)6.1 mainshock is very close to that of the 2013 Lushan M_(S)7.0 earthquake(about 10 km),and the seismogenic fault of the former event is still controversial nowadays.The focal mechanisms of the 2022 mainshock and its largest aftershock may shed more light on this problem.In this paper,firstly based on the waveform data recorded by the broadband seismic stations deployed by the Sichuan Seismic Network,we use ISOLA software for waveform fitting to obtain the double couple focal mechanism of two earthquakes.The result of 2022 mainshock as follows:222°/43°/104°(nodal planeⅠ:strike/dip/slip)and 23°/48°/76°(nodal planeⅡ),centroid depth 16 km,and moment magnitude MW5.9.The result of aftershock is shown as 237°/36°/109°(nodal planeⅠ),34°/56°/76°(nodal planeⅡ),centroid depth 17 km,and moment magnitude MW5.0.These two earthquakes are mainly trust type.Secondly,based on the continuous waveforms recorded by the seismic stations within 150 km epicentral distance during 2022-06-01 to 2022-06-10,we use LOC-FLOW software to identify and relocate the aftershocks.A total of 838 aftershocks are obtained,most of which are distributed in the northwest of the mainshock in the horizontal map.In the depth profile,on one hand most aftershocks are distributed along the up-dip direction of the mainshock,on the other hand it indicates that the fault plane southeastward inclines.Theoretically,the recorded waveform can be calculated by the convolution of source time function,propagation path effect and instrument response function.Assuming two events closely nearby each other are recorded in the same seismic station,the difference between two waveforms approximately reflects the difference of earthquake rupture process due to the same instrument response and similar propagation path effect.In this paper,the mainshock waveforms of two Lushan events recorded by 31 azimuthal stations are compared and analyzed,and it turns out two waveforms at each station show good consistence in the period range 10~20 s,indicating the rupture process for these two events may have similarities to some extent.The study area locates in the southern Longmenshan fault belt,the surface is mainly dominated by the faults with NE-SW strike and NW dip.However,according to the focal mechanisms and aftershock analysis in this study,we speculate that the 2022 Lushan earthquake should occur on a blind thrust fault inclining to SE.It is consistent with the NW-SE tectonic stress field in this area that has long been subjected to lateral extrusion on the Tibetan Plateau.