2022年6月1日四川芦山M_(S)6.1地震的震源参数及其构造启示

Source parameters of the M_(S)6.1 Lushan earthquake on June 1,2022 in Sichuan Province and its tectonic implications

2022年6月1日四川芦山发生M_(S)6.1地震.基于四川区域台网的地震资料,采用HypoDD(双差重定位)方法对芦山M_(S)6.1地震序列ML≥1.0的地震事件(2022年6月1日至7日)进行了重定位,利用gCAP(generalized Cut And Paste)波形反演方法获取了序列中M_(S)≥3.0地震的震源机制和矩心深度,同时用Bootstrap方法评估了主震震源机制结果的稳定性以及计算了不同机构得到的多个震源机制中心解的最小旋转角,计算了现今区域应力场体系在2022年芦山M_(S)6.1地震和2013年芦山M_(S)7.0地震震源机制节面产生的相对剪应力和正应力,并根据芦山M_(S)6.1地震序列重定位后的震源位置拟合了发震断层面,分析了该地震序列的发震构造.获得的主要结果如下:(1)芦山M_(S)6.1地震序列主要沿着双石—大川断裂呈现NE-SW向的优势展布,初始破裂深度主要集中在10~18 km,平均深度14.5 km,整体呈现西北浅、东南深的空间分布特征.芦山M_(S)6.1主震和M_(S)4.5余震均位于余震区东南端,序列中其余的地震大都位于主震的西北侧,呈现为单侧破裂的特征,发震断层面倾向为SE向.(2)Bootstrap方法估算的误差以及多个震源机制中心解的最小空间旋转角均显示gCAP反演得到的主震震源机制误差较小,结果较为稳定可靠,芦山M_(S)6.1主震震源机制解为节面Ⅰ:走向228°、倾角46°、滑动角106°;节面Ⅱ:走向26°、倾角46°、滑动角74°,矩心深度12 km,矩震级MW5.8,M_(S)≥3.0余震的震源机制为逆冲型,震源矩心深度介于11~13 km.P轴方位主要为NW-SE向,与区域构造应力场的方向基本一致.(3)震源机制与应力场关系模拟结果表明2022年芦山M_(S)6.1地震和2013年芦山M_(S)7.0地震发震断层面的形状均处于相对剪应力和正应力的抑制节面,且不利于应力的充分释放.结合已有地质构造和以上分析结果,认为2022年芦山M_(S)6.1地震震源机制的节面Ⅱ为该地震断层面,发震构造极有可能为走向NE且倾向SE的隐伏逆冲断层,可能和2013年芦山M_(S)7.0地震走向NE、倾向NW发震构造上的次级反冲断层有关,这与2013年芦山M_(S)7.0地震发震构造倾向NW的断层方向有所不同.

On June 1,2022,an M_(S)6.1 earthquake hit the Lushan County,Sichuan Province,southwestern China.Based on the data from Sichuan Regional Seismic Network,the M_(S)6.1 Lushan earthquake sequence with ML≥1.0(June 1-7,2022)was relocated by HypoDD method.Focal mechanisms and centroid depths of the M_(S)≥3.0 earthquakes were inverted by the gCAP(generalized Cut And Paste)method.Meanwhile,the stability of focal mechanism of the mainshock was evaluated by Bootstrap method and the minimum rotation angles of the central solution of multiple focal mechanisms from various sources were calculated.Relative shear stress and normal stress on nodal planes from focal mechanisms of the 2022 M_(S)6.1 Lushan earthquake and the 2013 M_(S)7.0 Lushan earthquake were calculated under the contemporary regional stress field,and the seismogenic fault plane was fitted by relocated distribution of Lushan earthquake sequence.The seismogenic structure of the 2022 M_(S)6.1 Lushan earthquake sequence was analyzed.The main results are as follows:(1)The relocation distribution show that the aftershocks are mainly along Shuangshi-Dachuan fault and the predominant direction is to NE-SW.The events distribute in the 10~18 km depth range and the average depth of relocated earthquakes is 14.5 km.The shallow earthquakes distribute in the northwest and the deep in the southeast,and Lushan M_(S)6.1 mainshock and M_(S)4.5 aftershock are located in the southeast of the aftershocks region.Most of aftershocks are located in the northwest of the mainshock.The earthquake sequence shows the characteristics of unilateral rupture,and the fault plane dips to SE.(2)Both Bootstrap method and the minimum spatial rotation angle of the central solution of multiple focal mechanisms show that the error of the mainshock focal mechanism obtained by gCAP method is small,indicating that the results are reliable and stable.The mainshock focal mechanism solution is strike:228°,dip:46°and rake:106°for nodal planeⅠ,and strike:26°,dip:46°and rake:74°for nodal planeⅡ,and the centroid depth is 12 km while the moment magnitude is MW5.8.The focal mechanism inversion results show that the M_(S)≥3.0 earthquakes are mainly thrust earthquakes and the centroid depths are in 11~13 km range.P-axis azimuths are primarily in NW-SE direction which is consistent with the regional stress field.(3)Simulation on relationship between focal mechanisms and stress regimes reveals that the shape of fault plane of the 2022 M_(S)6.1 Lushan earthquake and the 2013 M_(S)7.0 Lushan earthquake inhibits the full release of stress.According to available geologic structure and results above,it is concluded that nodal planeⅡof focal mechanism of 2022 M_(S)6.1 Lushan earthquake is the seismogenic fault of earthquake.The seismogenic structure of the 2022 M_(S)6.1 Lushan earthquake sequence is probably a NE-striking blind thrust fault which is dipping towards SE.It also may be related to the secondary backthrust fault on the NE-striking and NW-dipping seismogenic structure of the 2013 M_(S)7.0 Lushan earthquake.