【摘要】 2021年9月16日,四川省泸州市泸县发生M_S6.0地震,是继2019年6月17日长宁M_S6.0地震后在四川盆地发生的又一次6级强震.泸县M_S6.0地震震中位于NE走向的华蓥山褶断带内部,极震区烈度达Ⅷ度,共造成3人死亡,159人受伤.M_S6.0主震后余震活动频度低、强度弱,截止到9月23日,发生的最大余震为9月16日4时55分M_S2.8(M_L3.4)地震,与主震震级差3.2,呈现孤立型地震序列特征.利用四川区域地震台网宽频带波形资料,通过CAP波形反演,获取的本次M_S6.0地震震源机制解节面Ⅰ走向286°、倾角45°、滑动角103°,节面Ⅱ走向88°、倾角46°、滑动角77°,显示该地震为逆冲型.P轴方位187°、俯仰角1°,反映震源区主压应力呈近NS向水平推挤特征,与华南地块构造应力场NW-SE向主压应力方向存在显著差异,揭示本次地震应受局部应力场控制.泸县M_S6.0地震的矩震级为M_W5.36,远低于面波震级;震源矩心深度为3.5 km,与重新定位后的震源初始破裂深度5.1 km较接近,表明该地震发生在四川盆地上地壳浅部沉积盖层内,符合最近3年四川盆地中强地震活动深度分布普遍较浅的特征,也反映本次地震的震源破裂尺度相对较小.其震源机制解两节面走向均与震中附近的华蓥山褶断带西支断裂及附近的已知地表断层几何结构不匹配,结合重新定位的前震和早期余震空间展布近NWW向优势特征,本文初步判定走向NWW的节面Ⅰ为同震破裂面,发震断层倾角45°,推测此次泸县M_S6.0地震为沉积盖层内NWW向隐伏逆冲断层在近NS向水平主压应力作用下挤压错动所致.更多还原

【Abstract】 On September 16, 2021, an M_S6.0 earthquake occurred in the Luxian County, Luzhou City in Sichuan Province. It is another M_S6.0 event in Sichuan Basin after the M_S6.0 Changning earthquake on June 17, 2019. The epicenter was located in the NE-trending Huayingshan fold-thrust belt, and the intensity of the meizoseismal area were estimated to be around VIII degree. Three deaths and 159 injuries were reported after the earthquake. Until September 23, very rare aftershocks of the M_S6.0 Luxian earthquake sequence were recorded, and the largest one was the M_S2.8(M_L3.4) event at 04∶55 on September 16. The magnitude difference between the maximum aftershock and the mainshock was 3.2, showing the characteristics of isolated earthquake sequence. Based on the waveform data from permanent stations, the moment magnitude and focal mechanism solution as well as centroid depth of the M_S6.0 Luxian event were obtained by the CAP waveform inversion method. The focal mechanism indicates this earthquake is a thrusting event, with strike/dip/rake of 286°/45°/103° and 88°/46°/77° for nodal planes I and II, respectively. The azimuth of P axis is 187° with plunge angle of 1°, reflecting a horizontal compression mode that the principal compressive stress is featured by a near NS direction in the source area. This is significantly different with the principal compressive stress in the NW-SE direction of the regional tectonic stress field in the South China block, indicating that the earthquake may be controlled by the local stress field. The moment magnitude is M_W5.36, and it is much lower than the surface-wave magnitude M_S6.0. The focal centroid depth of this event is 3.5 km, close to the relocated initial rupture depth of about 5.1 km, consistent with the depth distribution characteristics of moderate and strong earthquakes in Sichuan Basin in the latest three years. All these observations imply this event occurred in the sedimentary layers of the shallow part of the upper crust with relatively small rupture size. The directions of the two nodal planes from the focal mechanism solution of the M_S6.0 Luxian earthquake are both inconsistent with the geometric structure of the western branch fault of Huayingshan fold-thrust belt or the known nearby surface faults. Considering near NWW-trending of the spatial distribution of the relocated foreshocks and early aftershocks, we suggest that the nodal plane I in NWW direction is the coseismic fault plane with dip angle of 45°. We further infer that the M_S6.0 Luxian earthquake was caused by the dislocation of the NWW-trending blind thrust fault in the sedimentary layers, and it was under the horizontal principal compressive stress in the near NS direction.更多还原