2022年墨西哥米却肯州7.6级地震序列的触发关系及发震动力学探讨

Study on the Triggering Mechanism and Earthquake Dynamics of the 2022 Michoacan MW7.6 Earthquake Sequence in Mexico

摘要:

2022年9月19日墨西哥米却肯州发生M_W7.6地震,该地震位于北美板块与科科斯板块交汇处。为进一步研究本次地震对周围断层和后续地震的影响,剖析该地震的孕震背景和发生条件,首先使用包括USGS在内的多个国外地震机构得到的墨西哥M_W7.6地震以及后续的两次M_W＞5.0地震的震源机制解,分别计算出震源机制中心解,然后通过计算主震产生库伦破裂应力变化来研究本次地震对后续两次地震的触发作用,最后收集了1976年1月1日至2022年5月31日发生在本次地震附近的M_W≥4.9的29条震源机制解数据,反演该地区的构造应力场,并模拟在局部构造及其作用下产生的各种断层形状及相对正应力和剪应力的分布情况。研究结果表明:①震源机制中心解表明主震和6.8级余震为逆冲型地震,而5.8级余震的震源机制为正断型地震; ②本次地震对5.8级和6.8级余震的库伦破裂力变化均超过0.01MPa的阈值,表明这两次强余震可能是在主震的触发下发生的; ③震源区应力场的主压应力轴为NNE-SSW向,主张应力轴近乎垂直。本地震序列的断层破裂近乎沿应力场的最大剪应力平面发生,最大限度地释放了构造运动积累的应力。

Abstract:

On September 19,2022,an M_W7.6 earthquake hit Michoacan,Mexico,where the Cocos plate subduct under the North American plate. To further study of the influence of the earthquake on the surrounding faults and follow-up earthquakes,and to analyze the background of earthquake preparation and occurrence of the earthquake sequence,by using the focal mechanisms of the mainshock and 2 M>5.0 aftershocks provided by USGS,and other foreign organizations,we firstly calculated the central focal mechanisms of the 3 earthquakes. Then,we calculated the Coulomb Failure Stress(CFS)change projecting on the fault plane and slip direction of the two strong aftershocks generated by the mainshock to analyze the triggering effect. In order to study the effect of the mainshock to adjacent area,we also calculated the displacement field and strain field generated by the mainshock. Finally,we estimated the stress field in source region by using 29 M_W≥4.9 earthquakes' focal mechanism collected from January 1,1976 to May 31,2022,and simulated the relative shear and normal stresses projected on various fault planes geometry under the estimated tectonic stress. The results show that:① According to the estimated central focal mechanisms,the mainshock and M6.8 aftershock are thrust type and the M5.8 aftershock is normal fault type.② The changes of CFS on the fault plane and slip direction of both M5.8 and M6.8 aftershocks exceeded the threshold of 0.01MPa,indicating that the two aftershock occurrences are triggered by the mainshock. ③ The compressional and extensional stress axes of the estimated stress in source region are NNE-SSW direction and vertical,respectively. The rupture of the earthquake sequence occurred nearly along the plane with maximum shear stress of the stress field,and released the accumulated stress caused by the tectonic movement to the utmost.