基于震源机制和剪切波分裂分析鄂尔多斯地块内部一次显著性地震:2022-07-12盐池M_(L)4.0地震

Analysis of a Significant Earthquake Occurred in Ordos Block Based on Focal Mechanism and Shear Wave Splitting:The Yanchi M_(L)4.0 Earthquake on July 12,2022

结合宁夏区域地震台网和预警台网资料,通过Hash方法计算2022-07-12盐池M_(L)4.0地震震源机制,采用gCAP方法得到该地震震源机制、矩张量解及矩心深度,确定震源机制中心解,计算中心解两个节面上的相对剪应力与正应力,最后对符合剪切波分裂窗口的台站进行快波偏振方向和慢波时间延迟分析。结果表明,盐池地震是一起天然走滑型地震事件,不同方法确定的震源机制中心解为节面Ⅰ走向99°、倾角71°、滑动角8°,节面Ⅱ走向6°、倾角83°、滑动角161°,P轴方位54°;应力张量在震源机制中心解节面Ⅰ及节面Ⅱ上产生的相对剪应力分别为0.982和0.942。近台剪切波分裂显示,快波偏振方向为55°,慢波时间延迟为1.7 ms/km。分析发现,盐池M_(L)4.0地震震源机制P轴方位及其快波偏振方向均受到区域构造应力场控制,且震源机制两个节面的滑动角均与剪应力的滑动角基本一致。结合已有研究认为,本次地震发震断层可能与鄂尔多斯地块内部EW向基底断裂附近的平行伴生构造有关,并在主压应力轴为NE-SW向的区域构造应力场作用下,经过长时间应力积累,在剪应力的最优释放节面发生破裂。

Based on the data of Ningxia regional seismic network and earthquake warning network,we calculate the focal mechanism of the July 12,2022 Yanchi M_(L)4.0 earthquake by the Hash method.We obtain the focal mechanism,moment tensor solution and centroid depth of the earthquake by the gCAP method.We determine the focal mechanism central solution,and calculate the relative shear stress and normal stress on the two nodal planes of the focal mechanism solution.Finally,we carry out the polarization of fast shear wave and time delay of slow shear wave on the station that conforms to the shear wave splitting window.The results show that the Yanchi earthquake is a natural strike-slip earthquake event.The focal mechanism central solution of different methods is strike 99°,dip 71°,slide 8°for nodal plane I and strike 6°,dip 83°,slide 161°for nodal plane II,and the P-axis azimuth is 54°.The relative shear stress generated by the stress tensor on nodal plane I and nodal plane II of the focal mechanism central solution is 0.982 and 0.942,respectively.The near-station shear wave splitting shows that the fast shear wave polarization direction is 55°and the slow shear wave time delay is 1.7 ms/km.We find that the P-axis azimuth of the focal mechanism and the polarization direction of the fast shear wave of the Yanchi M_(L)4.0 earthquake are mainly controlled by the regional tectonic stress field,and the slides of the two nodal planes from the focal mechanism are basically consistent with the slides of the shear stress.Combined with existing research,we consider that the seismogenic fault may be a parallel associated structure near the EW-strike basement fault in the Ordos block.Under the principal compressive stress of regional tectonic stress field in the NE-SW direction,after a long period of stress accumulation,the rupture occurred on the optimal release nodal plane of shear stress,triggering the M_(L)4.0 Yanchi earthquake.