2019年四川威远东两次MS＞5.0地震序列活动特征及震源区体波速度结构

自2015年以来, 四川威远及邻区的地震活动强度显著增强, 其中2019年9月8日和12月18日在威远东发生了M_S5.4和M_S5.2地震, 刷新了威远地区有记录以来历史地震震级上限.本文利用2019年8月至2020年3月期间威远及邻区25个流动台和17个固定台记录到的5638次M_L≥1.2地震Pg和Sg波到时以及连续波形资料, 采用双差层析成像方法获得了威远东这两次地震序列的重定位结果和震源区浅层地壳高分辨率三维体波速度结构.重定位结果显示震源区地震震源深度主要集中在3~4 km.基于两次M_S＞5.0地震震后10天余震序列空间展布, 本文推测两次地震的发震构造均为NNE走向且倾向SE(倾角40°左右)的盲冲断层.三维体波速度结构反演结果揭示: 震源区V_P异常变化较V_S异常变化显著, V_P/V_S比背景值普遍低4%~10%.在深度小于5 km的浅层内, 地震多分布在V_P低速异常和V_S弱高速异常以及二者的高低速异常过渡带内; 在5~7 km深度范围, 地震基本上都发生在V_P低速异常区.上述速度结构特征揭示震源区附近可能发育着富含气体的孔隙和裂缝且存在流体活动.此外, 2019年11月7日以后的地震活动具有明显的流体扩散驱动地震迁移特征, 沿两次地震发震断层走向和倾向的流体扩散系数分别为1.5 m²·s^-1和0.1 m²·s^-1.两次地震的发生机理很可能是流体扩散引起的孔隙压力上升.

Activity characteristics of the 2019 two MS > 5.0 earthquake sequences in eastern Weiyuan (Sichuan) and body-wave velocity structure of the source region

Seismicity rates in Weiyuan (Sichuan Basin, China) and its adjacent regions have increased significantly since 2015. In 2019, one MS5.4 earthquake and one MS5.2 earthquake occurred in eastern Weiyuan on September 8 and December 18, respectively. The two events broke the record of the largest magnitude of historical earthquakes in the region. To investigate the source characteristics and occurrence mechanisms of the two events, here we collected the Pg and Sg wave travel times and continuous waveform data of 5638 ML≥1.2 earthquakes recorded by 25 temporal stations and 17 permanent stations in the study area from August 2019 to March 2020. The double-difference seismic tomography method is used to relocate the two earthquake sequences and to obtain high resolution 3D body-wave velocity structure of the source area. Our relocation results show that most events are located in the depths of 3~4 km. The spatial distribution of aftershocks occurred in 10 days after the two MS > 5.0 earthquakes indicates that they ruptured two blind thrust faults striking to NNE and dipping to SE at an angle of ~40°. The inversion results of the 3D body-wave velocity structure show that variation of VP values in the source region is more significant than VS values, and that VP/VS ratio is generally 4%~10% lower than the background value. Above the depth of 5 km, most earthquakes occurred within regions with low VP anomalies and weak-high VS anomalies and transition zones from high velocity anomalies to low velocity anomalies. In the depth range of 5~7 km, earthquakes are mostly located in regions with low VP anomalies. The characteristics of velocity structure reveal that, in the source area, there are probably abundant of gas-filled pore and fractures that are favor for fluid flow. In addition, seismicity observed after 7 November 2019 shows obvious characteristics of spatiotemporal migration driven by fluid diffusion. The corresponding fluid diffusion coefficients along the strike and dip directions of the two source faults are 1.5 m2·s-1 and 0.1 m2·s-1, respectively. The major cause for the occurrences of the two events is probably pore pressure increase associated with fluid diffusion.