2022年四川泸定M_(s)6.8地震震源区横波分裂变化特征

Variations of shear wave splitting in the source region of the 2022 Luding M_(s)6.8 earthquake,Sichuan

本文对2022年四川泸定M_(s)6.8地震震源区横波分裂的时空变化特征进行了分析. 通过对横波窗内的近震横波波形进行横波分裂测量,得到了震源区各台站的快波偏振方向和慢波延迟时间. 结果显示,震源区上地壳各向异性存在着明显的空间分区特征和随时间的规律性变化. 空间上,靠近发震断裂——鲜水河断裂和余震密集区的台站的快波偏振优势方向呈NW或NNW向,与主压应力方向和发震断裂走向一致,且平均慢波延迟时间较大;位于震源区东北部大渡河断裂和龙门山断裂的台站呈现出两个快波偏振优势方向,一个是与主压应力方向一致的NW向,另一个是与断裂走向一致的NNE向,且平均慢波延迟时间较小. 上述分布特征反映了应力场和断裂构造对上地壳各向异性的综合效应,且泸定地震孕震过程中发震断裂和余震密集区积累的应力明显强于其他区域. 时间上,主震前,靠近发震断裂和余震密集区的台站的快波偏振方向较稳定,慢波延迟时间较小,在震后35天内,各台站的快波偏振方向离散度较大,快波偏振优势方向与主压应力方向存在一定夹角,慢波延迟时间变化剧烈,平均慢波延迟时间较大,主震36天后,各台站的快波偏振方向逐渐收敛,快波偏振优势方向与最大主压应力方向一致,慢波延迟时间逐渐减小并趋于稳定,反映了主震后的余震活跃期内的应力调整剧烈,随着主震和大量余震的发生,积累的应力不断释放,地壳应力逐渐减小,后期的余震活动逐渐变弱,泸定地震的发震断裂在震后的应力调整期可能存在着愈合过程.

In this paper, temporal and spatial variations of shear wave splitting in the aftershock sequence of the 2022 Luding M_(s)6.8 earthquake in Sichuan Province was analyzed. Based on the shear wave splitting analysis of local S-wave waveforms in the shear wave window, we acquired the fast wave polarizations and the delay times of each station in the source region. The results show that the anisotropy of the upper crust in the source region has obvious spatial characteristics and regular changes with time. As for spatial distribution, the predominant fast wave polarizations of stations near the seismogenic fault (Xianshuihe fault) and the regions of dense aftershocks are NW or NNW, which are consistent with the direction of principal compressive stress and the strike of seismogenic fault, and the average delay times are large;Stations located at Daduhe fault and Longmenshan fault in the northeast of the source region show two predominant fast wave polarizations, one is NW consistent with the direction of principal compressive stress, the other is NNE consistent with the strike of faults and the average delay times are small. The above spatial characteristics of anisotropy reflect the comprehensive effect of stress field and structure of faults on the anisotropy of the upper crust in the source region. And during the seismogenic process of the Luding M_(s)6.8 earthquake, the stress accumulated in the regions near the seismogenic fault and dense aftershocks was obviously greater than that in other areas. In the time domain, during the 35 days after the earthquake, the dispersion of the fast wave polarizations of stations near the seismogenic fault and dense aftershocks is large. There is an angle between the average fast wave polarizations and the direction of the principal compressive stress. The delay times fluctuate wildly, and the average delay times are large. After the 36th day of the mainshock, the fast wave polarizations gradually converge, the predominant fast wave polarizations show a strong consistency with the direction of the maximum principal compressive stress, and the delay times gradually decrease and tend to be stable. This reflects that the adjustment of stress during the active period of aftershocks was very intense. With the occurrence of the mainshock and a large number of aftershocks, the accumulated stress was released constantly, the crustal stress decreased gradually, aftershock activity weakened gradually, and there may be a healing process of the seismogenic fault in the stress adjustment period.