2008年汶川8.0级地震序列震源参数分段特征的研究

A study on segmentation characteristics of aftershock source parameters of Wenchuan M8. 0 earthquake in 2008

本文利用四川省区域固定地震台网观测记录到的2008年汶川8.0级地震序列的资料,从中挑选部分台站和地震资料,在精确扣除了余震区地震波衰减与台站场地响应后,计算得到了汶川地震序列中1070次M_L≥3.0级地震的震源参数,结果显示,地震矩与震级之间有较好的线性关系,应力降和视应力的大小与震级大小有关.利用M_L 3.0级地震资料得到的应力降的时空演变过程研究结果表明,汶川余震序列地震应力降总体上随时间是一个衰减过程,预示着主震发生后整个余震区应力降呈逐渐衰减的状态.主震发生之后,以虎牙—北川—安县为界,空间上龙门山断裂带上地震活动水平和应力降具有明显的分段性.5月17日之前M_L≥5.0余震主要集中在龙门山断裂带北川以西,由于地震释放了较多应力,该地区应力降较低,而北川至青川之间地震活动水平相对较弱,应力降一直处于高值水平.5月17日之后,M_L≥5.0余震活动主体地区则转移到北川至青川之间,在该段发生了5月25日青川6.4级最大余震,在这之后,整个余震序列应力降随时间变化开始趋于平稳.

Source parameters of 1070 earthquakes with ML≥3. 0 in Wenchuan aftershocks were calculated after taking away the effects due to earthquake anelastic attenuation and site response accurately, using partial seismic data selected from Wenchuan M8.0 aftershocks in 2008 recorded by the fixed seismograph network of Sichuan province. These results show that there is a linear relation between seismic moment and magnitude, and the stress drop and apparent stress increase with increasing magnitude. The temporal and spatial evolution process of stress drop derived from seismic data with ML3. 0 shows that the stress drop of Wenchuan aftershocks decays with time as a whole after the mainshock occurred. Divided along Huya-Beichuan-Anxian, the activity level and stress drop in the Longmenshan fault appear to be segmental. Aftershocks with ML≥5.0 mainly occurred at the west of Beichuan in the Longmenshan fault and the values of stress drop in this area are lower before May 17. Meanwhile, the activity level between Beichaun and Qinchuan is weak comparatively and the stress drop in there maintains a high level. However, aftershocks with ML≥5. 0 changed to occur between Beichaun and Qinchuan after May 17 and the biggest aftershock of M6.4 in Qinchuan occurred here on May 25 and the change of stress drop with time of the aftershocks appears to become smooth since then.