2014年2月12日新疆于田MS7.3地震序列及其构造背景研究

2014年2月12日新疆于田发生M_S7.3地震，该震前1天曾发生M_S5.4前震，震后余震活动频繁.截止到2月20日12时，该地震序列记录到4000多次余震，最大余震为2月12日M_S5.7地震，序列类型为前震—主震—余震型.该地震前震的b值明显低于该区域正常活动的b值和余震的b值.这次地震位于西昆仑断裂带与阿尔金断裂带的交汇区域的阿什库勒断裂北段，震源机制解为走滑型.余震区NE向长70 km、宽20 km，分为主余震分布区和次余震分布区，其中M_L4.0以上强余震基本位于NE向主余震分布区，N--S向的次余震分布区则以M_L3.0左右地震分布为主，显示该部分可能受到主震的触发作用.于田地区曾发生的2008年3月21日M_S7.3地震的震源机制解为正断型，距这次地震约100 km；2012年8月12日发生的M_S6.2地震的震源机制解为正断型，距这次地震约10 km.该地区的发震构造背景是:在NE向阿尔金断裂带尾端向SW方向延伸过程中，左旋走滑作用逐渐转换为拉张作用，形成多条左旋走滑兼具拉张作用的断裂. 2014年于田M_S7.3地震的发震模式表现为:左旋走滑的阿什库勒断裂北段与南段因速率差异而产生的小型构造盆地，在区域拉张作用力下顺时针旋转；2008年M_S7.3张性地震后区域的伸展作用增强，导致盆地南侧的苦牙克断裂发生2012年M_S6.2张性地震，该地震引起2014年M_S5.4前震，两者激发其后在盆地北侧阿什库勒断裂发生了2014年M_S7.3主震. 

Earthquake sequence of the Yutian MS7.3 earthquake on February 12, 2014 and its tectonic background

On February 12 2014, an M_S7.3 earthquake hit the Yutian County, Xinjiang. One day before, an M_S5.4 foreshock occurred on the same place. Also frequent aftershocks struck the place after the main event. The Yutian M_S7.3 earthquake sequence includes more than 4000 aftershocks before 12:00 on February 20, with the maximum aftershock magnitude of M_S5.7. The sequence type is mainly foreshock--mainshock--aftershock, and the b value of the foreshock is lower than that of the aftershocks, even lower than that of the regional normal seismicity. The epicenter of the 2014 Yutian M_S7.3 earthquake is located on the northern section of the Ashikule fault in the intersection area of the West Kunlun fault and the Altyn Tagh fault. The focal mechanism of the M_S7.3 earthquake indicates that the earthquake occurred as the result of strike-slip. The aftershock zone is extending northeastward with 70 km length and 20 km width, respectively, and the zone can be divided into the main and secondary aftershock areas. Almost all the aftershocks with magnitude larger than M_L4.0 are in the main aftershock area trending in NE direction. While the aftershocks in the secondary area is mainly M_L3.0 with nearly N--S trending, showing that the earthquakes in this zone were probably triggered by the mainshock. Another M_S7.3 earthquake occurred on 21 March 2008 with normal-faulting behavior is only 100 km west to the M_S7.3 earthquake in 2014. Also the M_S6.2 Yutian earthquake on 12 August 2012 with normal-faulting behavior is just 10 km away. The tectonic background of these earthquakes is concluded as follows: during the procession of the NE-striking Altyn Tagh fault extending toward southwest, several parallel faults with sinistral and normal slip components are generated in the region, with the behavior of the faults transferring from sinistral to normal type gradually. A simple model of a tectonic basin with boundary faults, which was caused by the slip rates difference between the northern and southern Ashikule fault, is used to illustrate the seismogenic mode of the 2014 Yutian M_S7.3 earthquake in our study. In this model, the regional extension from the Altyn Tagh fault tail effect is enhanced by the 2008 M_S7.3 earthquake, resulting in the 2012 M_S6.2 earthquake with normal slip on the south boundary fault of the basin. Then the 2012 M_S6.2 earthquake triggered the M_S5.4 foreshock in 2014, and finally promoted the occurrence of the M_S7.3 mainshock in 2014 on the Ashikule fault.