Earthquake sequence of the Yutian M_S7.3 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.