基于地震学资料探讨2013年岷县漳县6.6级地震发震构造

Study on Seismogenic Structure of 2013 Minxian-Zhangxian M_S6.6 Earthquake with Seismological Data

应用速度和Q值成像方法以及小震精定位方法获得了2013年岷县漳县6.6级地震震源区的深部结构和余震序列的空间分布图像。结果表明:发震断层为一隐伏断裂,走向NW,倾向SW,逆冲兼左旋走滑,在地表距临潭-宕昌断裂岷县段约20km,在深部可能与临潭-宕昌断裂交汇在一起,向南组成一条NW向的断裂带。2003年11月13日岷县MS5.2、2004年9月7日岷县-卓尼MS5.0和本次MS6.6地震的发震断层可能属同一发震断层,位于漳县南部块体的西南边缘;该块体为次级块体(三角形),近几年来块体西南边界强震活跃,其它两边平静。

Velocity and Q value tomography, in addition to precise positioning methods of small earthquakes, were used to obtain the deep structure of seismic source area and spatial distribution images of aftershocks. The data of velocity and Q value imaging consist of two parts. The first includes the observation data from a high-density portable seismic array consisting of 150 stations with a reporting period of November 2009 to November 2011 in the area of 32.2--36°N, 102.8-- 107. 1°E. The second includes data from a fixed network during the period of 1970--2010. From these datasets, we selected 6 334 seismic events recorded by at least 8 stations and conducted ve- locity and Q value inversion calculation. We applied the three-dimensional grid search method to aftershock relocation, in which 746 seismic events met with S--P arrival time differences detected by at least three stations. Finally, we obtained the relocation results of 400 seismic events, and we applied the same method to relocation of the Minxian Ms5.2 earthquake occurring on Novem- ber 13, 2003, and the Minxian--Zhuoni Ms5.0 earthquake occurring on September 7, 2004. We set the Pl profile perpendicular to the direction to research in the deep distribution of the seis- mogenic fault. The profile P1 crosscut the Lintan--Tanchang fault and the Minxian--Zhangxianseismic source area. We obtained the velocity profile, Q value profile, and aftershock distribu- tion, which were combined with the focal mechanism solutions of these three earthquakes and the distribution of smaller earthquakes recorded since instrumental record have been kept. The re- sults show that the seismogenic fault of Minxian--Zhangxian Ms6.6 earthquake is a buried fault with a NW strike direction, SW dip direction, and a dip angle steeper in the shallow region. In addition, the fault is thrust and sinistral strike-slip, and its exposure at the surface is 20 km from Minxian section of the Lintan--Tanchang fault. The seismogenic fault and the Minxian section of the Lintan--Tanchang fault may have met at a deep level to form a fault zone to the south with a NW dip direction. The seismogenic faults of the Minxian Ms5.2, Minxian--Zhuoni Ms 5.0, and Minxian--Zhangxian Ms6.6 earthquakes may belong to the same fault. The seismogenic fault of the Minxian--Zhangxian Ms6. 6 earthquake is located in the suture parts of two secondary blocks. The east block is relatively stable, and the west block moves eastward and is blocked by the east block to form the extrusion area. The Q value of the extrusion area is lower and shows velocity uplift. In the Earth＇s crust of the west block, the Q value is lower, which provides east- ward movement of west block with more favorable conditions. The occurrence region for earth- quakes of Ms^5 has included three events since 2003 with seismogenic faults at least 20 km from the Lintan--Tanchang fault; the dip direction is contrary. The seismogenic fault is located in the southwestern edge of Zhangxian＇s southern block, which is a secondary block （triangle） with a stable internal region. Three strong earthquakes occurred in western boundary of this block, which is conducive to the strain energy accumulation of the eastern region and the northern edge, which is the north rim fracture of western Qinling. The eastern region and northern boundary will likely be main regions of seismic activity of the southeast area of Gansu for the next several years.