2013年甘肃岷县漳县M_S6.6地震及其余震序列重定位

Relocation of Mainshock and Aftershocks of the 2013 Minxian-Zhangxian M_S6.6 Earthquake in Gansu

使用甘东南地区三维速度模型,利用三维网格搜索法和双差地震定位法对2013年7月22日甘肃岷县漳县MS6.6地震及其震后三天的余震序列进行了精确定位,结合地质构造资料对本次地震的发震构造进行了初步研究。其结果显示:主震的震中位置为34.54°N,104.189°E,震源深度13.5km;余震震中呈NW或NWW方向分布,与临潭-宕昌断裂的走向基本吻合,主要分布于5～20km的深度,震中在深度剖面上呈SW向;发震断裂为倾向SW的隐伏断层,位于临潭-宕昌断裂NE方向,距临潭-宕昌断裂约20km。

A 3-D velocity model of the southeast Gansu Province region, including the 3-D grid searching algorithm and double difference method, was used to examine the relocation the Minx- ian--Zhangxian Ms6.6 earthquake occurring on July 22, 2013, and its aftershocks from July 22 to July 25. The grid search method was used to determine earthquake location and the S--P arrival time of 400 seismic events, which agreed with data detected by at least three stations. Of these e- vents, 336 were used to calculate double-difference location, and 261 were used to obtain the relo- cation results. After relocation, the average estimation errors of the source location were 1498 m in the E--W direction, 1385.2 m in the N--S direction, and 2 492.3 m in the vertical direction. On the basis of these calculation results, the seism0genic structure was compared with the data of geological structure, and the results showed that the mainshock was relocated to 34.54°N, 104. 189°E, with a focal depth of 13.5 km. The aftershocks were distributed mainly between the Lint- an--Tanchang fault and the northern edge of the West Qinling fault; several were distributed in the southwestern region of the Lintan fault. The distribution of the aftershocks was in the NW or NWW direction, which is essentially consistent with the strike of Lintan--Tanchang fault, al- though the dominant distribution was not obvious. Therefore, we examined the A--Ar profileperpendicular to the direction to research the relationship between the distribution of earthquakes and fracture structure and determined that this profile crosscuts the Lintan--Tanchang fault. We obtained similar results using the 3-D grid searching algorithm and the double difference method, which revealed that the focus depths are mainly distributed in 0~10 km before relocation, and the average depth was 6 km. In contrast, the focus depths of the aftershocks were distributed mainly in 5~20 km after relocation, which indicates that the aftershock sequence occurred mainly in the upper and lower crust. The distribution of the earthquake was in the SW direction, thus, we speculate that the dip direction of the seismogenic fault was SW, and the deep distribution is dee- per. The focus depths determined by the 3-D grid-searching algorithm can reach to 25 km, where- as those determined by the double difference method were deeper at a maximum of 30 km. The Lintan--Tanchang fault, with a NW--NWW strike and a SW dip of 50°--70°, is in closest prox- imity to the earthquake swarm and is located between the East Kunlun fault and the northern edge of West Qinling fault. The transition fracture of the structural change and the geologic structure of the transition zone are complex and provide the regional geological background. The seismic profile A--A＇ shows that the basic characteristics of the seismogenic fault, with a SW dip direction, differ from those of the Lintan--Tanchang fault such that their exposures at the sur- face are 20 km apart. Therefore, we believe that Fault F3 is a seismogenic fault. However, other characteristics of Fault F3 are unclear and need to be developed through active fault detection re- search.