2013年三台M_(S)4.7地震震源机制及发震构造研究

Study on earthquake focal mechanism and seismogenic structure of the Santai M_(S)4.7 earthquake in 2013

2013年2月19日四川省绵阳市三台县发生M_(S)4.7地震,震中位于四川盆地中部基底断裂绵阳—三台—大足断裂与蒲江—三台—巴中断裂的交会区域。基于地震目录、震相报告和波形等资料,对地震序列进行重定位,采用近远震联合波形反演求解主震震源机制,结合野外地质调查对该地震的发震构造进行分析。余震序列重定位的空间展布方向为NW-SE向,与节面Ⅰ走向吻合,认为此次地震发震断裂走向为NW-SE,倾向为NE,且破裂面近于直立,发震断层在近NS向的近水平挤压应力作用下作右旋走滑错动。实地调查中未见地表破裂及地表断层行迹,认为发震构造为一隐伏断裂。重定位及通过近远震联合反演所得震源深度分别为21.6和19 km,震源深度与调整过程中,四川盆地川中地块内隐伏于前震旦恹纪结洃晶基底中的高角度断裂重新活动的结果。

Santai M_(S)4.7 earthquake occurred in Mianyang city,Sichuan Province on February 19,2013.The epicenter located in an intersection area which combined the basement fracture in central Sichuan basin,the Mianyang-Santai-Dazu fracture and the Pujiang-Santai-Bazhong fracture.Based on the earthquake catalogue,seismic phase report and waveform data,we relocated the earthquake sequence,and solved the focal mechanism of the main earthquake by joint inversion of local and teleseismic waveforms,and also studied the seismogenic structure of the Santai M_(S)4.7 earthquake according to the field geological survey.The spatial distribution direction of the relocated aftershock sequence is NW-SE and it highly fitted with the nodal plane I of the focal mechanism,the seismogenic structure of the earthquake can be considered as strike NW-SE,dip NE,and rupture plane is nearly vertical,and the seismogenic fracture was dextral strike-slip dislocation by the proximately horizontal confining pressure in the direction of nearly NS.During the field survey,any fracture marks nor surface ruptures can be obviously found,hence the seismic structure is concluded as a buried fault.The focal depths are 21.6 and 19 km respectively through the relocation and joint inversion of local and teleseismic waveforms.The focal depth reflects that the location of the source area is in the low-velocity layer which is in the bottom of the upper-crust.This earthquake is caused during the adjusted and changed process of the tectonic stress field caused by Wenchuan earthquake,and it is the result of the reactivation of the high-angle fault concealed in the Presinian crystalline substrate in middle-block of Sichuan basin.