Study on earthquake focal mechanism and seismogenic structure of the Santai M_S4.7 earthquake in 2013

Santai M_S4.7 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_S4.7 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.