Temporal And Spatial Evolution Characteristics Of Moment Tensor In Longmenshan Fault Zone Since Wenchuan Earthquake

In the interior of the crust, there are a lot of earthquake fault zones. Earthquake is the result of stress accumulation on faults. The place and time of the earthquake occurrence depend on the stress transfer amongthe faults. Seismic moment tensor analysis is an important wayto study the faulting stress properties. The May12th,2008Wenchuan8.0earthquakegreatly changed the regional stress field, and stress transfer happened among the multiple faults in theneighborhood. Since the mainshock, there hundreds of magnitude ML≥4.0aftershocks occurred in the Longmen Shan fault zone. On April20,2013, at the southwest segment of the longmenshan fault, a magnitude7.0earthquake happened again at Lushan. After the7.0mainshock, more than50(ML≥4.0) aftershocks were recorded. The Wenchuan and Lushan earthquake sequences, provide abundant data for us to understandthe nature of Longmen Shan fault stress evolution. Through the moment tensor analysis of the Wenchuan and Lushan earthquake sequences, to study the spatio-temporal evolution of the fault zone tectonic stress, will contribute to further understanding the interaction between the different blocks, as well as basic seismogenic and occurrence rule in the area.In this article, by means of seismic moment tensor analysis, taken the wenchuan8.0and the Lushan7.0earthquakes as two time node, We carried out the moment tensor inversion of Wenchuan and Lushan earthquake sequence, studied the spatio-temporal evolution characteristics of moment tensor; also anlyzed the spatio-temporal distribution of seismicity, seismicmoment, and b values on Longmen Shan fault since the Wenchuan earthquake, and tried to discussthe stress transfer evolution trend on the fault zone.. Considering the length of the rupture, the distance from epicenter to the station, and the inhomogeneity of crust structure, we established four velocity models for moment tensor inversion.We collected high signal-to-noise ratiowaveform data for moment tensor inversion, and got177high quality solutions. According to the method of Zoback (1992), we classified the moment tensors and studied their spatio-temporal evolution, inverted the stress field. With the recored seismicity catalogue since the Wenchuan8.0earthquake on Longmen Shan fault, as 0.2degree interval, we spatially segmented the Longmen Shan fault and studied the magnitude ML≥4.0earthquakes’time distribution feature. As six month time interval, we analyzed seismicmoment (Mo)’s spatio-temporal distribution. Asociating with the magnitude of completeness (Mc), we analyzed the spatio-temporal distribution of b value.Through study, we obtained the following main results:(1) The major type of aftershock moment tensor of Wenchuan8.0erathquake are thrust and strike-slip, with minority of nornal and unknow ones. The moment tensors have spatial segmentation feature, most of the strike-slip solutions are distributed at the S2(near Xiaoyudong) and S6(near Qingchuan) segments. These observations agree with the main shock rupture process. Temporally the moment tensors initially comprised all fault types and mainly as thrust and strike-slip, as the time elapse, the mainly type change to thrust, which may be related to a return to pre-event loading conditions.(2) On the Lushan mainshock, the depth is13km, for the best double couple focal mechanism, the strike, dip, rake of plane I respectively are216°,47°,103°, and correspondingly20°,44°,78°for plane Ⅱ, It is a thrust fault with little strike-slip component, and Mo is0.638e+19Nm. Most ML≥4.0aftershock s’focal mechanism are similar to mainshock as thrust feature, also their epicenters relatively concentrated, this may explain the seismogenic fault is simple and rupture scale is relatively small. The stress release to a certain extent, did not trigger activity of more nearby faults.(3)The Wenchuan magnitude8.0earthquake make the stress of the southwestern end of the Longmen Shan fault increased, and created condition for the occurrence of Lushan magnitude7.0earthquake, but compare to the influence from Wenchuan earthquake, the NNW tectonic stress take the leading role on the occurrence of Lushan earthquake. Although Lushan earthquake and Wenchuan earthquake may have a close relationship, there are still relatively independent.(4) After the Wenchuan earthquake, the stress increase on the Longmen Shan fault zone has a gradually transfer trend from north to south; the24th-26th segments (in the west of Dayi and Qionglai counties) that spacilly segmented as0.2degree interval is the seismic gap between the Wenchuan and Lushan sequences, where the seismicity distribution of ML≥4.0earthquake and Mo release almost be zero, and the change of b values in this region is very small in the5years after the Wenchuan earthquake. They distributed around the value of0.9and1.0, as the relatively high value area, which indicated that the regional stress accumulation is not very strong. It may be related to the low velocity anormaly body and the function of Xiaoyuedong fault in Longmen Shan fault energy transfer; On both ends of the study area has the value of b anomalies may be related to the coulomb stress increasing by the effect of Wenchuan earthquake.