| The Lushan Lushan Ms7.0 earthquake and Ludian Ms6.5 earthquake occurred in April 20 and 2013 August 3 2014, respectively, in Sichuan-Yunnan region after the Wenchuan Ms8.0 earthquake. Those two events had caused significant casualties and economic losses, and attracted widespread concern. After the above two events occurred, many scholars have investigated the focal mechanism, causative structure and seismic risk and other aspects of the two earthquakes.The process of earthquake occurrence involves stress accumulation in the crust and sudden releasing of stored energy. And the process will result in a change in the property or condition of the crust media. In the long term research goals, it is helpful to understand the process of earthquake occurrence by monitoring the property variation of the crust media associated earthquakes, which will be contribute to earthquake prediction. Previous studies show that we can retrieve the seismic wave velocity changes using the repeating earthquake and artificial seismic source, so as to monitoring changes in property or condition of the crust media. However, the ways above have a large limitations because of the anisotropy distribution of the repeating earthquake and the costly artificial seismic source. Now the seismic ambient noise correlation provides another effective method.The method that monitoring seismic wave velocity changes with seismic ambient noise is a new way presented in recent years. This new method has great advantages compared to repeating earthquake and artificial source because the seismic ambient noise can obtained more easily, and it can get the temporal variations of seismic velocity different from tomography with ambient noise. In this paper, we retrieve the seismic wave velocity changes associated with the Lushan earthquake or Ludian earthquake, respectively, with seismic ambient noise recorded by some seismic stations around the two earthquakes.We selected 11 stations distributed in Longmen Shan fault zone in the study on the seismic wave changes associated with Lushan earthquake. Depending on the quality of the results, we have excluded two pairs and analyzed the seismic wave perturbations of 53 pairs. The results showed that the Lushan earthquake causing dramatic wave velocity changes in south section Longmen Shan fault zone, and this change has significant regional differences: the wave velocity reduced in the west of the fault zone while increased in the east of the fault zone. In this paper, we also calculated the baseline of some seismic stations and plane strain of local regions with GPS data. We found that the seismic wave velocity decreased in the region where plane strain increased while increased in the region where plane strain decreased after the earthquake, by comparison the seismic wave velocity changes with the result of GPS. In addition, the correlation analysis between the seismic wave velocity and the plane strain or baseline showed that the seismic velocity changes linear correlation with the baseline and with the plane strain.In the study on Ludian earthquake, we selected 13 seismic stations in the study area. The result showed that 31 pairs showed seismic wave velocity decrease, 23 pairs showed velocity increased and the other pairs had no significant changes. In the spatial distribution of coseimic wave changes, we can find that the large changes of wave velocity focused on Zemuhe-Xiaojiang fault and Lianfeng fault zone. The seismic wave velocity showed coseismic increase in the area where stress increase after Ludian earthquake, and it showed coseismic decrease in the area where stress decrease, the magnitude of the changes also had a positive correlation between the wave velocity and the stress. In addition, we also get the wave velocity changes after April 5, 2014 Yunnan Yongshan Ms5.1 earthquake, and discuss its effect on the Ludian earthquake.According to the study on the two earthquakes, we believe that the changes of strain(stress) field in the source area, is likely to be the main factor that controlling the seismic wave velocity changes after the earthquake. |
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