The Studies On Wave Velocity Change Before And After Strong Earthquakes In Yunnan Region Using Ambient Noise Method

There are defects with different sizes, such as various flaws, fissure, fault and so on, in crust rocks. These defects are caused by many factors, for example, the opening and closing or expanding and contracting of the rocks by the crustal stress changes, the groundwater inflow and outflow and so on. It is an early idea of forecasting the earthquake in modern seismology by monitoring the wave velocity change in the crust to detect the change of stress or fluid in the crust.Though the scientists have researched on wave velocity change, it is not smooth for monitoring earthquake by studying on velocity change. In the early research, some observation data about the large change of V p /Vs is not reliable. Not all the change of seismic wave velocity can be observed. Using active seismic source method, such as air gun, we can find the wave velocity change related with tide stress, but these variations are detected only in the narrow space nearby the surface of crust. In the deeper and wider region, there have not been the new results until now. On the other hand, the Q value represents the attenuation character of earth media as a wave velocity in broad sense. When the seismic wave propagates in the region of active volcano or rock rupture, its energy will be attenuated strongly and there is low Q value. And when it propagates in the region of stable craton, there is a low attenuation and high Q value. The change of Q value can reflect the variation trend of tectonic activity. The variation of the Q value is especially obvious in the seismic active region. These show that it is possible for using wave velocity change on researching the variation of the underground stress and the fluid migration.However, there are many problems such as earthquake location and unequal ray distribution using the nature earthquake method to research on velocity change, thus, the nature earthquake method is mostly applied to the static wave velocity research. The active seismic source method is mainly applied to wave velocity change research in different periods. We studies velocity variation using ambient noise method in this paper article. The advantages are dispensing with seismic source information, equal ray distribution, feasible to study short-period surface wave and smaller expense and so on. Yunnan region locates on the highlighted position of orogenic movement with remarkable geology impact, active crustal action, and frequent seismic or volcano action. The adjustment of underground stress and fluid are very obvious, as a result, its wave velocity will be changed synchronously. This is the reason that we study wave velocity change in strong earthquakes periods in this region.We use the continuous wave data of 46 stations in digital telemetry seismic network in Yunnan from January 2008 to December 2010, and the ambient noise method to calculate the surface wave delay time in different station pairs in Yunnan region (21°~29°N,97°~106°E). And we inverse the surface wave velocity distribution of different time interval with the same time window using CT method, and gives the resolution checkboard result. It shows that the resolution ratio is 0.75°×0.75°in most of Yunnan region. By the experiment, we can find that the tomography results are obviously various in different time intervals. There are some characters before and after strong earthquakes. Firstly, there are the obvious adjustments of the wave velocity in Yunnan region after Wenchuan Ms 8.0 earthquake. Secondly, the Panzhihua area locates in the boundary region where velocity rises and reduces before the 2008 Panzhihua Ms6.1 earthquake. Thirdly, the velocity of the epicenter area rises before the 2009 Yaoan Ms6.0 earthquake and reduces after it.The regional tomography results show that the wave velocity changes are obvious in the distinguished area, especially the area nearby the earthquake source before and after strong earthquakes. In order to obtain the more accurate relationship between the wave velocity change and the earthquake, we choose the ray pairs passed by Dayao region (100°~103°E,24°~27°N) where there are many earthquakes in recent years. We study the wave velocity change and trend from January 2008 to December 2010 using correlated detection method, and give the error range of these changes. The precision of the results can reach to 0.01s.It is found that the velocity of each ray changes obviously before Yaoan Ms6.0 earthquake in July 2009. The rays with the increased wave velocity are mainly located in the northeastern part of Chuxiong-Jianshui fault, which shows that there is stress convergence in this part. When the earthquake happens, the ray velocities in the northwestern part of the fault reduce while those in southeastern part increase. It is good fitness with the dextral character of the focal mechanism of Yaoan earthquake. There are the reverse adjustments for the ray velocities after earthquake. The velocity variations are mainly that the ray velocities in northwest increase during Panzhihua Ms6.1 earthquake in August 2008. The precise and reliable measurement can be obtained using the correlated method to detect the velocity variation, and the continuous detection of velocity variation can be realized.