Elimination Of Seasonal Variations For Ambient Noise Cross-correlation And Investigation Into Temporal Velocity Changes Related To The Ms8.0 Wenchuan Earthquake

The temporal changes of seismic velocity related to the Ms8.0 Wenchuan earthquake have been investigated by applying the ambient noise cross-correlation to the broadband seismic data between Jan. 1st 2008 and Sep. 30th 2010 recorded at 39 stations operated by China Earthquake Administration near the Longmen Shan Fault. The data were first decimated to 5 samples per second, and then removed trend, mean and instrumental response, and last regulated. In this process the seismic events should be removed from the data. After that Empirical Green Functions were extracted from the results of cross-correlation computation of seismic data for all station pairs, and the temporal changes of seismic velocity were estimated within the bandwidth from 0.1-0.5 Hz by measuring the travel time shift between Empirical Green Functions and Reference Green Functions every day. We found that the temporal changes of seismic velocity revealed by ambient noise cross-correlation were strongly influenced by seasonal variations and the results showed that for some station pairs temporal variations of seismic velocity were even dominated by seasonal variations. After the correction for seasonal variations, the results revealed that most of the station pairs in the vicinity of Longmen Shan Fault exhibit rapid relative velocity drop after the Ms8.0 earthquake and the maximal relative velocity drop reaches to 0.5 percent. On analyzing the intrinsic correlation between the variation of seismic velocity and the strong aftershocks, we found that the relative velocity changes in some of the station pairs show good coincidence with the strong aftershocks. After the main shock the relative seismic velocity exhibits sharp drop associated with the strong aftershocks and rises rapidly along with the decrease on the aftershocks and finally becomes stable. Analyzing to the self-healing period of the crust after the earthquake reveals that the regions with large velocity drop usually need longer time to heal, and these regions always correspond with serious destruction during the main shock.