A Study On Temporal Changes In Medium Velocity Of Lushan Earthquake Region By Ambient Noise

On April 20th,2013, a Ms 7.0 earthquake happened on the southern segment of the Longmenshan fault zone, Lushan area of Sichuan province, which was a destructive earthquake that following Wenchuan Ms 8.0 earthquake occurring in the Longmenshan fault zone on May 12th,2008. Previous research showed that between the ruptured region of the two earthquakes, there was an empty section with high possibility of occurring another big earthquake. The purpose of this thesis focuses on using data recorded by stations around Lushan earthquake epicenter to monitor if there is a change in medium properties of Lushan earthquake source region before and after the earthquake, and whether there is an abnormal change in the media of the empty section.In this study, we applied the method of extracting the empirical Green function from ambient seismic noise data to obtain the velocity time variation of underground medium in Lushan earthquake zone. We used the vertical component of ambient seismic noise data recorded by the 9 stations near the Lushan earthquake zone, and the 9 stations formed 14 different paths that covered the source region of the Lushan earthquake and the empty section. The time length of our study was from January, 2012 to October,2014, a total of 34 months. Firstly, we need re-sample the raw data to 20Hz. Then, by doing multiple tests, we found that the empirical Green function in the period band of 10s～20s had a high similarity before and after the earthquake, and it was more stable and reliable. So we filtered the observed data to the 10s～20s frequency band, and normalized the wave using running-absolute-mean method. After doing whitening process, we computed the cross-correlation function between the two stations, and extracted the reference empirical Green function and current empirical Green function, respectively. Finally, making use of moving window cross spectral method, we calculated the temporal changes in seismic velocity of the selected 14 paths.The results of this research showed that, before the Lushan earthquake, velocity changes could not be observed along all the paths covering the focal area. This result was consistent with the previous tectonic earthquake examples of magnitude above Ms 6 studied by the same method. Some paths across the focal region existed rapid velocity decrease by 0.4% after the earthquake. The speed change process lasted 2 months to disappear. As to the paths through "empty block" area between Lushan earthquake aftershocks and Wenchuan earthquake aftershocks, we found no significant velocity change. Compared with Wenchuan earthquake, the magnitude of Lushan earthquake is much less, and its rupture range is much smaller with much weaker coseismic dislocation. We assume that maybe it is the reason that velocity change is only found out in the source area along a handful of station pair paths. Meanwhile, the wave period of 10s～20s frequency band we used reflects media properties in a wide range and depth. Therefore, if the change process of the medium after the earthquake is tiny, it is also likely to be covered.