| On 12 May 2008, the Mw 7.9 earthquake occurred on the LMS fault in Sichuan province of China which induced huge losses. Lots of work such as geological surveys (Xu et al., 2009), inversions of surface rupture process (Zhang et al., 2008; Ji and Hayes, 2008; Wang et al., 2008), aftershock relocation (Huang et al., 2008; Chen et al., 2009), and inversion of the GPS data (Shen et al., 2009) indicate that the Wenchuan earthquake is, up to now, the largest strong intra-continental earthquake on a high angle thrust fault. There are usually many events accompanying the generation and occurrence of earthquakes, such as variations of the co-seismic stress field, migration of underground fluid, destruction of shallow crust and so on, as a result, physical properties of the crust medium will be changed. Studies on changes of physical properties of the crust medium before and after a strong earthquake will help us to understand the process of its generation, occurrence and recovery, and also it will help realize the earthquake dynamic process. It is particularly important to research variations of media and the stress field in the epicenter region of the Wenchuan earthquake which has special seismogenic structures.Recently, a technique called passive image interferometry (PII) using the cross-correlation function coda of seismic ambient noise has become a new method to monitor the changes of crust media. Because of source-free and sustainable monitoring, it could be applied to many aspects. Researches on earthquakes (Wegler et al., 2009; Xu and Song, 2009; Chen et al., 2010; Cheng et al., 2010; Liu and Huang, 2010), volcanoes (Brenguier et al., 2008a) and fault zones (Wegler et al., 2007; Brenguier et al., 2008b) by PII indicate that it can help understand the process of gestation, occurrence and recovery of quakes, recognize the active process in the volcano and even forecast its eruption with time and intensity.A transportable array with 297 broadband seismic stations was deployed in the western Sichuan(100°～105°E, 26°～32°N)in October of 2006 by the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, called Western Sichuan Seismic Array(WSSA). The WSSA covers the CD block, SG block and SC basin, 2/3 of the fault system activated during the Wenchuan earthquake. It worked for more than 2 years and provided unique continuous recordings before, during and after the Wenchuan earthquake. These data allow a intensive study on the Wenchuan earthquake.Three-component seismic ambient noise data continuously recorded at 137 stations of WSSA to the north of 29°N from January of 2007 to October of 2008 were used in this paper to research the crust velocity changes caused by the Wenchuan earthquake. Doublet processing was applied to the auto-correlation functions coda and cross-correlation function coda for each single station with three different components. Time shifts were measured in the frequency range of 0.33~1HZ between the 50 days moving averaged correlation functions coda and the long time averaged reference correlation functions coda, then the crust velocity changes with time were obtained. With the help of the high density of WSSA, the spatial distribution of co-seismic velocity changes was determined.The results can be summarized as follows:(1) Remarkable co-seismic velocity droops could be found in the Sichuan basin and around the Longmenshan fault along the direction of fracture extending. Meanwhile, in the opposite direction, there are week velocity changes, and so do the western Songpan block and Chuandian block.(2) Comparing with the results given by Chen et al. (2010) who used cross-correlation functions coda between each two stations in the same area of WSSA, the co-seismic velocity changes presented in this thesis are larger but with similar trend. The ratios of the co-seismic velocity changes to the velocity perturbations before the earthquake around the LMS fault are 2 and 2.1, respectively. So there is no essential difference between the two methods but with different depths of the sensitive zone.(3) Although the spatial distribution of co-seismic velocity changes around the LMS fault is similar to the volumetric strain changes during the Wenchuan earthquake, 2 to 3 orders higher of magnitude in velocity changes indicate that the latter may be not the primary cause to the former. To shallow crust, extent of porosity or fluid migration caused by co-seismic stress changes could be the main reason for co-seismic velocity changes.(4) With higher spatial resolution of the ambient noise auto-correlation method, a distinct co-seismic velocity increase was found at the area of the intersection of LMS fault and Xianshuihe fault which coincides with the Coulomb stress increase predicted by co-seismic Coulomb stress changes and the observations of surface deformation. It reveals that the stress increase continued for about 2 months after which it dropped back into a general decrease.(5) There are similar characteristics of velocity changes with time in different areas especially around the LMS fault and in the SC basin where velocity increased to different degrees about 200 days before the Wenchuan earthquake. It seems not related to season but possibly a reaction to stress increase. Unfortunately, observation before the earthquake only lasted about one and a half years, so it is difficult to give a verdict. Relations between stress changes and the Wenchuan earthquake should be further studied. |
