Distribution Of Seismic Velocity Change Associated With The May 12, 2008 M7.9 Wenchuan Earthquake

We used continuous recordings in Sichuan, China to track the temporal evolution of the seismic velocity in a 2 year period which includes the great Wenchuan earthquake. The data are recorded by a temporary network of 84 out of 297 broad-band seismometers run by the Institute of Geology of the China Earthquake Administration. We analyzed the data from the stations in a 400*300km2 region that includes the southern 2/3 of the fault system activated during the Wenchuan event. We computed the cross correlation functions of seismic noise in a 30-day moving window for period between 1 and 3 seconds. We interpret them as an approximation of the actual Green function between the recorders. We then performed a doublet analysis to detect temporal changes of velocity with respect to a reference correlation. We deduce a relative average velocity change from the high quality delay measurements obtained for the ensemble of stations pairs. We found clear evidences that the seismic velocity drops after the earthquake by an average amount of about 0.1% in the fault region when measured with waves in the period range 1-3 seconds. We found that, according to our measurements, the velocity fluctuates within 0.02% in the months before the earthquake. The co-seismic variation is therefore well above the resolution of the measurements. We found that the co-seismic variation has similar amplitude for station groups in the Sichuan basin or in the Longmen Shan range, indicating that the co-seismic change is not fully controlled by the non-linear response of the shallow sediments. To investigate the velocity variations for different part in the region, we used a 0.5-degree station searching radius on 0.5°×0.5° grids to define sub-arrays, and measured the velocity variation for station pairs in the sub-arrays. We compared the measurements of velocity changes in different sub-arrays with a map of stress change deduced from a kinematic rupture model (Ji and Shao, personal communication) and the co-seismic deformation by GPS measurement. The velocity change, as the deformation, is concentrated in 100km width range and spread single-directional northeastward along the aftershock zone. These observations indicate that the velocity change distribution is more correlated with the deformation/stress map than with the surface geology map. The comparison with the results obtained at longer period is described in a companion paper (Campillo et al. 2009).