Seismic noise correlations to image structural and mechanical changes associated with the Mw 7.9 2008 Wenchuan earthquake

We locate temporal changes of seismic wave speed and scattering properties of the crust associated with the 2008 Mw 7.9 Wenchuan earthquake. To that end, we analyze ambient seismic noise records from 2007 until the end of 2008 in the 1-3 s period band and in the 12-20 s period band measured in a region that covers the southern two thirds of the fault activated during the earthquake. To locate the changes, we use a refined imaging procedure based on the sensitivity of scattered waves to weak perturbations. This inverse method uses the radiative transfer approximation to describe the intensity of the noise correlation coda. Our results show that the largest structural changes are observed northeast of the Wenchuan epicenter, around the fault zone where aftershocks are distributed. The spatial and temporal characteristics of the behavior of the crust at depth around the Wenchuan earthquake suggest a postseismic and geology-dependent signature of the middle crust. We also observe a clear seasonal signature within the Sichuan basin at depth that we relate to the heavy rainfalls during the monsoon season. This seasonality can also be observed at the surface in form of a waveform decoherence. Our study highlights that seismic velocity changes, and waveform decoherence are independent measurements that are sensitive to different parameters and can thus give complementary information. In our study we also point out the value of studying the evolution of the changes at different times in the coda.