Distribution of seismic speed changes associated with the 12 May 2008 Mw 7 . 9 Wenchuan earthquake

We used continuous recordings in Sichuan, China to track the temporal change in elastic properties of the medium, namely the seismic speed, at a regional scale in a 2 year period which includes the great Wenchuan Mw 7.9 earthquake. The data are recorded by a temporary network of 156 broad-band seismographs, in a 200km × 200km region that covers the southern 2/3 of the fault system activated during the Wenchuan event. Seismic noise cross correlation functions are computed in a 30-day moving window for periods between 1 and 3 seconds, and a doublet analysis is performed to detect temporal velocity changes with respect to a reference correlation. We found clear evidence that the seismic velocity drops by up to 0.08% in the fault region just after the earthquake while the measured velocity fluctuates within 0.02% before the earthquake. The co-seismic variation is therefore well above the resolution of the measurements. We compared the measurements in different subarrays to get a spatial distribution of the velocity change. This distribution is consistent with the stress change during the Wenchuan earthquake, as inferred from a kinematic model. We did not find that the co-seismic velocity variation is significantly larger for station groups in the Sichuan basin than in the Longmen Shan range, indicating that the co-seismic change is not controlled by the response of the sediments. On average the sensitivity to stress of the relative velocity change is of the order of 0.5 × 10Pa.