Coseismic Velocity Change in the Rupture Zone of the 2008 Mw 7.9 Wenchuan Earthquake Observed from Ambient Seismic Noise

We investigated seismic velocity and structural changes along the Long- men Shan fault associated with the 2008 Mw 7.9 Wenchuan earthquake using ambient seismic noise data recorded by the regional seismic network of the China Earthquake Administration. We computed cross correlograms of continuous records of station pairs from which we obtained clear Rayleigh-wave signals at periods of 10 to 25 s. We measured the travel speeds of these Rayleigh waves from ambient noise data recorded before and after the earthquake and obtained a total of 77 estimates of group velocity along paths running across the rupture zone of the earthquake. Most of the ray paths showed increasing travel times of Rayleigh waves right after the earth- quake. We inverted the measured differential travel times along different ray paths to constrain regions with significant changes in seismic velocity. Our 2D tomographic image showed a distinct region with an ∼0:4% velocity decrease. The strike and extension of this region match remarkably well with the deep rupture area of the earth- quake. Because the Rayleigh waves that we used in this study travel as deep as ∼25 km below the surface, it is difficult to explain the observed velocity changes solely with shallow damages (<100 m) resulting from strong shaking. Numerous laboratory studies have shown that the speed at which seismic waves travel through rocks changes with the level of stress. The velocity decrease observed here thus may be caused by a large stress drop associated with the earthquake. Assuming a stress drop of 3 MPa, we obtained a stress sensitivity of 1:3 × 10 9 Pa 1 , consistent with both laboratory and field observations. Our results suggest that time-lapse seismic imaging with ambient noise data provides a promising probe for monitoring temporal structural changes at seismogenic depths.