Seismic velocity changes in the epicentral region of the 2008 Wenchuan earthquake measured from three- component ambient noise correlation techniques

We investigate temporal changes of seismic velocity in the epicentral region of the 12 May 2008 M-w 7.9 Wenchuan earthquake using three-component continuous waveforms recorded by a seven-station small-aperture array. We use an ambient noise cross-correlation technique to compute the empirical Green's function between station pairs from August 2004 to September 2011. Our results show no obvious precursory change immediately before the main shock, clear coseismic reduction of seismic velocity of up to 0.2%, and initial postseismic recovery followed by a long-lived velocity reduction. The coseismic and postseismic velocity changes are most prominent in the period band of 2-4s (approximate depth of 1-4km), and the velocity changes are smaller in other period bands. The seismic velocity in the period band of 1-2s (i.e., top 2km) correlates well with the water level change of the Zipingpu Reservoir. The observed temporal changes likely reflect damage and healing processes with possible permanent deformation in the upper crust associated with the Wenchuan main shock.