Monitoring temporal variation of subsurface waveattenuation using active source

Monitoring subsurface medium properties is essential for understanding earthquake evolution process. After the occurrence of Wenchuan MS8.0 earthquake, a 1-month field experiment was conducted in Ningqiang County, Shaanxi Province, to monitor subsurface medium property variation. The experiment site is located at the northeast end of the main fault ruptured during the devastating Wenchuan earthquake. An electric drop hammer was used for highly repeatable excitation of elastic waves. We extracted small temporal variations of the attenuation parameter t* of subsurface medium using spectral ratio method, and investigated the correlation between the attenuation, wave velocity and atmospheric pressure. The result shows that the shallow fault zone, which is generally composed of sedimentary cover and unconsolidated rocks, exhibits strong attenuation with a quality factor Q of about 10; the temporal variation of t* is strongly correlated with the variation of atmospheric pressure and is conformable to seismic velocity change with atmospheric pressure, which is probably caused by the opening and closing of compliant microcracks associated with pressure variation; apparent co-seismic change, as well as different post-seismic variation patterns across different seismic stations, of t* associated with a moderate aftershock is identified. Our field experiment shows that it is feasible to monitor subsurface medium properties using active source.