Variations in seismic velocity and attenuation associated with seismogenesis: A numerical verification using ambient noise

Abstract

Seismic velocity variations associated with the seismogenic process have been studied worldwide for almost half a century since the dilatancy hypothesis proposed in early 1970s. However, the reports in seismogenesis-associated variations in attenuation are rare. Reports on simultaneous variation of velocity and attenuation are even rarer. The conventional way to obtain the seismogenic temporal variation in velocity and attenuation is through the observation of travel time and amplitude variations of microseismicity in a seismogenic zone. Anyhow, for some major earthquakes there may not always be microseismicity prior to the mainshock. Thus, obtaining a complete record of seismic velocity and attenuation variation from microseismicity is severely limited if pre-mainshock microseismicity exists. In contrast, seismic ambient noise is an ideal source for crustal stimulation for monitoring temporal variations in velocity and attenuation in a seismogenic zone.In this paper the seismogenesis-associated seismic velocity and attenuation variations shown as some observable parameters in ambient noise measurements are verified using a numerical simulation approach. First, based on the temporal variation in seismic velocity observed along the Longmenshan fault associated with the 2008 Wenchuan earthquake, we divided the seismogenic process into six phases upon velocity drop stages in an elliptic area in the crust. Second, using finite difference time domain method we generated 30-minute low frequency ambient noise over a vertical profile of 200×45km and recorded it with a 90-station array on the surface. Next, we processed the synthetic ambient noise records to get the auto-correlation function (ACF), cross-correlation function (CCF), Rayleigh wave dispersion curve, and horizontal-to-vertical spectral ratio (H/V). Finally, we examined the temporal variation of these parameters versus the phases of the seismogenic process and found the most pronounced changes occur in the phase with the largest velocity drop and the recovery phase directly before the mainshock.