Decade-long seismic velocity reduction in the upper crust after the 1999 Mw 7.6 Chi-Chi Earthquake in Taiwan

Abstract:

The 1999 M_W 7.6 Chi-Chi earthquake in Taiwan caused extensive damage and remains one of the best studied earthquakes. We present new observations of the gradual changes in seismic velocity (dv/v) in the intervening decades and provide the first spatial-temporal resolved window into the evolution of the crust after major earthquakes. Using data from the Broadband Array in Taiwan for Seismology (BATS), we calculate cross-correlations of ambient seismic noise at the stations and analyze the temporal changes in the coda of the resulting empirical Green's functions. We find that at least 6 BATS stations show clear dv/v drop immediately following the Chi-Chi earthquake. Using bandpass filters we are able to constrain the depth and spatial extent as well as two distinct mechanisms for dv/v. The closest station that is < 5 km away from the epicenter recorded the greatest velocity change as ~0.45%, and asymptotically approaches a new value 0.2% lower 18 years later. In contrast, all other stations that recorded changes in seismic velocity have subsequently returned to the pre-Chi-Chi baseline. The amplitude of dv/v drop is highly correlated with peak ground velocity (PGV) which, in turn, was significantly affected by the northward seismic directivity. We argue this reflects two different processes: In the near field, permanent crustal damage decreases seismic velocity while, separately, fluid flow due to dilational surface waves produced transient decrease in dv/v. Throughout the crust, there is an overall higher coseismic velocity drop in shorter periods, implying damage above 10 km depth. These observations are a novel test of competing hydrologic models for changes after the Chi-Chi earthquake. We show that broad changes of permeability and fluid flow throughout the crust are most consistent with dv/v changes. We also quantify the seasonal modulation in dv/v and show how upper crustal material properties are affected by regional earthquakes and rainfall.