Co- and Post-Seismic Responses in Ambient Seismic Velocity to the 1999 M7.6 Chi-Chi Earthquake in Central Taiwan

Session: Advances in Seismic Interferometry: Theory, Computation and Applications
Type: Oral
Date: 4/30/2020
Time: 10:45 AM
Room: 230 + 235
Description: 

The 1999 Mw 7.6 Chi-Chi earthquake in Central Taiwan caused significant damage across the entire island. In this study, we collected nearly two decades of continuous seismic records from the Broadband Seismic Array for Taiwan network to explore change in crustal seismic velocity in response to earthquakes. 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. From the 17 seismic stations that are available in Incorporated Research Institutions for Seismology (IRIS), we find that at least 6 stations show clear velocity drop immediately following the Chi-Chi earthquake. The closest station, SSLB (< 5 km away from the epicenter) recorded the greatest velocity change as 0.45%. In general, there is much greater velocity drop north of the Chi-Chi surface rupture, even for station TATO(0.2%) that is ~100 km north of the north end of the surface rupture. In contrast, station TPUB that is ~10 km south of the south-most surface rupture receives only 0.08% of velocity drop. The ambient seismic velocity drop is highly correlated with peak ground velocity (PGV) which, in turn, was significantly affected by the northward seismic directivity. There is an overall higher coseismic velocity drop in shorter periods (1.1 - 5 sec), implying damage in shallower crust. We also observe gradual recovery of ambient velocity after the earthquake for most of the stations. This may suggest healing of the upper crust and is consistent with observations of hydrologic recovery after the earthquake. However, the ambient velocity of station SSLB, where the maximum velocity drop occurred, remains ~0.2% lower than the pre-Chi-Chi level even 18 years after the earthquake. This may suggest permanent damage or long-term (decadal) crustal healing process in the near-field. We also find seasonal modulation in ambient seismic velocity that correlates with the mean annual precipitation in Taiwan. Our observations show how upper crustal material properties are affected by earthquakes and rainfall.

Presenting Author: Mong-Han Huang

Authors