A Comprehensive Study of Shear-Wave Splitting and Related Signals Using the Chi-Chi Earthquake Sequence

University Of Southern California, Los Angeles CA

Investigators

Abstract

This is a seismological study of shear-wave splitting in the source volume of the large Mw 7.6 Chi-Chi, Taiwan, earthquake of 1999. The study also investigates the guided fault zone waves, and non-linear wave propagation effects in damaged fault zone rock, using one of the best data sets in the world in a region of high natural seismicity. A comprehensive measurement is carried out on the stress-induced crustal anisotropy, augmented by other possible signatures of damaged fault zone material, and the project also evaluates the relationship of observations with (therefore predictive potential of) the occurrence of great earthquakes. The extensive data set is used that has been collected by the Taiwan Central Weather Bureau Seismic Network (CWBSN) from 1990 to 2000. Rigorous tests are conducted on the correlation of shear-wave splitting and on other signals with regional crustal stress variations and lapse time before and after the Mw 7.6 event. This data set extends back to 1970, providing a 30-year seismic database for monitoring the bulk crustal properties before, during, and after the Chi-Chi earthquake sequence. The CWBSN operates a network of more than 650-station (at 5-km station spacing) of 3-component, large dynamic range, and broadband accelerographs and a network of 75-station 3-component high-gain short-period seismometers. Thousands of M > 3 events from the middle-lower crust have been beautifully recorded with excellent S/N ratios. For most M > 3 earthquake in the data set, found that there are a number of recording stations directly above the source with epicentral distance less than 20 km. The software for this project performs particle polarization analysis and at the same time extracts the shear-wave spiltting delay times through an automated maximization of the cross-correlation function of the two matched horizontal component signals. This allows accurate measurements to be made for testing the idea of shear-wave splitting as a potential indicator for a large earthquake in the future. This research project has the potential of being more comprehensive and definitive than any shear-wave splitting work that has so far been done to date.