Detecting changes in long-period site responses after the M w 7.6 Chi-Chi earthquake, Taiwan, using strong motion records

Abstract

Temporal changes in site effects are obtained using the HVSR (horizontal-to-vertical spectral ratio) method and strong motion records after the M w 7.6 Chi-Chi earthquake, Taiwan. Seismic data recorded between 1995 and 2010 are used, comprising 3,708 data from 15 stations adjacent to the Chelungpu fault. Temporal fluctuations are determined by analyzing the site effect variation using a time–frequency variation (TFV) diagram based on these seismic data. Stations adjacent to the fault show significant disturbances in the resonance frequency at 16–26 Hz. Station TCU129 shows a 40% drop in fundamental frequency after the main shock, and a gradual return to the original state over nine years. For stations located farther from the fault zone, sudden changes in tectonic stress play a dominant role in temporal changes to the HVSR. An impact analysis of the directional factor confirms our finding that the proximity of the fault to seismic stations has the most influence on data.