Investigation of the Temporal Change in Attenuation Within the Ruptured Fault Zone of the 1999 Mw7.3 Chi-Chi, Taiwan Earthquake

A decrease in Qs values within the hanging wall of the ruptured Chelungpu fault two years following the 1999 Chi-Chi earthquake was revealed by Qs tomography images and an analysis of single-path Qs. The synthetic and sensitivity tests of the Qs determination were carried out accordingly to justify the temporal variation. A Qs value within the hanging wall above the hypocenter was determined to be 157 ± 18 two years following the Chi-Chi earthquake, which is significantly lower than the Qs tomography values of 238 ± 17 and 289 ± 13 prior to and two years after the main shock, respectively. Similar values using a signal-path Qs analysis from events within the ruptured fault zone to stations along the fault were obtained. The corresponding Qs values were 247 ± 85 prior to the Chi-Chi earthquake. After the earthquake we obtained Qs values of 158 ± 75 and 318 ± 80 for 2 years following and 2 years after the earthquake, respectively. Considering the two independent methods in determination of Qs, the reduction in Qs by 89 two years following the Chi-Chi earthquake in both methods is significant. Along with 1 % Vs reduction revealed by the analysis of repeating earthquakes our studies suggested possible reduction both in Vs and Qs values within the fault zone after the Chi-Chi earthquake. Temporal changes in Qs after the Chi-Chi earthquake imply variations of pore-fluid saturation in the ruptured fault zone. The reduction in Qs two years following the Chi-Chi earthquake indicates high pore-fluid saturation within the fractured fault zone due to the postseismic fluid redistribution.