Temporal change of Qs-1 in the 1999 Chi-Chi earthquake fault area, Taiwan

On the basis of magnitude deviation analysis, we investigated the temporal change of Q(s)(-1) in the 1999 Chi-Chi earthquake fault area in central Taiwan, by analyzing the seismograms of events occurring in a volume in the focal and aftershock areas. The temporal change in attenuation intensity around the fault area was measured. Attenuation change was estimated by analyzing the deviations between the station magnitude and the average local magnitude. The station magnitude M-L was calculated (earthquake report of Central Weather Bureau) from the simulated Wood-Anderson seismogram at each station and the average local magnitude was the mean of the station magnitude at the stations near the main shock. The results show that the deviations decreased at five stations, increased in one station in the vicinity of the focal area after the main shock compared with that before. Q(s)(-1), derived from magnitude deviation analysis, in the upper crust in the close vicinity of the focal region increased during 1 day to 4 months following the main shock and decreased to its normal value after then. The simplest interpretation of the results is that the attenuation intensity in the upper crust in the close vicinity of the focal region increased after the main shock for frequencies from 4 to 25 Hz: DeltaQ(S)(-.1)=1.378 x 10(-2) x f(.-1).