Abstract
On the basis of magnitude deviation analysis, we investigated the temporal change of Qs −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. Qs −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: ∆ Qs 1 −− =× × − 1.378 10 2 f 1
Highlights
Temporal changes of attenuation intensity in the vicinity of focal area - of interest to seismologists for the past decade - are of particular importance to better understanding of the change in rock properties during the whole process of preparation and generation of an earthquake
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
We investigate the temporal change of Qs−1 associated with the ChiChi earthquake in Taiwan of magnitude 7.3 in the vicinities of the main shock and the aftershock, by measuring the deviation of the station magnitude
Summary
Temporal changes of attenuation intensity in the vicinity of focal area - of interest to seismologists for the past decade - are of particular importance to better understanding of the change in rock properties during the whole process of preparation and generation of an earthquake. Sato (1986) made a more detailed analysis for the temporal change in attenuation intensity associated with a crustal earthquake of magnitude 6 in the vicinity of the main shock. We investigate the temporal change of Qs−1 associated with the ChiChi earthquake in Taiwan of magnitude 7.3 in the vicinities of the main shock and the aftershock, by measuring the deviation of the station magnitude. The attenuation intensity after the main shock was found to have become smaller than that before the main shock on the basis of deviation of station magnitude from the average magnitude calculated by routine observations.
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