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《地震学报(英文版)》 2009-01
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Intrinsic and attenuative dispersion characteristics of direct P-waves in and near the source area of the 1999 MW7.6 Chi-Chi,Taiwan,earth-quake before and after the mainshock

Xiqiang Liu 1,Chauhuei Chen 2 Yanwen Zhou 1 and Junhao Qu 1 1 Earthquake Administration of Shandong Province,Ji'nan 250015,China 2 Institute of Seismology,National Chung Cheng University,Chiayi 621,Taiwan  
Based on the measurement of the arrival time of maxima magnitude from band-pass filtering signals which were determined using a new Morlet wavelet multiple-filter method,we develop a method for measuring intrinsic and attenuative dispersion of the first cycle direct P-wave.We determine relative group delays of spectral components of direct P-waves for 984 ray paths from SML and ALS stations of the Taiwan Central Weather Bureau Seismic Network(CWBSN).Using continuous relaxation model,we deduce a new transfer function that relates intrinsic dispersion to attenuation.Based on the genetic algorithm(GA),we put forward a new inversion procedure for determining which is defined the flat part of quality factor Q(ω) spectrum,τ1 and τ2 parameters.The results indicate that 1 The distribution of Qm values versus epicen-tral distance and depth show that Qm values linearly increase with increasing of epicentral distance and depth,and Qm values is clearly independent of earthquakes magnitude;2 In the different depth ranges,Qm residual show no correlation with variations in epicentral distance.Some significant changes of Qm residual with time is likely caused by pre-seismic stress accumulation,and associated with fluid-filled higher density fractures rock volume in the source area of 1999 Chi-Chi Tai-wan earthquake.We confirm that Qm residual with time anomaly appears about 2.5 years before the Chi-Chi earthquake;3 A comparison of Qm residual for different depth range between SML and ALS stations show that the level of stress has vertical and lateral difference;4 The area near observation station with both anomalously increasing and decreasing averaged Qm residual is likely an unstable environment for future strong earthquake occurrence.This study demonstrates the capability of direct P-waves dispersion for monitoring attenuation characteristics and its state changes of anelastic medium of the Earth at short propagation distance using seismograms recorded from very small events.
【Fund】: supported by the National Science Council of Taiwan granted NSC-94-2816-M-194-004
【CateGory Index】: P315.31
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