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

• 来源期刊: 地震学报（英文版）
• 学科: 地球科学
• 年，卷（期）: 2009,（1）
• 页码范围: 33-44
• 页数: 12页
• 分类号: P315.3+1
• 语种: 英文
• DOI: 10.1007/s11589-009-0033-2

摘要

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 at-tenuative 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(w) spectrum, τ1 and τ2 parameters. The results indicate that ① The distribution of Qm values versus epicen-trai 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-seisrnic 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 re-sidual 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.