Details

Autor(en) / Beteiligte

• Liu, Xiqiang
• Chen, Chauhuei
• Zhou, Yanwen
• Qu, Junhao

Titel

Intrinsic and attenuative dispersion characteristics of direct P-waves in and near the source area of the 1999 M sub(W)7.6 Chi-Chi, Taiwan, earth- quake before and after the mainshock

Ist Teil von

• Earthquake science, 2009-02, Vol.22 (1), p.33-44

Erscheinungsjahr

2009

Link zum Volltext

Beschreibungen/Notizen

• 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( omega ) spectrum, tau sub(1) and tau sub(2) parameters. The results indicate that (1) The distribution of Q sub(m) values versus epicentral distance and depth show that Q sub(m) values linearly increase with increasing of epicentral distance and depth, and Q sub(m) values is clearly independent of earthquakes magnitude; (2) In the different depth ranges, Q sub(m) residual show no correlation with variations in epicentral distance. Some significant changes of Q sub(m) 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 Taiwan earthquake. We confirm that Q sub(m) residual with time anomaly appears about 2.5 years before the Chi-Chi earthquake; (3) A comparison of Q sub(m) 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 Q sub(m) 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.