Spatial distribution of coda Q estimated from local earthquakes in Taiwan area

Abstract

Abstract Attenuation of seismic wave energy and its spatial distributions in Taiwan area have been investigated using a large amount of short-period seismograms recorded from the local earthquakes with magnitude ranging from 2.5 to 4.5. The coda Q values were estimated in the frequency range 1.5∼18 Hz, applying the single backscattering model proposed by Aki and Chouet in 1975. The estimated average frequency dependent coda Q values within the crust give the relationship, Q C = 93f 0.75, while varying from 143±33 at 1.5 Hz to 904±164 at 18 Hz central frequencies. However, using the data from deeper earthquakes, the average coda Q relationship is given as Q C = 122f 0.71 in related to the material property of the crust and upper lithosphere, in which the average coda Q values vary from 180±67 at 1.5 Hz to 1000±144 at 18 Hz. The comparison of different lapse times starting at double the time of the primary S-wave from the origin time and corresponding to the datasets with different focal depths, reveals that coda Q values increase with the coda generating volume. As a whole, the regressive relationships of frequency dependent coda Q for Taiwan region approximately agree with those by previous investigators. In addition, the average coda Q values for three subregions of Taiwan were calculated and correlated with the geology and tectonic features, respectively. The lowest Q values were obtained in the region consisting of the Coastal Range and the Longitudinal Valley of eastern Taiwan, whilst the highest Q for the region underlain by the Peikang High beneath the western Coastal Plain. Such distinguishable difference has definitely explained that the seismic waves should exhibit lower decay rate in a stable region than within a seismoactive plate boundary.