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

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 codaQvalues 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 codaQvalues within the crust give the relationship,QC= 93f0.75, 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 codaQrelationship is given asQC= 122f0.71in related to the material property of the crust and upper lithosphere, in which the average codaQvalues 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 primaryS-wave from the origin time and corresponding to the datasets with different focal depths, reveals that codaQvalues increase with the coda generating volume. As a whole, the regressive relationships of frequency dependent codaQfor Taiwan region approximately agree with those by previous investigators. In addition, the average codaQvalues for three subregions of Taiwan were calculated and correlated with the geology and tectonic features, respectively. The lowestQvalues were obtained in the region consisting of the Coastal Range and the Longitudinal Valley of eastern Taiwan, whilst the highestQfor 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.