Abstract
AbstractAttenuation 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.
Highlights
Attenuation of seismic waves propagating through a realistic medium plays an important role on the prediction of ground motion induced by an earthquake
Using all the three components of each seismogram recorded from shallow event, the coda Q values were estimated from coda envelopes utilizing the single backscattering method (Aki and Chouet, 1975)
The result has clearly revealed the pattern with no significant dependence on the azimuth even for the frequencies ranging from 1.5 Hz to 18 Hz analyzed in this article. It seems to be reasonable as calculating the attenuation rate of coda waves in a similar lapse time for local earthquakes with similar epicentral distance and focal depth
Summary
Attenuation of seismic waves propagating through a realistic medium plays an important role on the prediction of ground motion induced by an earthquake. The characteristics of seismic coda waves are generally described by the average decay of the envelope in the tail portion of a seismogram (Aki, 1969). Based on the concept that the earth is assumed to be composed by the randomly distributed heterogeneities, the behavior of coda waves can be adequately explained by the scattering of primary elastic waves in a random medium. A generalization form of Aki and Chouet’s model were proposed by Sato (1977). In this model the scattering is a weak process, and use is made of the Born approximation in that the loss of energy from the primary waves as well as the multiple scattering is neglected in the original idea. Putting the controversy on the mechanism of energy dissipation aside, the contention of scattered S to S waves from heterogeneities in the crust and the upper mantle can be widely supported by the work of Rautian and Khalturin (1978), Aki (1980), Herrmann
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