S-Wave Attenuation Structure in the Taiwan Area and Its Correlation to Seismicity


S-wave amplitude data form the Taiwan Telemetered Seismographic Network (TTSN) and the Central Weather Bureau Seismographic Network (CWBSN) are used in a tomographical inversion for a three-dimensional S-wave attenuation structure beneath the Taiwan area. The data consist of three-dimensional, digital seismograms of 137 earthquakes with magnitudes between 3.0 and 5.0. The S-wave attenuation structure is obtained through the use of the damped least-squares method from attenuation time value (t*) and travel times. Attenuation time value is calculated by using the spectral decay method under the assumption that t* is independent of frequency. S-wave spectra are corrected for a w^2 source spectrum model. The frequencies of instrument reponse above 25 Hz or below 4 Hz usually have large uncertainty. Therefore, the least-squares line fitting is performed in the frequency band of 4 to 25 Hz. The results here show that the distribution pattern of the Qs values in the Taiwan area correlates well with seismicity. In the upper crust (depths of between 0 to 15 km), the region of lower Qs area correlates with higher seismicity. However, in the lower crust and upper mantle (15 to 75 km), the area of higher seismicity correlates with the regions of higher Qs values.

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