3-D seismic tomography, focal mechanisms, and Taiwan orogeny

Seismic Vp and Vp/Vs tomography is conducted to image the lithospheric structures under Taiwan, and earthquake focal mechanisms and the state of stress are determined to understand the mode of deformation under the Taiwan orogen. The main purpose of this study is to use the tomographic images and the state of stress and deformation inferred from the focal mechanism solutions obtained to understand the mountain building processes of a young and active arc-continent collision-induced orogen.; In the tomographic inversions, a total of 24,206 P and 10,823 S-P arrival times from 1197 events recorded by the 75-station Taiwan seismic network were used in three different grid configurations in the Taiwan region. The results of the resolution and error analyses demonstrate that the 3-D inversions employed are robust, and the tomographic images obtained are reliable and meaningful. For the obtained tomographic images, an inclined high P-wave velocity zone in northern Taiwan, coinciding with the Wadati-Benioff zone, can readily be identified as the subducted Philippine Sea plate. Along the Central Range, the high Vp and low Vp/Vs ratio at the upper crustal levels can be interpreted as quartz-rich rocks, such as granite, granitic gneiss, or granodiorite, with elevated temperatures associated with the quartz {dollar}alpha{dollar}-{dollar}beta{dollar} phase transition. The low Vp and high Vp/Vs ratio at lower crustal and upper mantle levels in the Central Range are consistent with lower crustal mafic rocks, such as gabbro, mafic granulite, or anorthosite, and may be related to the presence of partial melt and/or fluid. These images are interpreted to reflect the up-arching of middle crustal materials and the down-warping of lower crustal materials under the Central Range. The tomographic images show that a significant portion of the lithosphere is involved in the Taiwan orogeny.; To understand the mechanical processes under the Taiwan orogen, the focal mechanisms of 97 small-to-moderate {dollar}(2.7leq Msb{lcub}L{rcub}leq5.7){dollar} earthquakes under the Taiwan orogen were determined using P-wave first motions, SH/P amplitude ratios, and hypocentral locations obtained in a joint tomographic inversion. The results show that, of the 97 earthquakes studied, the focal mechanisms of 23 of them using 3-D velocity models are remarkably different from the routine 1-D results. The SH/P amplitude ratios are found to be very effective as constraints for choosing the best solutions along those consistent with the polarities. Qp and Qs are determined to correct for the effect of attenuation on P- and S-wave amplitudes using the spectral ratio method. Qp ranges from 289 to 330 and Qs, from 201 to 301. The resulting focal mechanisms show a high degree of variability across the island, but in terms of tectonic stress regimes, they show consistent patterns. They provide a glimpse of the mechanical processes under the Taiwan orogen.; The nature of faulting of the 97 minor earthquakes is characterized by a mixture of reverse, normal, and strike-slip faults. 30 of the 97 events studied are reverse faulting events, 24 of which occurred under the Western Foothills and the Central Range, mostly within a depth range of 10-32 km and a dip-angle range of 30-70{dollar}spcirc.{dollar} (Abstract shortened by UMI.)...