Three-dimensionalQp- andQs-tomography beneath Taiwan orogenic belt: implications for tectonic and thermal structure

Abstract

SUMMARY We determined the 3-D Qp- and Qs- structure of the Taiwan orogenic belt to enhance understanding of the related tectonic and thermal structure beneath the collision zone. The inversion used t* values measured from the spectra of P and S waves from the dense Taiwan strong motion network for moderate size earthquakes (ML 4.5–5.5) to avoid source complexity. The time period, 1991–2007, includes the aftershock sequence of the 1999 Chi-Chi earthquake that provides good ray coverage in central Taiwan. Over 18 000 velocity spectra from 883 earthquakes were analysed. A non-linear least square technique is applied to the spectra for t* determination by assuming a ω−2 source model for the frequency band of 1–30 Hz. A frequency-independent Q was assumed in this study. The corner frequency of a specific event was fixed for the corresponding stations, and a quality index was defined to assure good quality data for the inversion. The results reveal the sharp variation of Qp and Qs across the recently ruptured Chelungpu Fault, and the Kaoping and Chaochou Faults in Pingtung Plain. The Q values in the hangingwall are smaller by about 85 and 110 for Qp and Qs, respectively, relative to the footwall. The fault geometry is distinctly delineated by the contour of Qp/Qs of 1.2 that extends to the depth of the geologically identified decollement structure. Beneath the Central Range, the low Qp, low Qs and high Qp/Qs features coincide well with the aseismic zone. Comparison to the recent thermomechanical numerical models of Taiwan shows that the low Q zone corresponds to the exhumation of the lower crust. The low Qs regime (high attenuation) beneath the Central Ranges at the depth of 5–22 km coincides with predicted temperatures of 400–600 °C. The Qs comparison with the major tectonic and thermal mechanical models of Taiwan reveals that the shear wave attenuation model contains comprehensive rheological and thermal information of relevance to understanding mountain building processes. This technique appears particularly useful for distinguishing strong and weak crustal regions in the absence of other constraints.