S U M M A R YWe 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 (M L 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 décollement 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.
By analyzing the data from 28 seismic borehole stations deployed by the Central Weather Bureau Seismic Network throughout Taiwan from 2007 to 2014, we estimated the near-surface velocity (Vp and Vs) and attenuation (Qp and Qs) structures from surface to depths of approximately 300 m. To ensure that the deeper recordings were on the ray path between the seismic source and upper receiver, only events with an incidence angle of less than 35° were selected. Local magnitudes of analyzed events were between 1.1 and 6.6. The subsurface Qp and Qs were well modeled in the 5 -40 Hz frequency band using the spectral ratio of direct P-and S-waves, respectively, at each station, under frequency-independent Q and ω 2 source model assumptions. The estimated Vp in the Coastal Plain, the Western Foothills, the Longitudinal Valley, and the Yilan Plain were approximately 1000 -2000 m s -1 , which was lower than the Vp of 2500 -4000 m s -1 in the Central Mountain Range. In addition, the Vs in the plain areas were lower than that in the Central Mountain Range. The low Vp and Vs and high Vp/Vs ratio in the Coastal Plain and the Western Foothills can be attributed to the unconsolidated soil and high pore-fluid content of subsurface sediments in the plain areas. In contrast to the velocity distribution, low Qp and Qs were observed in the Central Mountain Range. The low Qp and Qs with low Vp/Vs and low Qs/Qp ratios in the Central Mountain Range was consistent with the high thermal temperature observed in the field investigation. The obtained velocity and attenuation structures near surface could also provide important constraints in validation of the crustal structure of Taiwan.
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