Near-Surface Attenuation and Velocity Structures in Taiwan from Wellhead and Borehole Recordings Comparisons

Abstract

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.