Anomalous Attenuation of High‐Frequency Seismic Waves in Taiwan: Observation, Model and Interpretation

Abstract

AbstractHigh resolution maps of seismic attenuation parameters in Taiwan have been obtained from a modified “Multiple Lapse Time Window Analysis” (MLTWA). At most of the stations in porous sedimentary and highly faulted areas in Taiwan, the conventional modeling of MLTWA based on the scalar theory of radiative transfer in a half‐space with isotropic scattering fails to explain the spatio‐temporal distribution of the whole S‐wave train. Using Monte Carlo simulations of wave transport, we demonstrate that this anomalous energy distribution in the coda may be modeled by multiple anisotropic scattering of seismic waves. In addition to the scattering quality factor , we introduce a parameter (independent of ) which determines the angular redistribution of energy upon scattering (scattering anisotropy). We inverted the attenuation parameters , and g in three frequency bands (1–2, 2–4, and 4–8 Hz). Overall, Taiwan is more attenuating than most orogens with a mean effective scattering loss and a mean intrinsic absorption of 2.5 × 10−3 and 9 × 10−3 at 1.5 Hz, respectively. Scattering loss varies over more than one order of magnitude across Taiwan while absorption variations reach approximately 30%. The more attenuating zones are the Coastal Range and the Coastal Plain where scattering dominates over absorption at low frequency, and inversely at high frequency. These regions are also characterized by strong backscattering ( < −0.85) at 1.5 Hz and rather high ratio. We propose that the observed strong back‐scattering at low frequency, related to large impedance fluctuations in the crust, is induced by the presence of fluids.