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Anomalous attenuation of high-frequency seismic waves in Taiwan: observation, model and interpretation
  • Marie Calvet,
  • Ludovic Margerin,
  • Shu-Huei Hung
Marie Calvet
Paul Sabatier University - Toulouse III

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Ludovic Margerin
Paul Sabatier University - Toulouse III
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Shu-Huei Hung
National Taiwan University
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High resolution maps of seismic attenuation parameters in Taiwan have been obtained by using 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 wavetrain. Using Monte Carlo simulations of wave transport, we demonstrate that this anomalous energy distribution in the coda may be modelled by multiple anisotropic scattering of seismic waves. In addition to the scattering quality factor Qsc, we introduce a parameter g (independant of Qsc) which determines the angular redistribution of energy upon scattering (scattering anisotropy). We determine the attenuation parameters Qsc-1, Qi-1 and g in three frequency bands (1-2, 2-4 and 4-8Hz). Overall, Taiwan is more attenuating than most orogens with a mean effective scattering loss (Qsc*)-1=Qsc-1(1-g) about 0.025 and a mean intrinsic absorption Qi-1 about 0.009 at 1.5Hz. Scattering loss (Qsc*)-1 varies over more than one order of magnitude across Taiwan while absorption fluctuations are about 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 (g<-0.85) at 1.5Hz and rather high VP/VS ratio. We speculate that the observed strong back-scattering at low frequency is related to strong impedance fluctuations in the crust induced by the presence of fluids.