Anomalous attenuation of high-frequency seismic waves in Taiwan:
observation, model and interpretation
Abstract
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.