冰島地幔熱柱的波速影像：波成像與波傳遞
Date Issued
2004
Date
2004
Author(s)
DOI
922119M002013
Abstract
Tomographic models based on hypothetically infinite frequency ray interpretation of
teleseismic travel time shifts have revealed a region of relatively low P and S wave speeds
extending from shallow mantle to 400 km depth beneath Iceland. In reality, seismic waves
have finite frequency bandwidths and undergo diffractive wave front healing. The
limitation in ray theory leaves large uncertainties in the determinations of the magnitude
and shape of the velocity anomaly beneath Iceland and its geodynamic implications. We
developed a tomographic method that utilizes the banana-shaped sensitivity of finite
frequency relative travel times from the paraxial kernel theory. Using available seismic
data from the ICEMELT and HOTSPOT experiments, we applied the new method to
image subsurface velocity structure beneath Iceland. Taking advantage that the sensitivity
volume of broadband waveforms varies with frequency, we measured relative delay times
in three frequency ranges from 0.03 to 2 Hz for P and 0.02 to 0.5 Hz for S waves. Given
similar fit to data, the kernel-based models yield the root-mean-square amplitudes of P
and S wave speed perturbations about 2–2.8 times those from ray tomography in the
depths of 150–400 km. The kernel-based images show that a columnar low-velocity
region having a lateral dimension of 250–300 km extends to the base of the upper
mantle beneath central Iceland, deeper than that resolved by the ray-based studies. The
improved resolution in the upper mantle transition zone is attributed to the deeper crossing
of broad off-path sensitivity of travel time kernels than in ray approximation and
frequency-dependent wave front healing as an intrinsic measure of the distance from
velocity heterogeneity to receivers.
Subjects
finite frequency travel time
tomography
wave front healing
Fre´chet kernel
Iceland
Publisher
臺北市：國立臺灣大學地質科學系暨研究所
Type
report
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