SUBDUCTION ZONE STUDIES
B.L.N. Kennett, T Furumura [ERI, University of Tokyo]
Recordings of deep seismic events in the Pacific plate at stations on
the eastern seaboard of northern Japan show a low-frequency
(f<0.25 Hz) onset for both P and S waves followed by large,
high-frequency (f>2 Hz) later arrivals with a long coda. This
behaviour is not explained by a simple subduction zone model comprising
a high velocity plate with low attenuation.
Figure 1: Pattern of anomalous intensities from a deep event
beneath Russia extending along the entire eastern seaboard of japan.
The waveform behaviour is accompanied by anomalous intensity paterns
with significant ground motion well away from the epicentre as illustrated
in figure 1 for an event at 598 km depth below Russia.
From the analysis of observed broadband waveforms and numerical
simulation of seismic wave propagation in the Pacific subduction zone
we propose that the high-frequency guided waves traveling in the
subducting plate arise from the scattering of seismic waves by
heterogeneity in plate structure. Our preferred model of the
heterogeneity has elongated scatterers parallel to the plate margin
described by a von Karmann function with a down-dip correlation length
of about 10 km and much shorter correlation length of about 0.5 km in
thickness. The standard deviation of wavespeed fluctuations from the
averaged background model is about 2% (see Figure 2)
Figure 2:
2-D cross-section through the model of plate heterogeniety
with an illustration of the differences seen between seismograms
in the fore-arc region (green) and behind the volcanic front (orange).
This new heterogeneous plate model generates significant scattering of
seismic waves with wavelengths shorter than correlation distance in
thickness, but low-frequency waves, with long wavelengths, can easy
tunnel through such lamina structure. The result is
frequency-selective propagation characteristics with a faster
low-frequency phase followed by high-frequency signals with very long
coda.
In figure 3 we illustrate the character of the seismograms recorded at
station KMU in Hokkaido in the fore-arc region. We see the
characteristic feature of a low frequency onset for both P and S
followed by an extended train of high frequency waves. The offset
in time is approximately constant for events below 150 km deoth
Figure 3: Observed seismograms at KMU for different depths
of source in the subduction zone.
Simulations of the behaviour using 2-D finite difference calculations
are shown in figure 4. A reciprocal calculation allows the response for
many sources at ta single receiver to calculated in one run. Two
profiles are shown. Profile a-a' deploys sources in the seismicity
zone along the top of the slab. We see the appearance of a
lower-frequency precursor for source depths below 150km; note also the
complexity of the simulated S wavetrain. Profile b-b' takes a line of
sources across the slab and shows that the character of the seismograms
propagating through the simple stochastic waveguide model is not
strongly dependent on source position within the slab.
Figure 4:
Finite difference simulations of propagation to station KMU for
arrays of sources along (a-a') and across (b-b') the slab.
Three-dimensional simulations using the Earth Simulator supercomputer
for modeling of high-frequency seismic wave propagation in the Pacific
subduction zone including plate heterogeneity demonstrate clearly the
scattering waveguide effects for high-frequency seismic waves traveling
in the plate. The region of large intensity for the heterogeneous
model migrates away from the hypocenter into northern Japan with an
elongated zone along the Pacific coast, comparable to the observations
from deep events in the Pacific Plate.
Back to the beginning
Questions about this topic to Brian Kennett:
brian@rses.anu.edu.au
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