Abstract
Seismograms from a dense, high-quality seismic network in Japan are
used to investigate the characteristics of the 26 December 2004 Sumatra-Andaman
and the 28 March 2005 Sumatran earthquakes. The onset of the P waveforms
are aligned through cross correlation, and a simple concept of back-projecting
seismic energy to a grid of potential source locations is applied.
The waveform alignment removes the effects due to lateral variations
in wave speed between the hypocenter and each station. To better
approximate the effects of three-dimensional heterogeneity for paths
originating from grid points away from the hypocenter, cross-correlation
results of the P waveforms from aftershocks are introduced. This
additional information leads to improved resolution of smaller-scale
features near many of the aftershocks by reducing wavefront distortion.
The back-projection analysis provides a quick assessment of the spatiotemporal
extent and variability of relative high-frequency energy release,
which can be translated into an estimate of the moment magnitude,
as well as an unparalleled view of high-frequency rupture propagation.
The results are, in general, consistent with those obtained from
more involved source inversion methods. The 2004 Sumatra-Andaman
earthquake released most energy in a region northwest of the Sumatra
island and the rupture extended to the northern Andaman islands,
about 1300 km from the epicenter. This northern portion of the rupture
radiated a considerable amount of energy, but there is little evidence
of slow slip. The 2005 event is imaged to have bilateral rupture
with northwestern slip occurring for about 50 s before it moved to
the southeast of the epicenter.
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