Abstract
The disastrous Sumatra-Andaman earthquake of 26 December 2004 was
one of the largest ever recorded. The damage potential of such earthquakes
depends on the extent and magnitude of fault slip. The first reliable
moment magnitude estimate of 9.0 was obtained several hours after
the Sumatra-Andaman earthquake, but more recent, longer-period, normal-mode
analyses have indicated that it had a moment magnitude of 9.3, about
2.5 times larger. Here we introduce a method for directly imaging
earthquake rupture that uses the first-arriving compressional wave
and is potentially able to produce detailed images within 30 min
of rupture initiation. We used the Hi-Net seismic array in Japan
as an antenna to map the progression of slip by monitoring the direction
of high-frequency radiation. We find that the rupture spread over
the entire 1,300-km-long aftershock zone by propagating northward
at roughly 2.8 km/s for approximately 8 minutes. Comparisons with
the aftershock areas of other great earthquakes indicate that the
Sumatra-Andaman earthquake did indeed have a moment magnitude of
9.3. Its rupture, in both duration and extent, is the longest ever
recorded.
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