Page 28 - Sumatra Geology, Resources and Tectonic Evolution

SEISMOLOGY & NEOTECTONICS 15

6 ~ ~ ~lc ~ : December 26 - Decemb'er 31, 2004 I .... ~ o "-, a,,,i,,,,~., 20osl
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...... ~ ..... ~ .............. April 10 - April;30, 2005
Fig. 2.8. Central Sumatra seismicity,
December 26 2004 to April 30 2005.
Epicentres plotted from catalogues available
on the Internet from http://
www.ngdc.noaa.gov. Note that the periods
covered (shown in the top fight hand corner
of each diagram) are not of uniform length,
being dictated in part by the dates of
initiation of significant earthquake swarms.
d The circles corresponding to the NEIC
epicentres of the two Great Earthquakes (in
2 the centroids of their Harvard CMT
. . . . . . . . . . . . . . . , ......................... ~,....; ............ ;.: ........ io~ plots a and e) are shaded and the locations of
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2~
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I ~) ~ ~ ...... .................... ,~ ~: , solution 'beachballs'.
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epicentre, and was still a considerable distance from the trench. A new train of events began still further south and just seaward
Also, and as might have been expected in view of the smaller mag- of Muara Siberut in the following weeks. There were a few rela-
nitude of the shock, and hence the probable smaller width of the tively weak shocks in this area in the period immediately after
slip zone, the displacement between centroid and hypocentre March 28 (Figure 28e), but the first major event (Mw=6.7) took
was considerably less than in December. The greater distance of place on April 10, and was followed three quarters of an hour
the centroid from the trench, together with the smaller magnitude, later by another strong (Mw=6.5) shock. Once again, the Menta-
may be sufficient explanation for the much smaller associated wai Fault appears to have controlled the location at which
tsunami, which was only about 3 metres high on exposed coasts failure was initiated. Both events were compressional but, in con-
of Nias and Simeulue and decreased rapidly in amplitude at trast to the two Great Earthquakes, the slip planes were much
more remote locations. It is also possible that submarine slides, steeper (from 30 ~ to 60~ There followed numerous weaker
which may have contributed to the destructive power the Decem- events in the same area but, again in contrast to the pattern associ-
ber wave, did not occur in March because of the absence of any ated with the Great Earthquakes, there was no significant rupture
remaining potentially unstable slopes. The aftershock sequence propagation (Fig. 28f). It is to be hoped that the earthquakes in
(Figures 2.8e and f) was notable for being much more tightly con- this isolated cluster will prove to be the last major events in the
strained to the region immediately beneath the forearc ridge than current phase of southward-propagating unzipping of subduction
had been the case following the December event. west of Sumatra.