Page 27 - Sumatra Geology, Resources and Tectonic Evolution

14 CHAPTER 2

Center website within a few days (NEIC 2005). The suggested 115 km of the trench axis and subsidence at all greater distances.
maximum displacement was 15 m, in a region where convergence Instantaneous vertical movements of tens of centimetres associ-
is more nearly orthogonal to the trench than it is further south (see ated with large earthquakes were superimposed on this pattern.
Figs 2.1 and 2.4). Bizarrely, in view of this latter fact, the results Individual islands in the northern part of the forearc often record
from the only GPS site NW of the change of strike, on Pulau Babi similar tilting. Islets shown on Dutch colonial maps as protecting
(PB on Fig. 2.4), suggest that during the 1989-1993 period the Sinabang harbour, at the eastern end of the north coast of Simeulue
forearc moved slightly further in a direction parallel to the (S in Fig. 2.4), are now permanently submerged, and palm trees
trench than did the Indian Ocean, the supposed driver of the are dying along much of the coast as salt water invades the soil
forearc motion. It also seems that about half of the Indian Ocean around their roots. Muara Siberut, the main town on Siberut
trench-normal motion was accommodated between Pulau Babi (MS on Fig. 2.4), is regularly flooded at high tide and some
and Sumatra, which is less than at Enggano, but much more nearby offshore 'islands' consist entirely of mangroves with
than predicted by simple sliver-plate models. The motion of their roots submerged even at low tide.
Simeulue, a few tens of kilometres to the NW, might, of course, On Nias the situation is more complicated, since the west coast
have been different but there is no bathymetric or other evidence can be divided into two very different sectors. In the north the
for the placement by NEIC (2005) of an extensional (or any other) coastal region is flat and swampy and the beach is broad and
boundary to a 'Burma Plate' immediately east of Pulau Babi. gently sloping, but in the south there are cliffs 50-100 m high
Fault-plane solutions for the Simeulue earthquake are consist- and the sea floor shelves steeply. This section of the coastline is
ent with either SW-directed thrusting dipping at about 10 ~ to the concave seawards and appears to be a scarp created by failure of
NE or NE-directed reverse faulting dipping at about 80 ~ (NEIC an unstable slope (see Fig. 2.1). The relatively low gravity field
2005). The first of these is much the more likely, but thrusting along the coast and offshore (see Fig. 3.5) suggests loss of mass
on a surface so nearly horizontal, when the Benioff Zone dips at from this region and also supports the concept of failure of a
about 30 ~ in the vicinity of the hypocentre, raises some questions. slope that has been uplifted to unsustainable elevations. On
The Harvard Centroid Moment Tensor solution, however, places the opposite (eastern) side of the island, rivers have been incised
the centroid west of the forearc ridge and beneath the eastern in narrow valleys to depths of 5-10 m within a broad coastal
wall of the trench (at 3.09~ 94.26~ cf. the NEIC epicentre at plain east of the Mentawai Fault, suggesting recent and rapid
3.30~ 95.96E~ Since, subject to errors introduced by faulty vel- uplift, but further north there is evidence of both uplift and
ocity models, hypocentres correspond to points of rupture intiation subsidence.
whereas centroids represent weighted average locations of The uplift of the coastal plain on Nias could have been associ-
moment release (Meredith Nettles pers. comm. 2005), the results ated with great earthquakes. Zachariasen et al. (1999) interpreted
can be interpreted as describing an event initiated in the vicinity the results of a detailed study of coral heads exposed around
of the Mentawai Fault and propagating oceanwards and also the Mentawai Islands of Sipora and North and South Pagai,
NW along the forearc. The complexity of stress patterns in the epi- south of Siberut, as recording aseismic subsidence followed by
central area is indicated by the multiplicity of previous smaller co-seismic uplift related to the great earthquake of 1833. In this
shocks, some of which had strike-slip solutions and others sol- area, and in contrast to areas further north, both aseismic and
utions similar to that of the December 2004 event (see co-seismic movements appear to have involved tilting towards
Newcomb & McCann 1987, Fig. 2). The fact that the region the trench. Deducing long-term regional displacement patterns
around the Mentawai Fault appears to respond to stress in different from measurements of movements over a few years, or even
ways at different places and at different times is consistent with the over tens of years, is clearly never going to be a simple exercise.
fault itself being the expression of a fundamental geological dis-
continuity rather than a simple break through an essentially homo-
geneous rock mass.
The Simeulue event also spectacularly confirmed the extreme Note added in proof
segmentation of the forearc. Aftershocks occurred along 1200
km of the arc, from the site of the main shock as far as the northern The earthquake activity in the central Sumatra forearc between 26
tip of the Andamans, but there was virtually no activity to the SE December 2004 and the end of April 2005 is summarized in Figure
(NEIC 2005). The bathymetric high northwest of Simeulue where 2.8. The first four plots show how the seismicity associated with
the epicentre was located may therefore be the surface expression the 26 December event gradually died away during the succeeding
of a discontinuity similar to those associated with the Banyak and three months. It is clear that even as late as March 2005, the
Batu highs further south. The extents of Great Earthquake ruptures majority of events were part of the aftershock sequence.
are strongly correlated with the extents of deep marine basins However, on 28 March 2005 there was a further Great Earthquake,
between Sumatra and the forearc ridge and, given that the NW with an epicentre just west of the Banyak Islands and an estimated
limit of the rupture zone of the 1861 event was not at Simeulue magnitude of 8.6. The distribution of aftershocks to this event indi-
but at the Banyak Islands (Newcomb & McCann 1987), it seems cated that rupture extended throughout the whole of the region
possible that stress is still building up in a 'Simeulue Basin' between the Banyaks and the December 26 epicentre. It was, in
segment, to be catastrophically released at some time in the not fact, being quite widely predicted in the first few months of
too distant future. 2005 that this would be where the next break would occur.
However, and unexpectedly, the zone of aftershocks also extended
south as far as the Batu Islands (Fig. 2.8e). It seems therefore that
not only had the last remaining segment that had no historic record
Vertical movements of Great Earthquakes failed, but that the segment that ruptured in
1861 moved with it.
It is more difficult to monitor vertical movements with GPS than Fault plane solutions by both the NEIC and the Harvard group
horizontal movements, both because of the generally smaller indicated a shallow thrust, at an even smaller angle of dip than
displacements and because the accuracy is inherently lower. had been the case the previous December. Once again, movement
At present, more reliable estimates of rates of vertical motion seems to have been initiated close to where the Mentawai Fault
are being obtained by observing short-term changes in relative (assumed to be near vertical) would reach the subduction fault at
sea level. Natawidjaja et al. (2000) studied the submergence and depth, and once again there was a significant displacement
emergence of corals and deduced a pattern of progressive between the calculated positions of the epicentre and the centroid.
landward tilting of the forearc ridge, with uplift within about In this case, however, the centroid lay south rather than west of the