Page 26 - Sumatra Geology, Resources and Tectonic Evolution

SEISMOLOGY & NEOTECTONICS 13

Fig. 2.7. SIO Line 42-43, showing the Mentawai Fault
immediately south of Nias. Section provided by Scripps
Institution of Oceanography.

position within the forearc basin. This fact, and the image itself, the linearity of the fault is a consequence of the linearity of the
are more compatible with transcurrent than vertical motion. original subduction trace, which would, in turn, have been con-
Indeed, Schltiter et al. (2002) suggested that the transcurrent trolled by the linearity of the former passive margin.
function of the Sumatran Fault might be in the process of The significance of the Enggano composite earthquake to the
shifting to the Mentawai Fault. This is an attractive hypothesis backstop concept is that the GPS results shown in Figure 2.5 indi-
but difficult to reconcile with the suggestion by Sieh & cate that in this area, and possibly only for short periods, the accre-
Natawidjaja (2000) that the total offset on the Sumatran Fault is tionary wedge moves with the subducting plate and must therefore
rather small, despite the abundant evidence (including occasional compress against the backstop, resulting in folding and reverse
large earthquakes; Untung et al. 1985) for recent and continuing faulting. Potential energy stored in this folded and faulted zone
offsets along it. can be released in large earthquakes in which the wedge moves
A further complication is introduced by a possible relationship oceanwards and deformation near the backstop is reversed. Pre-
between the Mentawai Fault and the Batee Fault. The latter is a sumably such reversals are only partial, so that deformation gradu-
dextral splay from the Sumatran Fault that trends offshore ally increases. At no point in this stick-slip cycle would large
near the Banyak Islands and was interpreted by Karig et al. earthquakes necessarily occur within the wedge, because accreted
(1980) as displacing or terminating the Mentawai Fault near material is usually too weak to sustain large local stress. Large
Nias (Fig. 2.1). The Mentawai Fault is often shown as either earthquakes will therefore be associated principally with the
ending near Nias (e.g. Diament et al. 1992) or merging with the unsticking of the wedge from the backstop or from the downgoing
Batee Fault, but a very strong gravity gradient indicates a major slab along the main subduction thrust and with relative lateral
structural discontinuity between the two westernmost islands in movement between locked and unlocked segments of
the Banyak group (see Fig. 3.5). This is roughly the position the forearc. Events of both types appear to have occurred in
where a Mentawai Fault continuation would be expected if June 2000, with the movement between segments of the Indian
the Batee Fault were not present. Moreover, the existence of Ocean plate increasing the stress and triggering failure along
Mentawai-type structures still further north has been confirmed the subduction thrust (Abercrombie et al. 2003).
by Izart et al. (1994) and by Malod & Kemal (1996) using The results of future GPS measurements in the Enggano-Beng-
single-channel reflection data. kulu area (there have, unfortunately, been no measurements on
Additional insights into the role of the Mentawai Fault in Enggano since the earthquake) are thus likely to be very different
the Enggano area were provided in June 2000 by an Mw 7.9 earth- from those obtained between 1991 and 1993. Amongst other
quake followed by a train of strong aftershocks (Fig. 2.5). P and S things, they can be expected to provide insights into the highly
wave studies of the primary event suggested that this comprised controversial question of the extent to which trench-parallel
two subevents, involving strike-slip within the Indian Ocean motion is accommodated by the Mentawai Fault. It seems prob-
Plate followed by thrust motion on the subduction fault able that the new vectors will resemble the vectors shown in
(Abercrombie et al. 2003). The events were too deep, and in the Figure 2.4 for the islands north of Siberut, i.e. they will show
wrong plate, to be due to failure on the Mentawai Fault, but almost entirely trench-parallel motion, implying a primarily trans-
they do provide important data on its relationship to the transition current long-term function. The characteristics of both the main
between the accretionary wedge and the continental margin. earthquake and the extensive aftershock sequence suggest that
Matson & Moore (1992) suggested that this transition occurs effects of the Enggano Great Earthquake are unlikely to be seen
near the east coast of Nias in the Central Domain and that the in the forearc north of Bengkulu (Abercrombie et al. 2003), and
subduction fault originally reached the surface in this area. Its in fact no such effects have been observed in post-earthquake
subsequent migration oceanwards was interpreted as a conse- GPS studies in the Central Domain (Bock et al. 2003). If this is
quence of the development of the accretionary wedge that now the case, then dangerous levels of stress must be accumulating
forms the forearc ridge. This is consistent with the Malod & in the region from South Pagai to Siberut.
Kemal (1996) interpretation of the Mentawai Fault along its The June 2000 Enggano earthquake was completely oversha-
entire length as marking the transition between the wedge and dowed by the December 2004 Simeulue event, information on
a rigid backstop of pre-existing basement. On this hypothesis, which was posted on the National Earthquake Information