Page 20 - Sumatra Geology, Resources and Tectonic Evolution

Chapter 2

Seismology and neotectonics

JOHN MILSOM

Sumatra is an active (Andean) continental margin that would be described by Sieh & Natawidjaja (2000) as a homocline and by
linked by land to SE Asia if sea level fell by as little as 50 m. Karig et al. (1980) as a 'fault-flexure'.
Present-day tectonic processes are controlled by three major Magnetic anomalies in the Indian Ocean south of Sumatra
fault systems, the most obvious of which is the subduction thrust trend east-west and were interpreted by Sclater & Fisher (1974)
which crops out in the Sunda Trench. The trench curves very as indicating Palaeogene ages for most of the crust adjacent to
little in the 800 km between Enggano and Nias, i.e. off central the trench, with a possibility of Late Cretaceous crust in the
Sumatra (Fig. 2.1), but is markedly convex towards the Indian extreme SE. Transforms such as the Investigator Fracture Zone,
Ocean both further north and further south. Water depths of which may offset the anomalies by several hundred kilometres,
more than 6000 m are reached in the south but the maximum in run almost precisely north-south. With the trend of the trench
the north may be less than 5000 m. The difference is usually, varying from N40~ to N60~ and the direction of the Indian
and convincingly, attributed to the presence on the Indian Ocean Ocean-Sumatra convergence vector being about N15~ (Fig. 2.1),
plate of the Nicobar Fan, consisting of sediments, derived Sumatra has long been recognized as a key area for studies of
ultimately from erosion of the Himalayas, which increase steadily the partitioning of strain between thrust and transcurrent faults
in thickness towards the north (e.g. Hamilton 1979). Continuing during oblique convergence (Fitch 1972; McCaffrey 1992, 1996;
subduction is attested by a Wadati-Benioff Zone (WBZ) that Malod & Kemal 1996). The suggestion, originally made by
extends to depths of the order of 200 km (e.g. Newcomb & Fitch (1972), that the oblique motion is to a first approxima-
McCann 1987) and by volcanic activity in the Barisan mountains, tion accommodated by orthogonal subduction at the trench and
the peaks of which generally lie within a few tens of kilometres dextral slip along the Sumatran Fault, is now widely accepted.
of the coast. The change, of more than 45 ~ in the trend of the To the extent that this is true, the forearc region must be decoupled
trench between 96~ and 97~ (the 'Nias Elbow') may have from both the Indian Ocean and Eurasia. The commonly used
been initiated by subduction of the 2 km high Investigator Ridge term 'sliver plate' (e.g. Curray 1989) suggests more strength and
(Investigator Fracture Zone), which trends approximately north- rigidity than could reasonably be expected of such a long and
south at about 98~ Sieh & Natawidjaja (2000) defined a narrow strip of lithosphere, and any analysis of subduction
'Central Domain' of mainland Sumatra between the Nias Elbow beneath Sumatra must take into account the probability of inde-
and the ridge intersection as anomalous in a number of ways pendent movements of forearc fragments (e.g. McCaffrey 1991).
(notably in the differing trends of the Sumatran Fault and the Estimates of the movements of the Indian Ocean relative
volcanic line) and as distinct from more regular Northern and to Sumatra are shown in Figures 2.1 and 2.4. Changes in magni-
Southern domains on either side (Fig. 2.1). tude and direction from NW to SE are dictated by the East
Inland, the dextral transcurrent Sumatran Fault runs the entire African location of the pole of rotation (Larson et al. 1997). If par-
length of the island, from Banda Aceh to the Sunda Strait titioning of orthogonal and transcurrent strain between, respect-
(Fig. 2.1). A variety of names have been used for both the ively, the trench and the Sumatran Fault were complete (and
overall fault system and parts of it, and new nomenclature movement occurred only along these features), then sites in the
developed by Sieh & Natawidjaja (2000) divided it into 19 indi- forearc sliver would move parallel to the Sumatran Fault relative
vidual segments. Even this detailed study failed to answer many to SE Asia, but at right angles to the trench relative to the
fundamental questions, and estimates of total lateral displacement Indian Ocean. Trench-normal relative motion implies that the
still vary from several hundred kilometres to as little as twenty forearc sliver 'tracks' across linear features on the Indian Ocean
kilometres. The 150km suggested by McCarthy & Elders Plate, such as the Investigator Fracture Zone, which have north-
(1997) seems to be about the mean of the published values. The south trends (Fig. 2.1). If the long term movement between the
fault trace coincides roughly with the watershed of the Barisans forearc and the Indian Ocean has actually been approximately
and with the volcanic line, although most of the volcanoes lie orthogonal, the intersection point of the Investigator Fracture
somewhat to the NE of the fault and only nine of the fifty youngest Zone with the trench, now near the Batu Islands, would have
centres lie within 2 km of it (Sieh & Natawidjaja 2000). A more been north of Nias less than 10 million years ago. The relief, of
precise correlation is with the subduction thrust, since for most more than 2 km, on the Investigator Fracture Zone might not
of its length the distance between the Sumatran Fault and the only impede such tracking but could be responsible for cyclical
trench axis differs by no more than 30 km from the average uplift and subsidence in the forearc basin and ridge.
value of 290 km. The largest deviations are a narrowing within Slip partitioning and subduction of Indian Ocean lithosphere
the bight of the Nias Elbow and a broadening in the region produce high levels of seismicity in the Barisan Mountains, in
further to the NW. the forearc basin and along the forearc ridge (Fig. 2.2). The poten-
The third and most enigmatic of Sumatra's major fault systems tial for extremely destructive earthquakes was most recently
is the Mentawai Fault, at the outer margin of the forearc basin demonstrated by the Magnitude 9 event near Simeulue in Decem-
(Fig. 2.1). In many publications the name is reserved for the ber 2004 and by the resulting tsunami, which gave rise to one of
segment extending from the Sunda Strait to Nias (Samuel & the worst natural disasters in recorded human history. However,
Harbury 1996) or the Batu Islands (Diament et al. 1992), but the and despite the geological evidence for a long history of subduc-
same disturbance zone continues at least as far as the Andaman tion (e.g. Page et al. 1979), shocks deeper than 200 km are rare
Sea (Malod & Kemal 1996) and possibly to the Andaman and (Fig. 2.3). Events below 300 km are confined to the extreme SE
Nicobar Islands. Movement has been variously interpreted as and may be associated with north-directed subduction beneath
normal, strike slip or reverse (Sieh & Natawidjaja 2000). There Java rather than NE-directed subduction beneath Sumatra. The
are considerable changes in appearance on seismic sections abrupt change in orientation of the active margin between these
even within the region from Nias southwards; the structure was two islands must produce considerable stress in the downgoing