The June 2000 Enggano earthquake, South Sumatra, and the role of the Mentawai Fault
Abstract
Sumatra is widely regarded as a type example of strain-partitioning during oblique subduction. In the simplest view, arc-orthogonal motion is accomodated at the trench and arc-parallel motion is accomodated at the strike-slip Sumatra Fault System along the volcanic line. There is, however, a third, more enigmatic, feature that runs the length of the oceanward side of the forearc basin and defines, in many cases, steep eastern margins to the forearc islands. This 'Mentawai Fault' has been variously interpreted as a fold and as a fault with strike-slip, normal or high-angle reverse motion. Seismic reflection images vary considerably from place to place, and the role of the feature in the subduction process remains controversial. It is, however, significant that repeat GPS measurements in the vicinity of Enggano, the southernmost of the forearc islands, indicated that much of the convergence between the Indian Ocean and Southeast Asian plates was being absorbed within the forearc basin during the period of observation. The Mentawai Fault is the only feature observed on the seismic sections that appears capable of accomodating this convergence. The subduction zone beneath Sumatra has been the site of repeated large earthquakes, most recently in June 2000 when a Magnitude 7.9 earthquake occurred near Enggano followed by a strong aftershock sequence. Analysis of the main shock, using body and surface waves, indicates that there were two distinct sub-events occurring a few seconds apart. The first, involving strike-slip within the subducting Indian Ocean Plate, evidently triggered reverse slip on the subduction interface. Both sub-events were too deep, and in the wrong locations, to be due to failure on the Mentawai Fault. They do, however, provide important insights into its likely role. The GPS results indicate that in areas such as Enggano, and possibly only for short periods, the forearc wedge moves with the subducting plate. It must therefore compress against the 'backstop' of the forearc basin. Shortening must be achieved by folding or reverse faulting or both. When, however, a subduction earthquake occurs, the wedge moves oceanwards relative to the subducting slab. Deformation at the backstop must then be reversed, but presumably only partially, so that permanent deformation gradually increases. At no point in the stick-slip cycle would large earthquakes be probable at the interface between wedge and backstop (i.e. at the Mentawai Fault), because the accreted wedge would likely be too weak to sustain large local stress. Large earthquakes will therefore be associated principally with the episodic unsticking of the wedge from the downgoing slab along the subduction thrust and with relative lateral movement between stuck and unstuck segments along fracture zones. Events of both types appear to have occurred in June 2000. The results of future GPS measurements in the Enggano area are thus likely to be very different from those obtained so far.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....9923M