Megathrust Earthquakes at the Western Sunda Arc of Indonesia

Darrel J. Drake

April, 2009

ES 767 Global Tectonics Emporia State University

Megathrust earthquakes are rare events for subduction zones, wherein the energy involved is accumulated over decades to centuries. A megathrust is the junction of two converging tectonic plates. One plate subducts beneath the other as they push against the other like a fault line, but on a large scale. The Western Sunda Arc has experienced 5 megathrust erthquakes measuring above 7.0 magnitude in the last five years and 10 in the last 175 years. Researchers anticipate the remaining two thirds of the arc area will experience megathrust earthquakes in the coming decades.

Image courtesy of Kerry Sieh, Tectonics Observatory, California Institute of Technology

Indonesia lies within an intricate collection of tectonic activity at the southern corner of the Eurasian Plate. The Pacific Plate borders to the east. To the south the Indo-Australian Plate converges with island arcs. The Sunda Arc Begins at the Andaman Sea. and stretches out in a sweeping curve, west to sast, along the south of Indonesia and finally bends north toward the Pacific Plate.

The eastern end of the Sunda Arc is the Banda Arc where the Australian Plate transition exhibits few or no deep earthquakes which indicates that deep trench subduction may not take place.The subducting slab is folded and causes bulging at the areal outer arc. According to McCaffery (1988), Seismology studies suggest the rock is being released out toward the eastern end of the Banda Arc. Strike-slip faults slide the Australian Plate to the east.

The Eastern Sunda Arc receives frontal subduction of the Indo-Austrailian Plate. Here tectonic activity can be categorized as average in comparison to other subduction zones. The less hindered thrust allows for an increased rate of subduction that accelerates in a westerly direction.

The Indo-Australian Plate approaches the Western Sunda Arc from an oblique position and slips past laterally in the form of strike faults. The Indian Ocean crust has a much different composition than the continental crust and the incresed density promotes a deeper subduction. Ocean sediment over the denser plate with an increased subduction rate amplifies build up of the accretionary prism thus producing greater uplift of the overlying Eurasian Plate.

The increased rate of subduction in the Western Sunda Arc also increases the torsion built in the rock formations of the overriding Eurasian Plate. A megathrust earthquake is the release of this energy produced by a sudden slip or rupture. Then a seismic recovery occurs (see figure below). The energy release is extremely destructive; X and above on the Mercalli Scale. Ocean crust and rock formations are shifted or uplifted. The crustal motion also pushes against the ocean water to produce tsunamis. The megathrust earthquake of 2004 near the Andaman Islands rupture zone reached 1600 km of the Sunda arc. The 2005 earthquake near Nias Island had a rupture zone of 400km.

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