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The Sumatran Plate Margin

The Indo-Australian plate is subducting along the Sunda trench at 65 mm/yr at Java to 50 mm/yr at the north tip of Sumatra. Convergence is nearly orthogonal to the trench axis along south of Java Island but is highly oblique along at southwest of Sumatra where strain is strongly partitioned into a dip-slip on the subduction interface and a dextral slip component on the overriding Sumatran plate margin. The Sumatran fault, traverses the hanging wall block of the Sumatra subduction zone, roughly coincident with the active Sumatran volcanic arc, accommodates significant amount of the strike-slip component of the oblique convergence.

I study both the large transcurrent Sumatran faultand the Sumatran subduction zone. Despite the Sumatran fault's ranking as one of the great strike-slip fault on Earth, it's high level seismicity and it's major role in active tectonic and seismic hazard of Southeast Asia has not well characterized. We construct a modern map of the active Sumatran fault on a scale large enough to discriminate major fault strands and discontinuities. McCaffrey (1991) calculated hypothetical trench-parallel extension rates for the block between the trench and the Sumatran fault. If one assumes that the extension rate is uniform along the entire Sumatran arc and that all of this extension is reflected by slip on the GSF, then the fault's slip rate should increase from about zero at the Sunda Strait to about 6 cm/yr at the northern tip of Sumatra. We have determined three slip rates along the fault, in order to test this model and to place additional quantitative constraints on the kinematics of the region (Natawidjaja and Sieh, in prep). Such determinations are relatively easy in Sumatra because thick pyroclastic deposits had been incised by river channels and offset along the Sumatran fault.

For the past few centuries, south of Equator, the giant earthquakes in 1833 (Mw~9) dominates the seismic record of the Sumatran subduction. North of Equator, the seismic record is dominated by the 1861 (Mw~8.5). In the intervening 70km section of the subduction interface, The 1935 event (Mw 7.7)is the principal event in the record of seismic strain relief. To understand the rule the giant earthquakes in the subduction processes and their seismic threat to surrounding big cities, including Jakarta, Singapore and Kuala Lumpur, we study the seismic sources for these giant subduction earthquakes as well as aseismic subduction processes during interseismic stages. We use massive corals, namely microatolls, that faithfully record both continuous and sudden vertical movements on the overriding plate

This page maintained by: Danny Natawidjaja, please send your comment to: danny@gps.caltech.edu