Slip vectors from thrust earthquakes at the Java Trench southwest of Sumatra demonstrate that the Sumatran fore arc is not a single rigid plate that is translated to the northwest by oblique plate convergence. Instead they indicate arc-parallel stretching of the fore arc at a uniform strain rate of 3-4 x 10-8/yr. The northwestward motion of the fore arc relative to the upper plate (Southeast Asia) increases from near zero at the Sunda Strait to 45-60 mm/yr in northwest Sumatra and should result in variable slip rates on the Sumatran fault.

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