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The "Simeulue Saddle" and Rupture Overlap in the 2002, 2004, and 2005 Sunda Megathrust Earthquakes

A. MELTZNER, Tectonics Observatory, Div. of Geological and Planetary Sciences, California Institute of Technology, meltzner@gps.caltech.edu; R. BRIGGS, Tectonics Observatory, Div. of Geological and Planetary Sciences, California Institute of Technology, briggs@gps.caltech.edu; K. SIEH, Tectonics Observatory, Div. of Geological and Planetary Sciences, California Institute of Technology, sieh@gps.caltech.edu; A. KONCA, Tectonics Observatory, Div. of Geological and Planetary Sciences, California Institute of Technology, ozgun@gps.caltech.edu; Y. HSU, Tectonics Observatory, Div. of Geological and Planetary Sciences, California Institute of Technology, yaru@gps.caltech.edu

The December 2004 and March 2005 Sunda megathrust earthquakes nucleated northwest and southeast of Simeulue island, respectively, and each ruptured bilaterally into the 100-km-long island. Uplift at the northwestern tip of Simeulue was 1.5 m during the 2004 earthquake, and uplift at the southeastern tip in 2005 was 1.5 m or more. Uplift associated with each earthquake diminished toward the center of the island, as did slip on the underlying megathrust, according to slip inversions. Cumulative uplift was as little as 0.5 m along the west coast of central Simeulue. Hence, although the 2004 and 2005 uplifted regions overlap, there is an uplift deficit, or saddle, on central Simeulue. Rupture during an M 7.3 earthquake in 2002 produced up to ~20 cm of uplift in central Simeulue, near the sites of lowest uplift in 2004-2005, but even including that uplift, the saddle persists. No events similar to 2002 can be found in the historical record for at least the previous 94 years. The occurrence of the 2002, 2004, and 2005 earthquakes, their relative locations and timing, and their associated patterns of uplift raise important questions about rupture boundaries and earthquake triggering. Why didn’t the 2004 earthquake continue southward to encompass the future 2005 rupture? Why didn’t the 2002 earthquake continue farther onto the 2004 or 2005 rupture planes? It does not appear that the 2002 and 2004 ruptures extended into regions of recent high slip. Could the rupture terminations have been controlled by permanent structural boundaries? Along the west coast of Simeulue, the 2004 and 2005 tsunami run-up heights did not exceed several meters, but an M ~7.6 earthquake in 1907 is associated with a tsunami that was significantly larger there. This, coupled with high 1907 intensities on Nias island, may indicate that the 1907 earthquake involved substantial slip on a portion of the megathrust updip of the 2005 rupture, west of Simeulue and Nias. Future work is required to determine whether the 1907 rupture transgressed the 2002, 2004, and 2005 rupture boundaries. We will discuss these questions and share preliminary findings, including a slip inversion for the 2002 earthquake.

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Last Modified: 2011 Aug 10

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