Modern Vertical Deformation above the Sumatran Subduction Zone: Paleogeodetic Insights from Coral Microatolls
Coral microatolls from the coast and outer-arc islands of Western Sumatra retain a stratigraphic and morphologic record of relative sea-level change, which is due in large part to vertical tectonic deformation above the Sumatran subduction zone. Low water levels, whose fluctuations produce measurable changes in coral morphology, limit the upward growth of the microatolls. Annual rings, derived from seasonal variations in coral density, serve as an internal chronometer of coral growth. The microatolls act as natural long-term tide gauges, recording sea-level variations on time scales of decades. Field observations and stratigraphic analysis of seven microatolls, five from the outer-arc islands and two from the mainland coast, indicate that the Mentawai Islands have been submerging at rates of 4–10 mm/yr over the last four or five decades, while the mainland has remained relatively stable. The presence of fossil microatolls up to several thousand years old in the intertidal zone indicates that little permanent vertical deformation has occurred over that time. Thus, most of the strain accumulated in the past few decades represents interseismic deformation that is recovered during earthquakes. Elastic dislocation models using these submergence data suggest that elastic strain is being accumulated in the interseismic period and that the subduction zone in this region is fully coupled.
Additional Information© 2000 Seismological Society of America. Manuscript received 27 January 1998. This work was supported by NSF grant NSF EAR-9628301. We are very grateful to John Beavan and Tim Melbourne for help with the elastic dislocation modeling. Mark Simons, Mike Gurnis, and Joann Stock offered valuable assistance and helpful comments. We thank Anne Lilje and Tony Soeller for computer and GIS assistance and Jim O'Donnell for library assistance. Lisa Grant, Yoko Ota, Paolo Pirazzoli, and an anonymous reviewer provided reviews that greatly improved this manuscript. We also thank the owner and crew of the Dani Putra, our Indonesian field vessel: Daniel Madre, Tobing, Doni, Bambang, Rapit, and As; and LIPI field assistants: Nono, Anto, Arief, Sri, and Dudi. Caltech Seismological Laboratory contribution number 8486.
Published - Zachariasen2000p897_Bull_Seis_Soc_Am.pdf