Modem vertical deformation above the Sumatran subduction zone: Paleogeodetic insights from coral microatolls

Coral microatolls from the coast and outer-are islands of Western Sumatra r etain a stratigraphic and morphologic record of relative sea-level change, which is due in large part to vertical tectonic deformation above the Sumat ran subduction zone. Low water levels, whose fluctuations produce measurabl e changes in coral morphology, limit the upward growth of the microatolls, Annual rings, derived from seasonal variations in coral density, serve as a n 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-are islands and two from the mainland coast, indicate that t he Mentawai Islands have been submerging at rates of 4-10 mm/yr over the la st 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 o ccurred over that time. Thus, most of the strain accumulated in the past fe w decades represents interseismic deformation that is recovered during eart hquakes. Elastic dislocation models using these submergence data suggest th at elastic strain is being accumulated in the interseismic period and that the subduction zone in this region is fully coupled.