| Corals from Western Sumatra retain stratigraphic records of relative sea-level change that can be used to infer vertical displacement rates in the hanging wall block of the Sumatran subduction zone. The upward growth of the coral "microatolls" is limited by low water levels, and fluctuations in water level produce measurable changes in the coral morphology. Furthermore, the corals contain annual growth rings, which act as an internal chronometer of coral growth history. The microatolls, which are widespread and can live for decades or centuries, can serve as natural long-term tide gauges, recording sea-level variations over wide intervals of space and time.; Relative sea-level records from living corals from the outer-arc islands and mainland coast constrain the nature of recent vertical deformation over the subduction zone. Emerged fossil corals from the islands, dated with U-Th geochronometry, constrain the paleoseismic and paleogeodetic history of the region.; Stratigraphic analysis of cross-sectional slabs cut from living coral heads reveals that the 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. Many fossil corals died in the early 1800's. Their age and morphological signature indicate they died as a result of coseismic uplift of more than 1m during the last great subduction-zone earthquake in this region, in 1833, following decades of interseismic submergence at rates similar to modern rates.; Other sampled corals died at earlier times throughout the late to mid-Holocene. Their presence in the intertidal zone suggests that little permanent vertical deformation has occurred here over the past several thousand years, and, therefore, most of the accumulated interseismic strain is recovered during earthquakes. The temporal distribution of coral deaths suggests an average earthquake recurrence interval for subduction zone events of about 230 years. Combining sea-level histories from modern, 1833, and older corals yields partial records of co-and interseismic vertical displacement for multiple earthquake cycles. Such displacement records from the hanging wall of a major plate boundary can help constrain models of subduction-zone deformation. |
