A Biostratigraphical Record of Repeated Great Earthquakes on the Sunda Subduction Megathrust, Northern Sumatra

The 2004 Andaman-Aceh earthquake and tsunami focused global attention on the northernmost part of the Sumatran subduction zone. Important questions were raised about the past geologic history of the Sunda megathrust offshore of northernmost Sumatra. In response, we have collected stratigraphic data from the northwest coast of Sumatra toward the following goals (1) determining the recurrence interval between great earthquakes; (2) quantifying the relative timing of coseismic and interseismic vertical land level changes; (3) documentation of subsidence precursory to the main coseismic subsidence event; (4) documentation of uplift after the main coseismic subsidence event caused by slow, down dip afterslip on the megathrust; and (5) development of a chronology of tsunami invading the Sumatran coast. In order to achieve these goals, we have begun analysis of the coastal wetlands of northwest Sumatra. Through careful site selection and litho- and biostratigraphical analyses, we aim to resolve both rapid and gradual changes in relative sea level at a resolution of ~10 cm. These data will help characterize earthquake-induced rapid subsidence and gradual sea-level changes precursory to, and subsequent to, coseismic land level changes. Preliminary results suggest two great earthquake events have occurred within the late Holocene, prior to 2004 Andaman-Aceh earthquake. The signatures of two older events, located approximately 2.5 and 3.5 m below mean sea level, are each recognized by the presence of a regionally extensive buried organic soil overlain by a medium to coarse grained quartz sand unit, interpreted as a tsunami deposit. Calcareous foraminifera are present in every lithostratigraphic unit, with noticeable assemblage changes between the medium to coarse grained quartz sand units and the underlying buried soil deposits. The 2004 event is also documented by a buried soil and a surficial coarse grained quartz sand tsunami deposit.