Department of Earth and Environmental Science

A Paleoseismic 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. The objective of the proposed research is to understand the long-term seismic behavior of the Sunda megathrust of northernmost Sumatra through a study that integrates forefront techniques of paleoseismology, geodesy, geochronology and paleoenvironmental reconstruction using microfossils. A subduction zone chronology of earthquakes and vertical land level changes will be established that extends through the Holocene. The reconstructed subduction zone history will be documented at the coast arcward of the inboard extent of the coseismic rupture, and will include investigation of several critical components that help define the nature of subduction zone tectonics at a convergent margin: (1) recurrence interval between earthquakes; (2) quantification and relative timing of coseismic and interseismic vertical land level changes; (3) documentation of any subsidence precursory to the main coseismic subsidence event; (4)  documentation of any uplift after the main coseismic subsidence event caused by slow, down dip afterslip on the megathrust; and (5) development of a chronology of tsunami that invades the Sumatran coast.

The proposed field sites have been chosen to allow the reconstruction of the best possible record of paleo subduction zone earthquakes and tsunami along the southern part of the plate margin that ruptured in December 2004. Because of the absence of offshore islands situated above the plane of rupture, work will be carried out on the adjacent coast to the east that subsided up to 2 m in the 2004 earthquake and probably sustained similar episodes of subsidence in earlier earthquakes.  The coastal wetlands preserve past sea-level changes, therefore yielding continuous coastal wetland stratigraphic sequences that span as much as the entire latter half of the Holocene. Through careful site selection and sampling, age determinations and microfossil analyses can be used to resolve both rapid and gradual changes in relative sea level at a resolution of ~10 cm, which in turn can be interpreted in terms of earthquake-induced rapid subsidence and gradual sea-level changes precursory to, and subsequent to, coseismic land level changes. These sea-level changes will help to constrain extent and magnitude of slip before and after the main earthquake.