Collaborative Research: A Paleoseismic Record of Great Earthquakes on the Sunda Subduction Megathrust, Northern Sumatra

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The loss of more than a quarter million lives on December 26, 2004 revealed the susceptibility of the Indian Ocean region to devastating earthquakes and tsunamis. Based upon preliminary fieldwork it now appears that newly discovered buried soils on the Sumatran coastal plain of northernmost part of the Sumatran subduction zone contain evidence for Holocene (10,000 years ago to the present) predecessors to the 2004 Andaman-Aceh earthquake. A research team from Central Washington University, Humboldt State University, the University of Pennsylvania, and the Indonesian Institute of Sciences and Research Center for Geotechnology are studying these deposits in order to understand and chronicle the Holocene seismic behavior of the Sunda megathrust of northernmost Sumatra. Through integration of paleoseismology, geodesy, geochronology, and paleoenvironmental reconstruction using microfossils, the team will establish a subduction zone chronology for the northernmost past of the Sumatran subduction zone offshore of northern Sumatra going back approximately five thousand years. The results will chronicle earthquake-induced rapid subsidence and gradual sea-level changes precursory to, and subsequent to, the coseismic land level changes. The documented relative sea-level changes will help constrain the extent and magnitude of slip before, during and after the main subduction zone earthquake. A long-term record of subduction zone earthquakes that spans at least three or four earthquake cycles will provide realistic estimates of the average time between earthquakes and tsunamis similar to the earthquake and ensuing tsunami that struck the west coast of northern Sumatra in December 2004. Preliminary work demonstrates that a paleoearthquake record is present in coastal wetland stratigraphy and that wetland sediment contains both datable material and abundant microfossils that will permit reconstruction of the timing and magnitude of vertical land level changes at the coast that precede, accompany and follow subduction zone earthquakes. Written history had failed to forewarn of the possibility of a giant Sunda megathrust earthquake and the resulting tsunami of December 26, 2004. Based on preliminary work, the research team has identified tropical environments where the record of former megathrust earthquakes is most likely to form and endure. These deposits will yield a long-term record of subduction zone earthquakes over thousands of years and will provide realistic estimates of the average time between earthquakes and tsunamis such similar to the 2004 earthquake and ensuing tsunami. Results from this project will be disseminated within the Sumatran coastal communities and with Sumatran and national governmental agencies. These results will establish that earthquake and tsunami are recurrent events that should be anticipated and that require disaster planning.