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.

Project Report

The loss of more than a quarter million lives on December 26, 2004 revealed tragic ignorance of Indian Ocean earthquake and tsunami history. Written history had failed to forewarn of the possibility of a giant Sunda magathrust event and resulting tsunami. Based upon our work it now appears that the geologic record of the northernmost region of the Sumatran subduction zone could have provided a warning: newly discovered buried soils on the Sumatran coastal plain contain evidence for Holocene ancestors to the 2004 Andaman-Aceh earthquake. Our study reveals paleoearthquakes and concurrent tsunamis through a study that integrates a variety of complementary disciplines within the Earth Sciences. Stratigraphical technique have identified paleoearthquakes and estimated the amount of coseismic subsidence in the temperate environments such as Alaska, Cascadia, Chile, Japan, and New Zealand. However, this technique has not been applied before in Sumatra or other tropical environments because of the many challenges that threaten the preservation of suitable sediment; we address this problem. Acceptance of stratigraphy as a paleoseismic tool in Sumatra will be vital for studies wishing to determine recurrence intervals of megathrust earthquakes, assess the amount of coseismic subsidence during paleo-subduction zone earthquakes, and constrain dimensions of paleorupture extent at the down dip end of rupture. Our work complements the investigations of microatoll records of sea-level changes. These investigations focused on the outer arc islands that overlie the part of the megathrust that typically ruptures and that, for the most part, uplifts in megathrust earthquakes and then subsides. Such an approach is not viable further north at the latitudes where we have studied in this project, because there are no large islands offshore positioned above the 2004 rupture area. We have presented the first and oldest evidence of a paleoearthquake using coastal stratigraphy from Aceh, Sumatra. We reveal rapid changes in relative sea level that are indicative of coseismic subsidence produced during a egathrust earthquake. The buried soil is overlain with a tsunami deposit. Radiometric dating constrain the earthquake to 6500-7000 cal. yrs. BP. Using a new multiproxy, micro- and macrofossil approach, we estimated coseismic subsidence to be up to 1.0 m, which is comparable to estimates of 0.6 m for 2004 Aceh-Andaman earthquake. We have published two peer reviewed publications and 12 conference proceedings with more to follow. The project has supported graduate and undergraduate students, and postdoctoral fellows. Candace Grand Pre uccessfully defended her PhD at the University of Pennsylvania in April 2011 and is now an Assistant Professor at Franklin and Marshall. Tina Dura completed her Masters at Central Washington University in 2010 and is now a PhD student at the University of Pennsylvania. Andrea Hawkes and Simon Engelhart were both post doctoral fellows at the University of Pennsylvania and are now Assistant Professors at the University of North Carolina and Rhode Island, respectively. Our study of great earthquakes on the Sunda subduction megathrust, Northern Sumatra has immediate and ongoing societal value in resilient and sustainable coastal communities of Southeast Asia. Our research has improved local awareness of hazard and preparation to meet the hazard of magathrust earthquakes and tsunamis. We have affiliated with a local host institution in Aceh, Syiah Kuala University. We have exchanged knowledge nd resources that can be used in planning and teaching. We have provided raining to Indonesian geologists in techniques of paleoseismological research specific to the tropical environment and specific to the Sunda subduction zone. We have produced posters and given talks to heighten understanding and awareness of the hazard and provide suggestions for actions that can save lives during earthquakes.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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David Fountain
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University of Pennsylvania
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