Application of Microfossils to Reconstruct a Paleoeseismic Record of the Sunda Subduction Megathrust, Northern Sumatra

The 26th December 2004 tsunami affected a large part of the Indian Ocean with 280,000 killed, 14,000 missing, and 1,100,000 people displaced. This disaster provided a horrific reminder of a practical problem: written and instrumental records do not span enough time to warn of the full range of earthquake and tsunami hazards in this region. To address this problem, we have begun to reconstruct the paleoenvironmental record for coastal northwestern Sumatra, utilizing foraminifera and pollen as proxies. The long-term goals of our project are (1) determine the number of great earthquakes and the average recurrence interval; (2) determine if subsidence occurs precursory to the main coseismic subsidence event; (3) determine if there is uplift after the main coseismic subsidence event that may be caused by slow, down dip afterslip on the megathrust and (4) quantify the relative timing of coseismic and interseismic vertical land level changes. We have collected litho-, bio-, and chronostratigraphic data from the northwest coast of Sumatra to resolve both rapid and gradual changes in relative sea level (RSL) and to begin building a taxonomic database of microfossils. The study sites, in Pulot and Seudu, Sumatra, are located approximately 20 km south of Banda Aceh in northwestern Sumatra. The sites are located on the coastal plain in near proximity to the shoreline. Both study sites are estuarine (or former estuarine), low energy depositional environments, which are protected from open-ocean wave attack and minimal fluvial input. Seudu is a site of shrimp ponds partially destroyed by the 2004 tsunami. Pulot is reclaimed agricultural land, probably a former Mangrove swamp. To date, we have found, at both the Pulot and Seudu sites, evidence of two regionally extensive organic-rich buried soils each overlain by a sand to silty-sand clastic deposit. The buried soils date to ~7,000 and ~6,000 cal yr BP and are separated by 1-2 meters of clastic sediment. Each buried soil is interpreted to represent subsidence from a large megathrust earthquake. The dominant mangrove pollen taxa in the organic buried soil strata is Rhizophora, which is replaced by very low counts of the dominate mangrove taxon Bruguira/Ceriops in the overlying clastic unit. Benthic foraminifera were only found in the overlying clastic unit. The foraminifera appear to represent an inner shelf environment, with taxa including several species of Elphidium, Ammonia parkinsoniana, and secondary species including Fissurina sp., and many species of Quinqueloculina and Bolivina. Based on these preliminary identifications of both the foraminifera and pollen, we interpret that the organic-rich buried soil hosted a mangrove environment, the buried soil-clastic contact represents a short-lived relative sea level rise, and the overlying clastic unit was deposited in an inner shelf environment.