THE COMBINED APPLICATION OF FORAMINIFERA AND POLLEN TO RECONSTRUCT PALEOEARTHQUAKES IN NORTHERN SUMATRA

The 2004 Sumatra-Andaman earthquake and tsunami was a devastating reminder that written and instrumental records are not adequate 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.

Our stratigraphical investigations from two coastal lowlands (Seudu and Pulot) of northwest Sumatra reveal evidence for an early Holocene precursor to the 2004 Aceh-Andaman earthquake. At both study sites, the organic-rich buried soil is dominated by a Rhizophora and Ceriops pollen assemblage, which suggests a mangrove environment. The soil is overlain by a thin (5-10 cm) coarse-grained clastic unit identified by a reduction in organic carbon and an increase in grain size. The pollen assemblages of the sand unit reveal an increase in species diversity with a mixture of freshwater and mangrove taxa. Foraminifera are dominated by shallow marine taxa such as Asterorotalia millettii, Pararotalia venusta, Operculina sp., and Pararotalia domantayi. The chaotic pollen and foraminifera assemblages of the sand unit are indicative of a tsunami because these high energy events inundate coastal and inland areas transporting, and redepositing deep water, estuarine and freshwater sediment. The microfossils of the overlying silty-sand clastic unit indicate a transition into a subtidal/estuarine environment with the introduction of foraminiferal taxa that occupy shallow water depths such as Quinqueloculina sp. and Triloculina sp. The pollen assemblage consists of freshwater spores and mangrove taxa but in low concentrations.

The buried soils hosted a mangrove environment that formed at approximately mean high high water (MHHW) during a period of interseismic deformation. The soil was abruptly overlain by a thin clastic unit of tsunami origin. Radiocarbon dates from the sharp upper contact of the buried soil suggest this rapid sea-level rise event, associated with coseimic subsidence, occurred at c. 7 cal kyr BP. The tsunami deposit is overlain by subtidal/estuarine deposits forming below mean tide level (MTL), suggesting a coseismic subsidence of at least 0.85 m. Further interseismic strain and gradual sea-level fall over a period of c. 1000 years returned the depositional environment to a mangrove.