EXTENDING PALEO-TSUNAMI RECORDS SOUTH OF THE 2004 INDIAN OCEAN TSUNAMI PATCH, SUMATRA, INDONESIA

Since the 2004 Indian Ocean tsunami, we have built a ~7500 year tsunami history for the northern patch of the Sunda Megathrust. However, little is known about paleo-tsunami activity south of the 2004 rupture patch. Here we use geological field methods to build a comprehensive paleo-tsunami record of the west coast of Sumatra, Indonesia. The geological record will be used to better define the boundaries of rupture patches along the Megathrust.

During a 2020 pilot study, we used satellite imagery to identify sites that were likely to archive evidence of co-seismic land-level change and tsunami deposits. We have trained a team of Indonesian geologists and devised methods for Singapore researchers to enable effective remote collaboration. Despite the challenges posed by Covid-19, the pilot study of topographic surveys, mapping of landforms and stratigraphical surveys, successfully identified six new sites on the west coast of Sumatra, including two sites located south of the 2004 tsunami inundation zone. We identified local paleo-tsunami deposits of anomalous beds of sand in low-energy environments where they would not normally occur, such as mangroves, peaty lowlands, and/or swales.

Here we present the results from one such site, at Susoh in Aceh province, approximately 90 km to the southeast of Meulaboh, the southernmost site previously investigated in Aceh. We collected cores to a depth of 4.75 m documenting the stratigraphy to identify potential tsunami layers. The top 2.5 meters is dominated by muds typical of quiet sedimentary settings, but it is interrupted by three pulses of sandy muds. At 2.55 m we encountered a 0.2 m thick layer of course sand, underlain by a 0.5 m thick mangrove peat with a gradual transition into organic sandy muds and sands. These sandy layers are reminiscent of the pulses of tsunami sands interrupting mangrove or wetland deposition, typical of other paleo-tsunami sites found further north on the same coastline. If confirmed to be of marine origin, these layers will expand our paleo-tsunami record geographically and lay the foundation for a better understanding of the paleo-tsunami history of this part of the coastline.