CORRELATION OF COASTAL GROWTH PATTERNS IN NORTHERN SUMATRA AND LARGE SCALE TECTONIC EVENTS ALONG THE SUNDA TRENCH

The December 2004 Sumatra-Andaman earthquake severely affected the morphology of the northern Sumatran coastline. Widespread coastal subsidence of about 1 m combined with erosion led to a complete destruction of the beach and displaced the coastline up to 500 m inland. We carried out an auto level survey and took up to 50 sediment cores to understand the long term coastal progradation and short term coastal recovery of a beach ridge plain north of Meulaboh, Sumatra, Indonesia. In addition to radiocarbon dates obtained from peaty marsh deposits, we sampled a number of beach ridges for optically stimulated luminescence (OSL) dating. The obtained field data is compared to satellite images, modern topographic maps, and Dutch colonial maps and yields evidence of coastal progradation over the last 1000 years and the response of the coast to large subduction zone earthquakes like the one in December 2004. Long term progradation rates of the beach ridge plain estimated from radiocarbon samples and OSL samples are in the order of 1-2 meters per year. Coastal progradation estimated from spatial data over the last 90 years, prior to the 2004 tsunami are 3-4 meters per year. During the 2004 tsunami the coastline receded ~200 meters, but a new prominent beach ridge built up within only 5 months indicating growth rates of 100 meters per year. Recently obtained data shows coastal growth in the order of 5 meters per year for the time period of April 2005 to March 2009. Our data shows that prominent, rapidly forming beach ridges potentially correlate with large earthquakes along the Sunda Trench subduction zone. Therefore, the coastal growth pattern of beach ridge plains can potentially serve as a tool in seismic hazard assessment.