MEGATHRUST EARTHQUAKES ALONG THE JAVA TRENCH: DISCOVERY OF POSSIBLE TSUNAMI DEPOSITS IN THE LESSER SUNDA ISLANDS, INDONESIA

The known historical record does not document any megathrust earthquakes in the Eastern Sunda Arc along the Java Trench for the past 400 years. We investigated the geological record of the south coasts of islands throughout the eastern Sunda Arc for tsunami deposits to test if they are preserved and if so, how far along strike they may correlate. Based on the findings at 37 sites in Bali, Lombok, Sumba and Timor, 2 layers of candidate sands and other marine material interbedded with mud are consistently present at depths of around 1 and 2 meters, which also may correlate with similar layers found in Java (Deng et al., 2016). Five imbricated boulder deposits were also found throughout the region. The size of the boulders range up to 4 x 3 x 1 meters.

Analysis of the samples are in progress. However, the consistent stratigraphic depth across greater then 1000 km of orogenic strike suggest at least 2 mega tsunami events in the last millenia.

Several outreach experiments were conducted during the field investigation that demonstrate a lack of awareness of tsunami hazards by those most at risk. Several different approaches to community-based mitigation were tested and found to vary with local culture and religion.