A 7500-year paleotsunami record from the coastal sediment of Sri Lanka and Indian Ocean trans-basin tsunami history.

Directly facing the Sumatra and Andaman segments of the Sunda subduction zone, Sri Lanka provides an ideal location to study the large trans-basin tsunamis that occur there. Its location close to the southern boundary of the northern cyclone belt, and the directional advantage of the southern and eastern coastlines makes Sri Lanka one of the best places in the eastern Bay of Bengal region for distinguishing tsunami and cyclone deposits. Although several paleotsunami records have been constructed for the region since the 2004 Indian Ocean Tsunami, most extend only to the last 3000 years, and discrepancies exist among records. Therefore, this study was carried out to construct a longer Holocene tsunami record for the western Bay of Bengal region and to develop a more robust record of large trans-basin tsunamis that occurred at the Sumatra and Andaman trenches, to understand the recurrence interval between events. Thirteen back-barrier environments spanning the southern and eastern coastline of Sri Lanka were selected, and 35 sediment cores (1–5 m long – Total over 100 m length) were extracted along landward transects. Also, one pit was excavated to study abrupt sand layers. After initial logging, 19 sediment cores were selected for further analysis due to visually observed abrupt event layers. Particle size was used as the primary proxy to confirm the observed abrupt sand layers. Sediment texture, sedimentary structures, chemical composition, magnetic susceptibility, and micofossils were used to distinguish marine overwash layers having potential tsunami origin. Stratigraphy and age were used to establish intra- and inter-site, as well as regional, correlations among layers to distinguish regional tsunamis. In addition to five 2004 tsunami sand layers, among the 29 identified ancient abrupt layers with marine origin, 20 were distinguished as high potential paleotsunami (T _HP ), and 06 were as medium potential (T _MP) tsunami layers deposited during nine events that occurred around 1100, 3000, 4000, 4600, 5700, and 7300 yrs BP (T _HP ), and 2000, 5100, 6300 yrs BP (T _MP ) based on local and regional-correlation analyses. Abrupt layers deposited during another two low-potential tsunami (T _LP ) events and other marine overwash events (OM) were also recognized by this study. In addition, published regional records show widely distributed, chronologically correlating evidence for a paleotsunami that occurred around 600 years BP, which was not observed at our study sites in Sri Lanka. Including the 2004 tsunami, when all 11 trans-basin events (excluding T _LP ) reported over the last 7500 yrs are considered, tsunami frequency in the Indian Ocean has increased during the last 1000 yrs and during the interval between 4000–5000 yrs BP. Increased seismicity between 500–1500 yrs BP in the Sunda trench correlates with increased tsunami frequency during the last 1000 yrs.