THE PRE-2004 TSUNAMIS IN THE BAY OF BENGAL INFERRED FROM GEOLOGIC EVIDENCE IN THE ANDAMAN AND NICOBAR ISLANDS AND SOUTHERN INDIA

Geologic evidence from the Andaman and Nicobar Islands and Southern India suggest that the source area of the giant 2004 Aceh-Andaman earthquake previously generated major tsunamis. We identify the evidence for sea surges, in the Andaman and Nicobar Islands during the 5th-7th, 10th, and 15th centuries AD, by drawing analogy with the land-level change and tsunami deposits in 2004. Our investigations also identify the field evidence for smaller historical tsunamis like that of the 1762 and 1881 events. Proxies from the northern (800-km-long) rupture segment of the 2004 earthquake include subsided mangrove swamps, uplifted coral terraces, transported coral debris and sand deposited over humic paleosols. We classify them based on various discriminating criteria, including their reliability as an event proxy in relation to their depositional environment. The chronology of these proxies and their spatial distribution reveal the larger regional impact of a tsunami dated at AD ~10 century, which is also supported by the evidence from the coastal stratigraphy of the southeastern coast of India. Our data also suggests the probability of two other major events, in AD 15 (late medieval) and AD 5-7 centuries (early medieval), but their evidence is more localized and thus, not as compelling as that of the AD ~10 century tsunami, which may have been the greatest Aceh-Andaman earthquake of the past 1500 years or more.