HR: 17:45h
AN: S24A-08    [Abstracts]
TI: Predecessors of the 2004 Indian Ocean Tsunami: Inferences Based on Historical, Archeological and Geological Evidence From the Indian Coast and the Andaman-Nicobar Islands
AU: * Rajendran, C
EM: cprajendran@gmail.com
AF: Centre for Earth Science Studies, Geosciences Division, Trivandrum, 695031, India
AU: Rajendran, K
EM: kusala@civil.iisc.ernet.in
AF: Indian Institute of Science, Centre for Earth Sciences, Bangalore, 560 012, India
AU: Machado, T
EM: terry.machado@gmail.com
AF: Centre for Earth Science Studies, Geosciences Division, Trivandrum, 695031, India
AB: The 2004 tsunami is an unprecedented event in the Indian Ocean. Never in the recent or distant history of the region has such a transoceanic event of devastating proportion is known to have been reported. Obviously, apparent lack of historical references in the culturally ancient Southeast Asian region suggests rarity of such events. Therefore, a major question that has been posed since the 2004 tsunami is whether similar events have occurred in the region in the past. If there are predecessors, what is the frequency of such events? Resolving this question is of crucial importance in developing the recurrence history of megathrust earthquakes and assessing the tsunami hazard of the region. Our strategy has been to tackle this problem using historical and archeological data, combined with geological investigations in the affected regions of the Indian coast, including the Andaman- Nicobar Islands. Citations from south India on ancient tsunami include classic Tamil texts, which mention about a devastating sea surge around A.D. 950 in the southeastern coast of India. Our studies were focused on two ancient port cities on the east coast of India: Mammallapuram and Kaveripattinam, the latter being a major township during the first millennium. The 2004 tsunami had scoured Mammallapuram beach exposing the basements of older temples. We have identified a discordant sand deposit sandwiched between two bricklayers at a site where the ruins of different generations of temples have been excavated. The radiocarbon dates suggest that this was deposited during 955+/-30 yr B.P., close to the historically documented period of devastation of this site by a sea surge. Excavations at Kaveripattinam, located 200 km to the south, revealed a widely distributed occupation horizon of A.D. 8-10 century, marked by a superjacent sand layer. We suspect that this layer represents the A.D. 950- sea incursion mentioned in the in the classic Tamil texts, also in line with the archeological inferences made earlier from this site. The supporting geological evidence for the afore-said approximations of a 1000-year-old sea surge, possibly a tsunami, need to be found from the Andaman-Nicobar Islands, closer to the source zone. Although the database is currently preliminary, the ages of the younger terraces in the islands suggest uplift events around 900 and 2000 yr B.P. Stratigraphic evidence of the previous tsunami events should reveal either as peat-sand couplets or as drowned ancient mangrove forests. Our investigations have revealed several potential candidates, which we are exploring further. The GPS estimates (Subarya et al., 2006) suggest an average recurrence interval of 230-600 years for the 2004-type earthquakes, if all the slip is released only through similar-size events, but double that if the slip was aseismic or expended by M 8 events like those in 1881 and 1941. Our inferences agree with the latter estimate.
DE: 5475 Tectonics (8149)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Seismology [S]
MN: 2007 Fall Meeting