Stratigraphic evidence for pre-2004 tsunamis in southwestern Thailand

The 2004 Sumatra–Andaman earthquake and the associated Indian Ocean tsunami vastly exceeded the size of their predecessors from the previous two centuries or more. We sought clues on how often large tsunamis occur by taking 9 shallow cores 0.5–2.0 km inland from the modern shore on a Holocene beach-ridge plain of Phra Thong Island, 125 km north of Phuket. We tentatively correlated one sand sheet among these cores and found a second sand bed beneath in one of the cores. We infer that these deposits represent two pre-2004 tsunamis. The island's setting precludes river floods and tends to rule out storms as causes of the sand deposition. Establishment of the beach-ridge plain requires that both the inferred tsunamis occurred after Holocene sea levels approached their present position. Radiocarbon ages of bulk soil samples are consistent with this conclusion. Though root penetration into soil samples probably makes the ages appear younger, one inferred tsunami occurred before AD 1300 and the other occurred before AD 1900, based on the age estimates. The tsunamis probably originated along the Sunda megathrust, from where tsunamis can travel directly to the Thailand coast.