Geologic evidence for two pre-2004 earthquakes during recent centuries near Port Blair, South Andaman Island, India

Abstract

Coastal stratigraphy near Port Blair, Andaman Islands, where the A.D. 2004 Sumatra-Andaman earthquake was accompanied by ∼1 m of subsidence, provides evidence for two prior earthquakes, perhaps both from the past 400 yr. The first of these (event I) is marked by an abrupt mud-over-peat contact best explained by subsidence similar to that in 2004. Event II is evidenced by an overlying chaotic layer composed of mud clasts in a sandy matrix that is connected with feeder dikes. These mud clasts, probably produced by liquefaction, are capped by laminated sand and mud that we ascribe to an event II tsunami. Radiocarbon ages of plant remains in the peat give discordant ages in the range 100 B.C. to A.D. 1950. Event I probably resembled the 2004 Sumatra-Andaman earthquake in that it was accompanied by subsidence (as much as 1 m) but not by strong shaking near Port Blair. If event II was the A.D. 1762 Arakan earthquake, the laminated sand and mud provide the first evidence that this earthquake was associated with a tsunami.