Geological signatures of mega-earthquake and tsunami along Andaman-Arakan Segment and evidence post-relaxation from West Coast of Andaman Island, India

Abstract Tirur village located ~2.5 km inland along the western coast of Andaman Island experienced marginal subsidence during the 2004 Sumatra-Andaman (Mw 9.3) earthquake, causing inundation by tidal waters. Observations made in 2005, 2009, 2010 and 2011 revealed reduction in inundation limits of the tidal-marsh. Geomorphic and GPS observations suggest that the area experienced gradual but faster uplift during post 2004 till 2009, followed by gradual uplift with slower pace, indicating process of post-seismic relaxation. Shallow stratigraphic records exhibit four earthquakes including the 2004 Sumatra-Andaman earthquake. Unit a massive clayey-silt suggests a deeper environment before 3500 yr BP. Overlying thick peat (Unit b) represents tidal-marsh or wetland, suggesting emergence at or above mean sea-level during Event-I, this uplift was during 3022-3500 yr BP (BCE 1897-1369). Unit c medium-coarse sand with prominent bi-directional structures, and sharp contact with underlying Unit b, indicates a tsunami by Event-II during 1147 ± 108 yr BP (CE 674-1098). We infer that this was a mega-earthquake occurred along Andaman-Arakan Segment producing a transoceanic tsunami. Event-II caused subsidence followed by gradual uplift during post-seismic relaxation as marked by comparatively finer (bottom) and coarser (upper) sediments in Unit d. The post-seismic process was like the 2004 event which did not leave tsunami deposit. The Event-III during CE 1712-1852 was a tsunami marked by fine yellowish medium-fine sand (Unit e), inundated the area with relatively low-energy conditions, correlated with CE 1762 Arakan earthquake. Event-III resulted uplift facilitating formation of overlying peat (Unit f). Finally, the area subsided during 2004 earthquake (Event-IV).