Abstract
The 2004 Sumatra-Andaman tsunami emphasized the catastrophic nature of such disasters and exposed our knowledge gap of the historical and palaeo events. In the aftermath of this deadly event, the thrust in palaeotsunami studies was restricted to areas in the Indian Ocean, affected by this tsunami. The northern Arabian Sea, which hosts a similar tsunamigenic source i.e. the Makran Subduction Zone (MSZ), has so far remained ‘Terra-Incognita’. Here, for the first time, we report geological evidence of the 1008 AD tsunami, also mentioned as ‘an enigma’ in the historical reports, by identifying a >250 km long sand sheet with a landward extent of more than 250 m from the Indian coastline. Detailed sedimentology and geochemistry reveals an offshore origin of this sand sheet, from where it was eroded by a high energy wave and deposited in a supratidal environment. Optical and AMS 14C chronology constrains its age of deposition around 1000 AD. The shear size of the sand sheet, laterally and across the coast, along with grain size, a characteristically different provenance, are some of the major indicators, which can be useful in palaeotsunami/palaeostorm deposit distinction. Our report of the AD 1008 event from the Indian coastline, also supports the claim that the Western MSZ, albeit at longer intervals, has experienced major thrust earthquakes (Mw > 8) in the historical past. The proximity of this sand sheet to the shoreline does not discount the role of extremely unlikely, large storms as its causal mechanism.
Highlights
The giant trans-oceanic Sumatra-Andaman earthquake (Mw ~ 9.3) on 26th December 2004 and the Tohuku earthquake (Mw ~ 9.0) on 11th March 2011 resulted in tsunami events, which caused massive destruction[1,2,3]
Setup[16,17], the Kachchh coastline in western India, qualifies to be one of the key sites for reconstructing the palaeotsunami history emanating from the Makran Subduction Zone (MSZ)
Based on OSL chronology and archaeological evidence, the sea level during the 6–3 ka period was inferred to be marginally higher along the Kachchh coast, by at least 1–2 m19–21
Summary
The Kachchh coastline facing the northern Arabian Sea in the west and Gulf of Kachchh in the south (Fig. 1a) consists of tide and wave dominating landforms like beach-beach ridge-dune-backswamp complex in the western and wide mudflat-swale dominating segments in the eastern parts[16]. The top of the sequence is a fine grained massive sand layer (Unit-4) of 10 cm thickness, representative of a beach-intertidal environment of the present day coastal system Another shallow pit PG, dug in the backswamp environment, is at an elevation of 4 m from the present day high tide level in the western Kachchh coastline (Fig. 1b and Supplementary Fig. 1b). The grain size distribution shows a fine skewed bi-model distribution with a mean grain size of 0.7 Ǿ (Supplemantary Table 1) This sand layer is capped by another clayey silt layer (Unit-3) at the top, with typical intertidal/mudflat sequence characteristics (Fig. 2). The sand layer has a coarse grained nature with poor sorting and abundant broken shell fragments
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