Identification of palaeotsunamis using ground penetrating radar, sedimentological and micropaleontological techniques; implications for Sri Lankan tsunami risk

Abstract

One of the most catastrophic natural hazards which can devastate coastal zone communities is the tsunami. The risk of tsunami devastation can be mitigated by reconstruction and quantification of past tsunamis, but this requires identification and analysis of past tsunami magnitudes and dates even from historical times. The interpretation of geological records of tsunamigenic deposits is the core theme explored in this thesis for the characterisation of palaeotsunamis. Three key elements: identification of palaeotsunamites, dating their deposition and determination of magnitude are needed for reconstruction and quantification of palaeotsunamis. While several studies have described criteria for identification of tsunamigenic sediments, much less research has been carried out on reconstruction of palaeotsunamis.

Sediment characteristics, their depositional configuration and extent of the inundation area on the coast give infOlmation on hydrodynamic conditions of tsunami waves. This study has developed a method to estimate tsunami risk by reconstructing and quantifying palaeotsunamis from tsunamigenic sediments on the Sri Lankan coast rising sedimentological and paleontological characteristics, Ground Penetrating Radar (GPR), and optically stimulated luminescence (OSL) dating techniques. The 2004 tsunami records were used as a control for the study. Distinctive tsunamigenic sediment signatures were recognised. Three palaeotsunami events were identified and dated at ISO (Krakatua), 2550±190 and 3170±320 years BP and correlated with historical and archaeological records. The recurrence interval of c 600 years postulated for the Indian Ocean was confirmed by this study and the oldest event recorded to date in the Indian Ocean was recognised at 3170±320BP. The method developed here based on reconstruction of palaeotsunamis using sedimentological records, GPR and OSL techniques enables estimation of inundation distance, recurrence interval and consequently the wave characteristics of palaeotsunamis which can be used to estimate tsunami risk for any coast.