Traces of Tsunami Events in off- and on-Shore Environments. Case Studies in the Maldives Scotland and Sweden

Abstract

A tsunami wave is characteristic by its large diameter (up to a couple of kilometres) and high speed of displacement (in the order of 700 km/h). Tsunami waves are generated by submarine earthquakes, by submarine slides and, occasionally, even by meteor impacts. When the wave starts to trim the seabed off a coast, it may rise to considerable heights and break in over the coast with disastrous force. The 1755 Lisbon earthquake and its disastrous tsunami effects are classical. The Boxing Day tsunami in 2004 in the Indian Ocean woke up the world in realizing what terrible effects a tsunami event may cause (NOAA, 2010). The death toll was around 230,000 persons. Suddenly, the word “tsunami” became known to a broad audience. The tsunami hazard of a region can only be assessed in a meaningful way if we have a reasonable record of the past events in that region. Consequently, there is an urgent need of establishing such records; i.e. a database of the paleotsunami events of the region in question (Morner, 2009a, 2010). This, in its turn, calls for a methodology of how to record past tsunami events. This implies a careful search of imprints left in morphology and stratigraph, and means of interpreting those imprints in terms of a past tsunami event. This paper is devoted to the traces and imprints left in nature from former tsunami events. We explore such signals in the off-shore sediments as well as in the on-shore deposits left in three different regions from where we have primary studies of our own. Hence, we may be talking about three case studies: the Maldives, Scotland and Sweden. As other tsunami studies are lacking in those areas, this paper tends to become a compilation of our previous work with recent additions.