Roles of Reflection, Energy Trapping and Secondary Undulations in the Tsunami on Kerala Coast

Abstract

The tsunami of 26th December 2004 in the Indian Ocean impacted the coast of Kerala in southwest India, which is in the shadow zone of the tsunami, to varying degrees, depending upon the location. It was possible to do a reasonable post-mortem of this tsunami on the Kerala coast, making use of data from eye witness reports, post-tsunami field surveys and tide gauge records. A few different physical processes are invoked to account for the behaviour of the tsunami on the Kerala coast, with reference to the time of arrival of various sets of waves, maximum tsunami amplitudes at the coast and coastal inundation. Here, the roles of reflection, energy trapping and secondary undulations are specifically examined. It is shown that, whereas the direct waves accounted for the water level oscillations during the first part of 26th December, reflected waves from the Lakshadweep-Maldives ridge (which is the relatively shallower region of the Arabian Sea encompassing these island chains) and the east coast of the African continent, could account for the water level oscillations during the late afternoon and evening of 26th December. Water level variations from 27th December onwards are attributed to energy trapping on the continental shelf and secondary undulations.