The December 26, 2004 tsunami left its imprints along the southern coast of India especially the coastal areas of Manakudy in Kanyakumari district of Tamil Nadu. In the study area - Manakudy estuary - the post-tsunami sediment texture is predominantly coarser as inferred from textural analysis. Granulometric analysis indicates a shift of well-sorted, coarse skewed and platykurtic nature during the pre-tsunami season, to moderately sorted, fine skewed and leptokurtic behavior, after the tsunami. Violent hydrodynamic conditions have prevailed during the post-tsunami deposition of sediments. The unimodal nature of the post-tsunami sediments as distinct from the bimodal pre- tsunami sediments is reflected from the frequency curves. The CM diagram and the log probability curves confirm these observations.

Manakudy estuary --- Tsunami India --- Tsunami Sediment Signaures

The great Tohoku earthquake of March 11, 2011 generated a very destructive and anomalously high tsunami. To understand its source mechanism, an examination was undertaken of the seismotectonics of the region and of the earthquake’ focal mechanism, energy release, rupture patterns and spatial and temporal sequencing and clustering of major aftershocks. It was determined that the great tsunami resulted from a combination of crustal deformations of the ocean floor due to up-thrust tectonic motions, augmented by additional uplift due to the quake’s slow and long rupturing process, as well as to large coseismic lateral movements which compressed and deformed the compacted sediments along the accretionary prism of the overriding plane. The deformation occurred randomly and non-uniformly along parallel normal faults and along oblique, en-echelon faults to the earthquake’s overall rupture direction – the latter failing in a sequential bookshelf manner with variable slip angles. As the 1992 Nicaragua and the 2004 Sumatra earthquakes demonstrated, such bookshelf failures of sedimentary layers could contribute to anomalously high tsunamis. As with the 1896 tsunami, additional ocean floor deformation and uplift of the sediments was responsible for the higher waves generated by the 2011 earthquake. The efficiency of tsunami generation was greater along the shallow eastern segment of the fault off the Miyagi Prefecture where most of the energy release of the earthquake and the deformations occurred, while the segment off the Ibaraki Prefecture – where the rupture process was rapid – released less seismic energy, resulted in less compaction and deformation of sedimentary layers and thus to a tsunami of lesser offshore height. The greater tsunamigenic efficiency of the 2011 earthquake and high degree of the tsunami’s destructiveness along Honshu’s coastlines resulted from vertical crustal displacements of more than 10 meters due to up-thrust faulting and from lateral compression and folding of sedimentary layers in an east-southeast direction which contributed additional uplift estimated about 7 meters - mainly along the leading segment of the accretionary prism of the overriding tectonic plate.

Japan --- Honshu --- Sanriku --- earthquake --- seismotectonics --- tsunami --- source-mechanism --- tsunamigenic efficiency --- Japan Trench