The 2011 Japanese 9.0 magnitude earthquake: Test of a kinetic energy wave model using coastal configuration and offshore gradient of Earth and beyond

• Published in: Sedimentary geology 2011-07, Vol.239 (1), p.80-86
• Main Authors: Mahaney, William C., Dohm, James M.
• Format: Article
• Language: English
• Subjects: Coastal; Coastal environments; Coastal geomorphology; Grains; Marine; Offshore; Offshore engineering; Offshore structures; Sand; SEM microtextures of tsunami blanket deposits; Tsunami modeling; Tsunamis
• ISSN: 0037-0738; 1879-0968
• DOI: 10.1016/j.sedgeo.2011.06.001

Based on geological investigations of coastal geometry and stratigraphy, offshore gradient variation, wave velocity/energy, and quartz grain microtexture analysis of key sites inundated by tsunamis during the Holocene, we have developed a postulated model that could be used to assess modern and ancient onshore environmental conditions during tsunami emplacement. Given that tsunami waves can travel up to ~ 900 km/h, the kinetic energy of the wave is determined by frictional release on the offshore slope to seabed, nearshore sand and sediment supply, and coastal geomorphology, specifically coast linearity or embayments that constrict wave energy. With gentle offshore gradients and relatively linear coastlines, the kinetic energy of the tsunami is decreased by friction with the seabed and radiated outward along the coast, thus producing reduced flooding of the affected coast and fewer collisions of entrained grains. In contrast, steep offshore gradients reduce friction to mere milliseconds of wave impact, and embayments, specifically narrow coastal constrictions, increase wave energy, both of which increase on land run-up heights, and importantly to our proposed model, grain collision effects. Damage to quartz grains deposited by tsunamis along coastal extents ranges from slight on linear coasts with gentle offshore gradients to highly resurfaced mineral surfaces on coasts with steep offshore gradients and narrow embayments. Our model can be further tested through collection, analyses, and comparison of quartz grains in relatively low- and high-energy environments of the recently tsunami-impacted northeastern coast of Japan. For example, the location of the Fukushima nuclear plant along a linear coastline with a less steep offshore gradient may have attenuated the energy of the incoming wave, and thus yielding less resurfaced grains when compared to the coastline located to the north of Sendai City where coastal embayments of variable size coupled with steep offshore gradients would produce higher-energy waves of greater magnitude, as attested by greater penetration inland with considerable loss of life and property damage, and resulting highly deformed grain surfaces. The model could be applicable for assessing modern and ancient coastal environmental conditions on Earth and postulated ancient marine conditions on the Red Planet. ► Japanese 9.0 magnitude earthquake. ► Geomorphic controls on tsunami blanket sedimentation. ► Microscopic analysis of tsunami sed