Tsunami



Tsunami

Introduction

Tsunami is a Japanese word meaning harbor wave. The tidal waves, or tsunamis, are a series of waves of long wavelength that originate from a violent movement of the water mass oceanic. In the deep ocean waves propagate at the speed of about 800 km/h and have a height of a few tens of centimeters. The wavelength is a factor that distinguishes the tsunamis by normal marine waves generated by the wind: in fact for tsunamis it is much higher and can even exceed the 200 km. As the depth of the sea decreases, also the wavelength decreases and simultaneously the height of the tsunami increases (McCarthy, 2010).

Mechanism of Tsunami

The main mechanisms that generate tsunamis are:

Dislocations in the seabed caused by an earthquake of magnitude greater than 6.5 on the Richter scale, which causes sudden uplift or subsidence of the crust with a consequent displacement of the water column. The tectonic causes 96% of the observed tsunami.

Underwater volcanic eruptions that are responsible for 3% occurrence of tsunamis.

Landslides on the continental slope, with 0.8% of occurrence (Whitmore, 2003).

Destructive power of a tsunami

The destructive force of the tsunami in coastal areas depends on the combination of the following factors:

Magnitude of the phenomenon that it induces. In the case of a submarine earthquake should consider the magnitude and depth of focus.

Influence of submarine topography on the propagation of the tsunami.

Distance to the coast from where the phenomenon occurred.

Configuration of the coastline.

Influence of the orientation of the axis of a bay on the epicenter.

Presence or absence of coral or breakwater, and the state of the tide at the time of the arrival of the tsunami.

Influence of surface topography, including slopes and roughness degree derived from buildings, trees and other obstacles on the ground.

The arrival of a tsunami on the coasts is manifested by an abnormal change in sea level, usually occurs to prior recollection increased or water, the latter situation is often left uncovered large areas of the seafloor. Subsequently, there is a rapid succession of sharp rise and fall of water level, the height can vary between one and four meters, however, there have been isolated cases where the waves reached heights over 7-10 meters (McCarthy, 2010).

The tsunami wave accumulates a large amount of energy when it reaches the coastline, this wave moves on the earth reaching significant heights above mean sea. The wave and flow that follows, when they encounter an obstacle impacting release their energy very strongly. The dynamics of a tsunami on the ground is usually quite complex and unpredictable; this is due to many different factors influencing such as: the period, the wave height, topography and underwater land, determining damage of varying intensity (www.nws.noaa.gov).

The effects of a tsunami are different depending on the length. In short, the wave reaches land with a strong current, and slow flooding occurs with little current. Moreover, the greater the wave height, the greater the accumulated energy and therefore, depending on the slope of the terrain and morphology, the greater the extent ...