The mostly hyphened word megatsunami (mega-tsunami), is known as iminami or “wave of purification”and is described as an informal term often used by popular media and popular scientific societies to explain a very huge tsunami wave beyond the typical size reached by most tsunamis. A megatsunami is associated with waves beyond the norm for tsunamis, ranged from over 40 metres (131 feet) to giants over 100 meters (328 ft) tall. Remember that the waves are often much higher while meeting land, as the water often floods upwards from the force of impact.

Megatsunamis are caused by a very large impact or landslide into a body of water when the water cannot disperse in all directions. Thus, they are usually a highly localized effect, either happening when the source of a tsunami is extremely close to the coast or in deep, narrow inlets, lakes or other water passages.

Tsunami alert system

A tsunami alert system is a system to detect tsunamis and issue warnings to avert loss of life and property. It consists of two equally important components: a network of sensors to identify tsunamis and a communications infrastructure to issue timely alarms to permit evacuation of coastal regions.

Two distinct types exist: international tsunami warning systems, and regional warning systems. They both depend on the fact that, as tsunamis travel at between 500 and 1,000 km/h (around 0.14 and 0.28 km/s) in open water, earthquakes can be identified immediately as seismic waves travel with a typical speed of 4 km/s (around 14,400 km/h) which gives time for a tsunami forecasting to be made and alerting to be issued to the areas at risk, if warranted. Unfortunately, until approaching a reliable model capable to predict which earthquakes will generate significant tsunamis; this issue will lead to many more false alarms than verified warnings. In the correct operational paradigm, the seismic alerts are used to send out the watches and warnings. Then, data from recorded sea level height (either shore based via tide gauges or deep ocean DART buoys) are used to detect the existence of a tsunami. Other systems proposed as warning paradigm. For instance, it has been suggested that the duration and frequency content of t-wave energy (which is earthquake energy trapped in the ocean SOFAR channel) is indicative of an earthquakes tsunami potential [Salzberg, 2006]. The first rudimentary system to warn communities of an impending tsunami was given a try in Hawaii in the 1920s. More modern systems were developed in the wake of the April 1, 1946 and May 23, 1960 tsunamis which struck the shoreline in Hilo, Hawaii and caused giant disaster.