Civil Protection Department

Tsunami risk

 

• Introduction
• Description of the risk
• Tsunami history in the Mediterranean and Italy
• National alert and monitoring system
• Tsunami Warning System for the Mediterranean area
• Tsunami Warning System. the Ocean Pacific model

Tsunami Warning System. the Ocean Pacific model

Since 1948 the Pacific Tsunami Warning System - PTWS, a tsunami alert system, is active in O’ahu island, Hawaii. Thes system is composed by a network of Pacific States, in which 30 tidal stations - that measure the tide level - operate.

Ptws may be supported by another hundred stations managed by the National Oceanic and Atmospheric Administration - NOAA and can receive data from hundreds of seismic stations from all over the world through the National Earthquake Information Center - Neic -in Colorado. Moreover, the Ptws coordinates the activities of tsunami alert centre in Alaska, Polynesia, Chile, Japan and Russia.

The system is able to calculate the arrival time of the first tsunami wave though a model, that elaborates the wave speed, considering the basin depth. When in an area of the Pacific Ocean an earthquake with magnitude 7 or higher (Richted scale) occurs, the Ptws is able to locate the epicentre and earthquake magnitude in less than half an hour. Thus, the tsunami alert may start in those areas that run the risk to be hit within 3 hours. Since the time necessary to have a confirmation of the tsunami arrival is longer that the one needed to activate the evacuation procedures, the latter are immediately set off.

Usually, the first indication of a tsunami wave arrives from the tidal station closest to the place where tidal waves appear higher and faster, compared to the ones normally registered. However, the signal from a single station is not enough: if the Ptws confirms the anomaly through the registration of other anomalous signals, the alert and notice are cancelled. On the other hand, if the tsunami is confirmed, the attention phase becomes immediately an alert one and then an alarm, following this procedure:

• Three hours before the arrival of the first wave, sirens launch the alert signal. The populations is instructed to turn on the radio and follow the evolution of the phenomenon on the news and guidelines diffused on every radio station.
• Sirens sound again two hours before the estimated arrival, and then again one hour and half and hour before. These signals are always accompanied by information diffused on the radio.

If necessary, the Civil Protection staff manages the operations to evacuate the population, starting from the lower coastal areas, that may be first hit.

The Pacific tsunami alert system, beyond the scientific community capacity to estimate the wave arrival, is based on the risk awareness of the population and the knowledge of how to behave in case of emergency. Along with structural prevention measures adopted on the Hawaii islands - e.g. building inhabited only from the 1° floor up and ground floor dedicated to an open parking  - the strategy chosen by the Governement provides for a capillary diffusion of information on both risks and emergency plans, among the residents and tourists.

Image source: Intergovernmental Coordination Group for the Pacific Tsunami Warning System (ICG/PTWS)

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