Beginning of the alert
An earthquake occurring at sea or in coastal areas with magnitudes greater than 6 can trigger a seaquake alert. This is activated with an initial Red or Orange Alert message (Watch or Advisory level in English language messages for Mediterranean countries).
The alert levels indicated in the initial message are associated with the estimated magnitude of the impact of the tsunami on the various points of coast (the so-called forecast points): the alert levels Orange (Advisory) and Red (Watch) define thresholds for possible flood levels, i.e. by how much the sea level can rise compared with the normal situation. The orange alert (Advisory) indicates that an earthquake has occurred whose effects on the coast could be expected to be minor (expected run-up less than 1 metre). The red alert (Watch) instead indicates that the effects on the coast due to a possible tsunami could be more damaging or destructive (expected run-up greater than 1 metre).
These messages trigger the Alert phase, which involves the rapid removal of the population from potentially flooded areas and securing the infrastructures on the ground, where possible.
To date, the time to calculate a first solution and to send the initial warning message can vary between seven and fourteen minutes, but once the message is sent, the CAT-IGV shift workers, under the supervision of the official on duty call, continue to monitor the data both of the seismic stations and of the tide gauge stations for an improved understanding of the nature and evolution of the phenomenon.
The initial message may then be followed by a series of messages which mark the entire cycle of the alert at all stages, from the issuance of the first message to the end of the event (when sea level anomalies have actually been observed) or to the revocation of the alert (if no anomalies are observed).
The initial message may be followed by an update message, which is issued only if the subsequent and more in-depth analysis of the seismic data may suggest a larger event than initially estimated in the first few minutes after the shock. The update message is issued only when the alert levels increase at least for one forecast point.
The confirmation message is sent out when the tsunami is detected by the tide gauges, usually by those closest to the epicentre of the earthquake. In addition to the event data, the confirmation message also reports the heights and, when possible, the periods of the tsunami waves observed. The data collected by the tide gauges are essential in order to understand whether a tsunami has occurred or not, thus allowing to decide whether to confirm the alert phase begun with the first message, or to revoke it and allow a return to normal as soon as possible. However, once a seaquake, even a small one, has been detected, a few hours must elapse before the alert phase can be closed.
Should the tide gauges not record significant sea level anomalies for a sufficiently long time, it means that the earthquake did not generate any tsunami. In this case a cancellation message is issued, which revokes the alert.
End of event
The end of event message is issued at the end of an actually detected seaquake event, when the sea level changes observed by the tide gauges are once again comparable with the levels before the seaquake event itself. It is always worth remembering that a seaquake is a complex and evolving phenomenon, with wave sequences which can continue to hit the coast for several hours. It should also be remembered that often the first wave is not the largest or the most destructive, and that therefore the phenomenon must be monitored throughout its duration.