Earthquake in Zakynthos on 25 October 2018 (M6.8). The red star represents the epicenter of the earthquake, the red triangles are the "forecast points", i.e. points where the tsunami forecast was of maximum alert (RED, expected runup greater than 1 meter), while the yellow triangles are the forecast points where the expected alert level was less than 1 meter (ORANGE alert). The lines with numbers (10, 20, 30, etc.) indicate the estimated propagation times of the first tsunami wave. It can be seen that the coasts of Puglia and Calabria were reached after about 35 minutes.
Sea level anomalies caused by the Zakynthos earthquake induced tsunami (25/10/2018) observed at the Le Castella tide gauge. The variations are centimeters (+/- 10cm) with respect to the average level of the moment. Above the unfiltered signal (tidal modulation can be seen), below where the tidal signal has been removed.
In the Seismic Surveillance and Tsunami Alert room of the CAT-INGV in Rome, seismic and tide gauge stations are continuously monitored in order to assess as quickly as possible whether the characteristics of a given earthquake are compatible with the generation of a tsunami.
As part of its surveillance and monitoring activities, the Center uses data from the INGV National Seismic Network and from the seismic stations of other international research centres, as well as data from the ISPRA tide gauge network and those from tide gauges located on coasts of other Mediterranean countries.
To date, the knowledge and technologies available only allow monitoring of tsunamis caused by earthquakes and do not allow us to operate on other possible causes of a tsunami. However, tsunamis of seismic origin make up about eighty percent of known events.
The alert system is divided into three fundamental activities. Seismic monitoring, operated by the INGV Tsunami Alert Center, detects earthquakes with their epicenter at sea or in the immediate vicinity, assesses the possibility that a specific earthquake could generate a tsunami, estimates the arrival times of the wave along the exposed coasts and, as soon as possible, communicate the alert to the Civil Protection Department.
The monitoring area of competence of the SiAM (National Warning System for Tsunamis of seismic origin) includes all the coasts of the Mediterranean and extends from one hundred km west of the Strait of Gibraltar to the Sea of Marmara, embracing the coasts of twenty countries . Furthermore, CAT-INGV, in its capacity as Tsunami Service Provider, works in close collaboration with other warning centres: the Center d'Alerte aux Tsunamis (CENALT) in France, the Hellenic National Tsunami Warning Centre, set up at the National Observatory of Athens, Institute of Geodynamics (NOA/HL-NTWC) in Greece and the Boğaziçi University Kandilli Observatory and Earthquake Research Institute - Regional Earthquake-Tsunami Monitoring Center, (KOERI-RETMC) in Turkey. To date, the alert messages have also been issued to various Mediterranean countries, and in particular to the agencies and institutions of Cyprus, Egypt, Germany, Israel, Lebanon and Spain, with the future aim of covering all countries in the area .
There are currently three activation levels envisaged in the NEAMTWS (North Eastern, Mediterranean and connected seas Tsunami Warning System) area, the first of which (called INFORMATION/VERDE) does not correspond to a real alert but to information on the occurrence of an earthquake with a magnitude greater than 5.5 in a marine or coastal area, but for which a tsunami is not expected. The other two levels (ADVISORY / ORANGE and WATCH / RED) provide for an alert of the civil protection system, with expected flooding lower and higher than 1 run-up meter (topographic height of the flood).
For training and testing of procedures, the CAT monitors earthquakes worldwide, as large earthquakes in the Mediterranean are fortunately rare. In the course of a year, around 250 earthquakes on a global scale cause the activation of the system and the emission of information or warning messages. Since the CAT has been operational, there have been about ten events in the Mediterranean that have triggered the alert procedures.
The last tsunami of major seismic origin that affected the Italian coasts was that of 1908 in the area of Messina and Reggio Calabria, following an earthquake with a magnitude just over 7, which flooded many towns in Calabria and Sicily up to the topographic altitude of 13 metres. The last event that generated a tsunami warning for Greece, Italy and Albania occurred on 25 October 2018 following an earthquake measuring 6.8 with its epicenter on the island of Zakynthos, in the Ionian Sea. In that case the first warning message was issued by the CAT 8 minutes after the earthquake. Fortunately, the type of fault movement during the earthquake meant that a major tsunami was not generated. However, tsunami-related sea level anomalies have actually been observed in Greece and Italy at tide gauge stations in Puglia and Calabria.