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Tsunami waves hit Malta
1st November 2018
Sea level oscillations at Portomaso
Trace of the sea level oscillations registered by the station at Portomaso marina

The Department of Geosciences has been actively looking at seismic and sea level data from its monitoring stations to decipher any local impacts from the earthquake triggered in the open sea on Thursday 25 October at 22:54 UTC, west of Greece (37.49°N; 20.6°E) some 580 km away from the Maltese Islands.

Tectonic activities of this kind can give rise to rapid displacements in the sea bed and consequently to vertical shifts in the water column leading to disturbances that travel away from the source at more than 700 km per hour (the typical speed of a commercial jet), depending on the depth of the sea, and reducing their speed by a factor of 4.5 as they cross the Malta Escarpment on to the much shallower continental shelf area. Upon reaching shallower water areas, these waves gain in amplitude and manifest as a tsunami with often very devastating outcomes on the coast. The eastern coast of Malta is particularly vulnerable to tsunami waves originating from earthquake epicentres near Greece, and traversing all the way across the deep Ionian Sea to reach our islands in just about an hour.

The figure shows an extract from the sea level data collected by a gauge positioned inside the Portomaso Marina in St Julian's. The station is maintained by the Physical Oceanography Research Group and has been operating with real time transmission of data for the last 17 years. It samples sea level measurements every 15s thus permitting the well known seiches, known as the 'milgħuba', to be monitored in detail. These seiches are related to atmospheric disturbances and have nothing to do with tsunami; they are a resonance response of the marina water body to open sea long period waves triggered by atmospheric gravity waves.

Tsunami-related oscillations have the same typical frequencies as those of the seiches, which therefore camouflage tsunami signals, rendering detection very difficult except in the case of extraordinarily strong events. In the particular case of this earthquake event, the tsunami signal manifested as an intensification of the marina oscillations starting from midnight of 26 October, when the tsunami waves reached the Maltese Islands giving rise to a superimposed effect on the already existing seiches. This led to very modest sea level vertical excursions of the order of 25cm, and sea level movements occurring in repeated short cycles of the order of minutes.

Luckily not a big deal this time, and a weak tsunami without consequences! But Prof. Aldo Drago mentions that 'these apparently insignificant events are important since they present real case situations and provide typical datasets that we can use to experiment on how to best automate the detection of tsunami signals against the background seiches [...] not a mean challenge at all.'

This study will contribute to the Italia-Malta project SIMIT-THARSY, during which the University of Malta will be strengthening its earthquake and tsunami monitoring capabilities through seismographic and sea level gauge instrumentation and software, evaluating tsunami scenarios, and providing input to the Civil Protection department.

Meanwhile aftershocks of the earthquake, some with magnitude greater than 5.0, continue to occur even one week later, and are being monitored by the Seismic Monitoring and Research Group.