Scientists at the University of Portsmouth have found geological evidence that historic tsunami waves have swept over Malta’s north east coasts up to 20 metres above sea level in some places.
The Portsmouth scientists, led by Dr Malcolm Bray, have spent three years examining the landforms on the north-eastern Maltese coast with the support of colleagues from the Department of Geography at University of Malta.
They identified boulders inland that had been detached from the shoreline and clifftops, some which had split after great impact, and others for which they were able to find their source, before they were carried inland for distances of up to 100 metres.
The scientists have found evidence that the historic tsunami reached onshore speeds exceeding 20 miles an hour lifting 70-tonne boulders, some of them shattering into smaller rocks as they crash-landed.
Dr Bray said: “What all these measurements point to is that an enormous assailing force was responsible. Our calculations show the tsunami wave would probably have been at least four metres high in some places and substantially more powerful than the biggest storm waves on the islands.”
The Mediterranean basin is seismically active and tsunami are a regular occurrence, with a catalogue of them extending back to classical times. Studies of neighbouring coastlines show there has been a major tsunami, on average, every 400 years. The last major ones in 1169 and 1693 were triggered by earthquakes centred on the Malta escarpment, southeast of Sicily, which directly faces Malta’s north-eastern coast and is only 100 km, or 54 nautical miles away. A tsunami in Malta could also be caused by seismic activity in Greece, where a major event occurred in southwest Crete in AD 365. This implies that Malta would have at most only 35 minutes (Malta escarpment) or 80 minutes (western Greece) warning of an advancing tsunami.
The north-east coast is now Malta’s most densely populated region and if a similar sized wave struck today, it could threaten thousands of people. Many residents and holidaymakers live by or visit the north-eastern coasts, where the towns are built right up to the edge of the shoreline.
The researchers now hope to establish the age of the deposits to determine exactly when the tsunami took place.
Professor Derek Mottershead said: “There was a tsunami in 1908 but reports tell that it had only a limited impact on Malta, simply causing localised flooding around Msida and Marsaxlokk. It appears that no event of the magnitude indicted by the new evidence has occurred within the past 300 years and so there is no historical memory of any such devastating event. It is highly likely that there have been other equally devastating waves previously and that there will be more in the future.”
Dr Bray said: “The most important thing we can do is to educate people about the risk and give the Maltese government the evidence needed to prepare and protect residents and holidaymakers.
“However, if a repeat tsunami occurred now, the risks are high. In the light of these recent findings it now appears that Malta would benefit from a re-evaluation of the tsunami threat
“If people feel an earthquake, they need to know that there is a risk of a tsunami, and if they see the sea recede they should know to get to higher ground. Fortunately in Malta, higher ground is never far away, but people do need to be made aware of the risks.”
Dr Bray is hoping the new evidence will help the Maltese government make contingency plans to protect lives and livelihoods on the vulnerable north-eastern coast.
The researchers are now considering whether a tsunami wave was responsible for the formation of one of Malta’s landscape curiosities – the Dragonara – the window in the ceiling of the Ahrax Point sea cave. The entry of a tsunami wave several kilometres in length into the enclosed sea cave might well have been a trigger, forcing both air and water upward to disrupt the cave roof, and creating rocky debris which was either carried away by the wave or fell back into the cave itself.
The research is published in the international journal Zeitschrift für Geomorphologie.