Tsunami, huge water

Tsunami early warning systems are set up in all oceans and large seas. How can the conclusions be applied and the tsunami risk assessed in Israel?

The tsunami phenomenon has received extensive public relations in recent years, especially after December 2004, when the sea surge occurred after a very strong earthquake, the epicenter of which was in Sumatra in Indonesia. The loss of life and the damage caused were unprecedented, as was the grief and frustration, because the surveillance centers around the world saw the event unfolding in real time on the screens before their eyes, but in the absence of proper communication, they could not warn of it in the places prone to disaster.

Now, under the auspices of UNESCO, early warning systems for tsunamis are being set up in all the world's oceans and seas, including the Mediterranean Sea, in the hope that in the future it will be possible to announce it ahead of time and minimize the harm to the soul. How can these lessons be applied and the tsunami risk assessed in Israel?
In this article we will describe the guiding principles and work method in risk assessment: starting with building a catalog of historical tsunami events and locating the factors that caused them; Continue with computer simulation of tsunami scenarios; And including building maps of the areas prone to flooding from a tsunami, giving recommendations to reduce the risk and guidelines for behavior in a real case.

And if the sea is rough

A tsunami is a very large wave that develops in the sea due to an acute and sudden disturbance in the body of water. The disturbance can occur at the bottom of the sea due to a strong earthquake, a large underwater slide or a volcanic eruption, or alternatively a disturbance in the sea surface, which occurs due to the collapse of a mountainside, cliff or glacier sliding into the sea, or even a meteorite strike.

The higher and/or longer the tsunami wave, the greater its ability to flood land areas. The places where the coast is moderate and the land cover is relatively sparse, will be exposed to more extensive flooding than places where the land cover is dense. From this it is clear that the coastal cliffs of Israel, which are common in the Sharon region, will form a protective wall against the tsunami waves, while in the coastal area and at the outlet of the creeks to the Mediterranean Sea, a deeper flooding will occur.

The sources of information about tsunami

Descriptions and reports from historical sources, registration and observations - From many descriptions it is evident that humans have known the phenomenon since historical times. Although the historical descriptions are biased according to the worldview of the describer and are partial, as they have survived the course of history, they are irreplaceable.

Geological evidence - In most cases, the evidence left behind by the tsunami is relatively meager. In recent years, a new research field has developed in geology, which deals with locating evidence and findings for the occurrence of ancient tsunamis.

Archaeological evidence - Damage and destruction discovered at archaeological sites, coastal or marine, may also indicate a tsunami, but things are not simple here either. The temptation to find evidence of a historical catastrophe is much more appealing than attributing the damage to a mere engineering failure, or even to unusual winter storms.

Models, calculation software and laboratory experiments - Due to the paucity of observations, the researchers turn to the image of the tsunami waves and their spread in space with the help of mathematical and physical models, which are used to run scenarios with the help of computer programs, and the results are checked against real data, measured in recorded events. Tsunami simulations are also done with the help of controlled experiments in the laboratory, and here too the results are used to calibrate models and adapt them to reality.

Tsunami in Israel and in our region

The historical analysis shows that Israel was hit by a wide variety of types of tsunami. The earliest report known to us is especially surprising: a clay tablet was found in Egypt, on which is written in cuneiform a message from the king of Tire to the king of Egypt, that Ugarit, a city located on the Mediterranean coast in northern Syria, was flooded and half destroyed. It was not written what was the cause of the flood, and so far this event has not been known from alternative historical sources. One can of course assume that at the same time an earthquake happened in the area, but it is also not impossible that the flooding was caused by a spontaneous underwater slide, not as a result of a seismic event.

It turns out that most of the tsunami events that hit Israel's coasts occurred after a continental earthquake, not necessarily a particularly strong one. Field studies of ancient earthquakes and archaeological sites, where earthquake damage has been identified, can in some cases indicate the possible location of the epicenter.

At the same time, the topography of the continental slope in Israel shows many scars in the shape of a crescent, a shape typical of landslides. From this, it can be concluded that a strong earthquake in the Jordan Valley, the Dead Sea and the Arabah (along the Syrian-African fault) may cause the ground to fail on the continental slope, and the landslide is the one that will cause the tsunami.

Tsunami risk assessment in Israel

As with earthquakes, it is impossible to predict the exact time when a tsunami will occur, but there is no doubt that they will occur in the future. Despite this, the existing technology makes it possible to detect earthquakes a few minutes after they have occurred, assess whether they can cause a tsunami, and in some cases even verify or deny this assessment in real time.

The sources of risk
The analysis of the list of historical tsunami events, the geology and bathymetry of the eastern basin of the Mediterranean Sea, revealed that the Aegean Sea and Crete, the surroundings of Cyprus and the replicas in the sea off the coast of Lebanon, are the main source of tsunami-causing earthquakes.

Maps of areas prone to flooding
In the first stage, maps of the areas prone to flooding were constructed. The maps reflect the risk arising from the total of all possible extreme events, and present the most severe predicted risk. They make it possible to locate the buildings and infrastructures exposed to flooding, as well as to plan and build with reference to tsunami risks.
In conclusion, although it is impossible to prevent the occurrence of a tsunami, it is possible to anticipate its dimensions, estimate how often it will occur, and outline the areas prone to flooding. In the last decade, it is possible to identify the formation of a tsunami in real time, and warn of its arrival before it reaches the coast. This information makes it possible to prepare better for the long term, to warn of the coming of the tsunami, to plan and build according to the risk.

Dr. Amos Salmon is a researcher at the Geological Institute, Jerusalem.
The author would like to thank Dr. Dov Tzvieli, from the Leon Recanati Institute of Marine Studies, Haifa University, for his help and helpful comments.