Wednesday, July 20, 2022

How vulnerable Is the Mediterranean to tsunami risks?

A sign on the Tel Aviv beach informing of the tsunami risk.
Credit: Beverly Goodman-Tchernov

From Haaretz by Ruth Schuster

Over 2,500 years ago a Greek general identified the link between quakes and tsunamis, but how often have tsunamis plagued the Mediterranean?
An expert tells us how to identify mega-waves of the distant past – and the future risks

On Saturday evening, an underwater volcano in the South Pacific erupted.
As steam, volcanic ash and debris shot 20 kilometers (12 miles) into the air, the shock waves sent tsunamis spreading in all directions, first hitting the adjacent island nation of Tonga and later Hawaii, California, New Zealand and Japan.

The waves making landfall weren’t huge, but damage did ensue.
As of this writing, three possible casualties in Tonga had been reported, though not confirmed.

But actually any undersea eruption of noticeable magnitude is bound to unleash tsunami waves, says Prof. Beverly Goodman-Tchernov, head of the University of Haifa’s Marine Geosciences Department at the Charney School of Marine Sciences.

The question is what sort of tsunami waves.
Experts refer to two categories of sources: “near-field,” a nearby source such as a landslide, which tends to have a more localized effect, and “far-field” large events that occur offshore, which can affect a greater range of coastlines.
The Hunga-Tonga-Hunga-Ha’apai volcano was a far-field type.

We who live on the balmy Mediterranean Sea may feel we’re safe from the mega-quakes and tsunamis that have wreaked devastation in the Pacific, and more rarely in the Atlantic.
We are not.

The Mediterranean region is seismically frisky, boasts a few impressive volcanoes (even a super-volcano, Campi Flegrei) and has a history of tsunami action, not that there are clear records of the anomalous waves.
The fact is, most people in a position to closely observe a paleo-tsunami probably drowned, and the survivors wouldn’t necessarily have understood the context.

So what applies worldwide applies to the Mediterranean as well.
Undersea volcanism is only one of many potential causes of tsunamis, which are usually not a single event but a series of waves – in the ocean, a sea, even lakes.
And most dangerously for the likes of Tel Aviv, they spread inland via river valleys.
The most common cause that people have heard of is an underwater earthquake.
When a portion of seabed rises or falls in a quake, so does the water column above it.
The displaced water “wants” to spread.

As the waves advance from deep to shallower water and approach the shore, their volume and energy is preserved, thus they erode the seafloor, their speed slows and their height grows.
A tsunami mere centimeters high in the open ocean may be meters high when it hits the shore.
The highest tsunami in recorded history reached 524 meters (1,720 feet), dousing the Lituya Fjord in Alaska in 1958 after a quake.

That quake didn’t directly cause the tsunami at Lituya Bay.
What did was the ensuing landslide into the sea.
That calamity originated on land, but underwater landslides are a thing too and a key cause of tsunamis on the Israeli coast.

“The reality is that anything that causes significant-enough water displacement can result in a tsunami,” Goodman-Tchernov says.
“If sediment on a submerged slope builds up to a point that it fails, as that material moves, it pushes the water and displaces it – that can create quite large tsunamis.”

How large? Potentially enormous; a collapse along the Norwegian continental shelf 8,000 years ago is believed to have triggered a series of mega-tsunamis that washed over the Doggerland land bridge and rolled high onto the Paleolithic British coast.

The coast at the Israeli city of Caesarea, which "has been a tsunami magnet."
Credit: Beverly Goodman-Tchernov

Another potential cause is meteorite impact.
The asteroid that killed almost all the dinosaurs slammed into the Gulf of Mexico, setting off world-spanning mega-tsunamis as much as a mile high, scientists estimate.
The wave could have been higher if the meteorite had hit deep ocean rather than shallow sea.

One last cause is we.
“There have been man-made tsunamis – minor, but still – from testing bombs,” Goodman-Tchernov says.
At least we’ve never created one that’s been terribly damaging, she adds.
Well, the day is young.

Shaky in the Mediterranean ‘ocean’

Though it hasn’t suffered a mega-tsunami like those seen in the Indian Ocean or Japan in recent history, the Mediterranean has had, and will have, its share of these waves, including – it seems – one monster that’s believed to have hit Dor on the Israeli coast about 4,500 years ago.

“The Mediterranean has all of the hazards. People think of it as a quiet vacation destination, but the reality is, it has all the features of the large ocean, including the tectonic activity and deep basins.
It has the bathymetry [underwater topography] that includes the potential for landslides, and it even has a few volcanoes,” Goodman-Tchernov says, referring chiefly to Italy’s Etna and Thera on Santorini island in the Aegean.
“Meteorite impacts are, of course, equal opportunity worldwide, including the Mediterranean.”

Historical records of tsunamis are rare and oblique, but what else could the Athenian general and historian Thucydides have been talking about in his “History of the Peleponnesian War” written 2,500 years ago?
The Peloponnesians and their allies invaded but were deterred by earthquakes, he wrote.

As Thucydides put it, “About the same time (Euboea being then troubled with earthquakes), the sea came in at Orobiae on the part which then was land and, being impetuous withal, overflowed most part of the city, whereof part it covered and part it washed down and made lower in the return so that it is now sea which before was land.
And the people, as many as could not prevent it by running up into the higher ground, perished.” (Translated by Thomas Hobbes in 1843.)

Thucydides then described similar events in Atalanta and Peparethus.
The historian may have been among the first to deduce the underlying trigger: “And it seemeth unto me that without an earthquake such an accident could never happen”

Goodman-Tchernov notes that a terrestrial quake may trigger not only landslides on land and at sea, but also a sequence of quakes along the same system.

Caesarea, tsunami magnet

An expert on identifying paleo-tsunamis, Goodman-Tchernov has done a great deal of work in Caesarea, a port city on the Mediterranean that goes back more than two thousand years and flourished in the Roman, Islamic and Crusader periods.
There she has spent over 20 years recording and analyzing deposits both on land and in the sea with the Israel Antiquities Authority and other partners, identifying both historically noted tsunamis as well as ones without any known written record.

Absent historical records either because there were no witnesses or because the observers died, fled or were illiterate, records did not survive, or the witnesses couldn’t write in the first place.
Thus the discovery of physical remains is the only recourse.

Students doing tsunami research in the lab.
Credit: Beverly Goodman-Tchernov

Identifying such prehistoric or later unrecorded tsunamis relies on identifying anomalous sediment layers, debris and destruction with the hallmarks of tsunami inundation; for example, seashells a couple of kilometers inland.
Identifying paleo-tsunamis relies on basic sedimentology.

“We look at what is deposited on the landscape and try to reconstruct what its source was,” Goodman-Tchernov explains.
Researchers compare with bona fide tsunami deposits, going through an exhaustive elimination process regarding what could have caused anomalous deposits; for instance, agglomerations that seem to have elements of land, sea, river, garbage and debris.
“We’re not proving anything; we’re disproving every other possibility.”

Her work has led her to the observation that near-field tsunamis can have quite localized impacts; you might find evidence of an ancient inundation along the shore in one place but not in nearby sites.

“Caesarea has been a tsunami magnet.
It’s unusual,” she says, though immediately qualifies for the benefit of the city’s residents that this could be due to sampling bias; that is, that’s where the researchers have been looking.
Or it could be that Caesarea really is in the sweet spot for more than one of the offshore slumps along the coast.

Why might the impact of a landside-triggered tsunami be lesser than, say, the Tongo volcano blowing underwater?
The models suggest that a slump pushes water away from it outward; the least-impacted places seem to be immediately in front of where the slide happened, Goodman-Tchernov notes.
When you have an undersea earthquake or eruption, the shock spreads as concentric circles from a single point.

So there are slumps off Israel’s coast – at Palmahim, the Dor disturbance – offshore mapping shows where there have been past sediment failures.
And it’s possible that these cause the tsunamis at Caesarea.

“The bottom line is that there are different sources. Some will affect a larger part of the Mediterranean – which doesn’t mean the waves are bigger, necessarily. They may be bigger in the distribution of the effect but not necessarily in terms of the impact on each coastline,” she says.

“Take the far-field 1956 quake along the Hellenic arc near Crete that caused very large tsunamis nearby.
Crete was hit by waves some 10 meters high or more. The waves reached the Israeli coast too but had petered out; the only evidence for it was a perturbance recorded on tidal gauges. The normal pattern changed. But it didn’t cause any local damage that we know of.”

Tsunami-tinted glasses

Whether a tsunami roaring toward Israel was caused by earthquakes near Greece or Turkey, or an offshore slump near Caesarea, what matters to us is how much warning time we have.

Can they be predicted? Nope, not until a triggering event has occurred.
Not that there is such thing as estimating how frequent tsunamis can be anywhere.

“When talking about frequency in the Mediterranean, we hear a lot of people throwing out a value of a significant tsunami around every 600 years. This number is a little bit misleading,” Goodman-Tchernov puts it courteously, then proceeds to rubbish the theory.

“It presumes that, one, we know about all the tsunamis, and, two, it gives us the false impression that these things occur in some kind of consistent pattern, which is absolutely untrue. An average of 600 years means nothing. In fact, what I’m seeing through my research is that the variation in the gaps between events are extreme.”

Goodman-Tchernov at work on the coast.
Credit: Evi Nominkou

In some cases, she sees evidence for tsunamis so close in time that they may actually have been from the same triggering event, or within a year of each other.
Or nothing for a thousand years.
Caesarea, for example, seems to have had quiet for 1,600 years; there is no evidence of a tsunami in that time.

So does that mean one is overdue? Again: No.
Could be.
We don’t know, and Tel Avivians, snigger not.
We’re at special risk because if and when, displaced seawater could come roaring up the Yarkon River where it feeds into the sea.
So averages and suchlike don’t help us whatsoever.

“As people we love to have averages, but it’s really artificial. We do it all the time, talking of 50-year storms, but these numbers are relevant mainly to insurance companies, not the natural world,” Goodman-Tchernov says.
Indeed, in this era of climate change, such figures are rendered more irrelevant than ever before.

Could global warming and ocean warming affect the tsunami risk? Apparently not; finally something isn’t a function of climate change.

But doesn’t it seem, just as our planet is warming, that tsunamis are on the rise?
They are not – not necessarily – the professor reassures us.
It’s just that after the horrendous Indonesian tsunamis of 2004, more attention – including by the Israeli government – is being paid to the phenomenon.
People are wearing tsunami-tinted glasses, Goodman-Tchernov laughs.

But as sea levels rise, and they are, more and more communities are at risk with the commensurate potential for property damage and loss of life.
And if you’re in Tel Aviv, don’t snort at the “tsunami evacuation route” signs.
You may think you’re running madly for higher ground, but actually you’ll be racing full speed toward the Yarkon River Valley.
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