On the hunt for ancient disasters? Call in the paleotsunami detectives

This story was originally published by Hakai Magazine and appears here as part of the Climate Desk collaboration.

A boulder weighing more than 40 tonnes sits on the sand high above the ocean. Dwarfing every other rock in view, it is conspicuously out of place. The answer to how this massive outlier got here lies not in the vast expanse of the Atacama Desert behind it but in the Pacific Ocean below. Hundreds of years ago, a tsunami slammed into the northern Chilean coast — a wall of water 20 metres high, taller than a six-storey building, that swept boulders landward like pebbles.

The tsunami that lobbed this behemoth happened before written records existed in Chile. But we know about it today thanks to the detective work of a small group of researchers who are uncovering the signs of ancient tsunamis around the globe. Using a diverse array of scientific techniques, these paleotsunami researchers have found evidence of previously undocumented colossal waves. In the process, their work is revealing that coastal communities could be in far more danger from tsunamis than they realize.

As scientists expand their search, they have continued to find ancient tsunamis bigger than those found in historical records, says James Goff, a paleotsunami researcher at the University of Southampton in England. The implications are clear: if a huge tsunami happened once in a given location, it could happen again. The question is whether we’re prepared for it.

A tsunami is more than just a big wave. Conventional waves, even those tens of meters high, are usually generated by the wind and involve only the uppermost layers of water. They carry relatively little energy, and typically crash harmlessly on the shore.

A tsunami, by contrast, is spawned by geological forces — an earthquake, volcanic eruption, or the side of a mountain crashing into the sea. A tsunami involves the entire water column. While large tsunamis can measure 20 metres or more in height — with some particularly monstrous ones rising hundreds of metres — they need not be exceptionally tall to cause widespread damage. Instead of collapsing on the beach, a tsunami rushes ashore like a battering ram. After racing hundreds of metres or more inland, the water recedes into the depths, carrying away nearly everything in its path. But tsunamis almost always leave evidence of their passage — like an out-of-place boulder high in the desert.

Goff has been searching for ancient tsunamis for almost three decades, mostly in countries bordering the Pacific Ocean. He’s one of just a few scientists worldwide who specialize in finding evidence of paleotsunamis, or tsunamis that predate written records.

The easiest way to tell that a tsunami hit hundreds or thousands of years ago is to look underground, Goff says. When the wave recedes, it leaves traces of everything it contained strewn across the surface. This thin layer of silt, rocks, tiny shells, and other marine deposits gets buried over time, preserving the tsunami’s path between layers of sediment. In some places, the layers are so well-preserved that researchers can see evidence of multiple tsunamis stacked on top of each other like a layer cake.

In southern Chile, you can dig a hole near many coastal rivers and count the bands. “One, two, three, four,” Goff says. “And you can just see these layers, and you know that they’re paleotsunamis.”

In places with rocky or more barren terrain, a paleotsunami’s track can be harder to discern, and the techniques used must be tailored to the environment. Goff and other researchers also look for microscopic marine organisms like diatoms and foraminifera, ancient DNA from marine life, changes to geochemistry, and, as in the Atacama, unexpected boulders.

That Atacama tsunami likely happened in 1420, says Tatiana Izquierdo, a paleotsunami researcher based at the University Rey Juan Carlos in Spain who helped to discover it. She and her colleagues dug underneath the boulder to find undisturbed sediment. They radiocarbon dated some of the marine shells they found, giving a range of potential dates from the 14th to the 16th centuries. With further research, the team found historical records of a tsunami in Japan in 1420 that fit with their dates. Izquierdo says their tsunami likely originated off the Chilean coast following a large earthquake and crossed the Pacific to Japan.

In other cases, paleotsunami researchers have drawn insights from the archeological record. Izquierdo says archeologists in Chile previously noted that suddenly, around 3,800 years ago, a number of coastal sites were systematically abandoned, with new sites soon appearing farther inland. Additional evidence, like shell middens that bore evidence of having been eroded by strong currents, hinted at a potential paleotsunami.

Those dates line up perfectly with a huge paleotsunami that Goff found evidence for an ocean away, in New Zealand, where boulders the size of cars had been tossed almost a kilometre inland. It’s a disaster that doesn’t appear in historical records, Goff says, and it’s a tsunami that likely affected islands all across the South Pacific, including Vanuatu, Tonga, and the Cook Islands. Paleotsunami researchers have yet to look for corroborating evidence on those islands, so they don’t yet know the full scale of the destruction it caused.

Finding out how big and how bad a paleotsunami was is more than a matter of historical interest. That data has a lot of value for contemporary coastal communities.

Predicting tsunamis is impossible. At best, residents might have minutes to hours of warning from agencies like the National Tsunami Warning Center in the United States and Canada that use buoys and seismometers to detect potential tsunamis before they reach land. The resulting alerts are based on computer models fed data on how past tsunamis behaved. If they’re missing key events that don’t show up in the historical record — like the ones paleotsunami researchers are steadily uncovering — the warnings may not be fully accurate.

Goff points to the 2011 Tōhoku tsunami in Japan as a prime example of the perils of ignoring evidence of past events.

That 2011 tsunami, generated by a magnitude 9.0 earthquake in the seafloor off Japan, spawned waves up to 40 metres high that travelled as far as 10 kilometres inland. The water overwhelmed sea walls and inundated more than 100 designated tsunami evacuation sites. It destroyed entire towns and crippled the Fukushima Daiichi Nuclear Power Plant. More than 15,000 people died.

Part of the problem was Japan’s inadequate defences. Researchers knew of three large tsunamis from historical records dating back as far as the 17th century, one of which produced waves nearly as tall as the 2011 tsunami. Yet officials based their tsunami defence preparations, including the construction of a sea wall and the location of tsunami evacuation zones, on a 1960 tsunami generated by an earthquake on the Chilean coast that produced waves in Japan just six metres tall.

“We knew how big they could be [in Japan]. We knew that these things must have been generated just off the Japanese coast. And yet, we were completely unprepared for it,” Goff says.

The 2011 Tōhoku tsunami was more destructive than nearly any other in modern times. But as paleotsunami research is showing, it was hardly unprecedented.

Back in Chile, Izquierdo says she’s particularly worried about what would happen if a tsunami comparable in size to the one that flung boulders into the Atacama Desert hit today. In popular vacation spots, like outside the city of Caldera, people have built homes right near the beach. Should a tsunami hit, those homes could be in grave peril.

Paleotsunami researchers are revealing that the tsunamis we don’t know about were often more destructive than the ones we do. Those disasters may have happened thousands of years ago, and those locations may never see such big waves any time soon. But somewhere, sometime, we will.