This article is from Hakai Magazine, an online publication on science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.

A rock weighing more than 40 tons is found in the sand above the ocean. Dwarfing every other rock in sight, it is conspicuously out of place. The answer to how this huge 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 coast of Chile: a 20-meter-high wall of water, taller than a six-story building, washing rocks ashore like pebbles.

The tsunami thrown by this giant occurred before there were written records in Chile. But we know today thanks to the detective work of a small group of researchers who are discovering the signs of ancient tsunamis Worldwide. Using a wide range 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 broaden their search, they continue to find ancient tsunamis larger 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 large tsunami happened once in a certain place, it could happen again. The question is whether we are ready for it.

---

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

A tsunami, on the other hand, is generated by geological forces: an earthquake, a 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 meters or more in height, with some particularly monstrous rising hundreds of metersThey don’t need to be exceptionally high to cause widespread damage. Instead of collapsing on the beach, a tsunami rushes toward the coast like a battering ram. After running hundreds of meters or more inland, the water recedes into the depths, washing away almost everything in its path, but tsunamis almost always leave evidence of their passage, like a misplaced rock high up in the desert.

Goff has been looking for ancient tsunamis for nearly three decades, mostly in countries bordering the Pacific Ocean. He is one of the few scientists in the entire world 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 scattered across the surface. This thin layer of silt, rocks, small shells, and other marine deposits is buried over time, preserving the path of the tsunami 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 layer cake.

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

In locations with rocky or more arid terrain, the footprint of a paleotsunami may be more difficult to discern and the techniques used must be adapted to the environment. Goff and other researchers are also looking for microscopic marine organisms such as diatoms and foraminifera, ancient DNA from marine life, changes in geochemistry and, as in the Atacama, unexpected rocks.

That Atacama tsunami likely occurred in 1420, says Tatiana Izquierdo, a paleotsunami researcher based at the Rey Juan Carlos University in Spain who helped discover it. she and her colleagues dug under the rock to find undisturbed sediments. They radiocarbon dated some of the seashells they found, giving a range of potential dates from the 14th to the 16th century. With further investigation, the team found historical records of a tsunami in Japan in 1420 that fit their dates. Izquierdo says his tsunami probably originated off the Chilean coast after a large earthquake and crossed the Pacific to Japan.

In other cases, paleotsunami researchers have obtained information from the archaeological record. Izquierdo says that archaeologists in Chile previously noted that suddenly, around 3,800 years ago, several coastal sites were abandoned, and new sites soon appeared inland. Additional evidence, such as shell middens, which showed evidence of having been eroded by strong currents, hinted at a possible paleotsunami.

Those dates line up perfectly with a massive paleotsunami that Goff found evidence of an ocean away in New Zealand, where rocks the size of cars it had been thrown nearly a kilometer inland. It’s a disaster that doesn’t appear in historical records, Goff says, and it’s a tsunami that likely hit islands throughout the South Pacific, including Vanuatu, Tonga and the Cook Islands. Paleotsunami researchers have yet to search 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 was more than a matter of historical interest. These data are of great value to contemporary coastal communities.

Predicting tsunamis is impossible. At best, residents may have minutes or hours’ 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 make landfall. The resulting alerts are based on computer models fed with data on how previous tsunamis behaved. If they are missing key events that do not appear in the historical record, such as those paleotsunami researchers are constantly discovering, the advisories may not be completely accurate.

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

That 2011 tsunami, generated by a magnitude 9.0 earthquake on the seabed off Japan, generated waves up to 40 meters high that traveled up to 10 kilometers inland. The water overflowed the levees and flooded more than 100 designated tsunami evacuation sites. It destroyed entire villages and crippled the Fukushima Daiichi nuclear power plant. More than 15,000 people died.

Part of the problem was Japan’s inadequate defenses. Researchers knew of three large tsunamis from historical records dating back to the 17th century, one of which produced waves nearly as high as the 2011 tsunami. However, officials based their tsunami defense preparations, including building a boardwalk and the location of tsunami evacuation zones, in a 1960 tsunami generated by an earthquake off the Chilean coast that produced waves in Japan only six meters high.

“We knew how big they could be [in Japan]. We knew these things must have spawned off the Japanese coast. And yet we weren’t ready for it,” says Goff.

The 2011 Tohoku tsunami was more destructive than almost any other in modern times. But as paleotsunami research shows, it was unprecedented.

Back in Chile, Izquierdo says she is particularly worried about what would happen if a tsunami comparable in size to the one that hurled rocks in the Atacama Desert hit today. In popular vacation spots, such as outside the city of Caldera, people have built houses near the beach. If a tsunami hits, those houses could be in serious danger.

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

This article is from Hakai Magazine, an online publication on science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.

Related Stories from Hakai Magazine:

Recommended videos