# Paleotsunami

A paleotsunami is a tsunami that occurs prior to written history where there are no documented observations. Paleotsunamis are evidenced by modern technology and scientific research. One of the largest was a megatsunami resulting from the asteroid that wiped out the dinosaurs.
Studying paleotsunamis is an emerging science to identify and interpret paleotsunami deposits. There are several recorded paleotsunami records, but though some are known only by historical mentions, such as tsunamis resulting from the 1700 Cascadia earthquake which is known only from oral traditions among the Native Americans of the Pacific Northwest.

## Historical occurrences
### Chile

On the coast of Chile, boulders have been found that "suggest directionality from sea to land," and they "could not be transported by rolling."
On the northern Chilean coast, probable evidence of a tsunami exist as one boulder on the sand high above the Pacific can be found, dwarfing every other rock in view in a conspicuous manner. Based on the effects of a tsunami that hit Japan, a tsunami 20 m (66 ft) probably hit the Chilean coast in AD 1420, which swept boulders inland as if they were pebbles.
The 1420 Caldera earthquake generated tsunamis reaching Japan.
In the sea off of the Atacama near Caldera, on April 11, 1819, there was a magnitude 8.5 earthquake. It lasted roughly 7 min and almost completely demolished the city of Copiapó. A tsunami with waves up to 4 metres (13 ft) high was registered. It had reached all coasts within a radius of 800 kilometres (500 mi), including Hawaii.

### New Zealand

In New Zealand, large boulders have been found close to 1 kilometre (0.62 mi) inland. No tsunami appears in historical records, but it is estimated to have occurred around 1777 BC. It likely hit islands all across the South Pacific, including the Cook Islands, Tonga, and Vanuatu. Paleotsunami researchers do not yet know the full scale of the destruction the tsunami caused.

### China

A tsunami struck in AD 1076 in southern China, during the Song dynasty and nearly wiped out civilization in what is now Guangdong. On Lincoln Island of the Paracel chain in the South China Sea, large rocks and coral have been deposited on the island far away from the coast which can be explained to be moved there due to the tsunami. The earthquake causing it was probably in the Manila Trench. Historical record show that earthquake activity was largely cut off and major activity did not resume for centuries.
There is evidence of paleotsunami events occurring on Taiwan.

### The Cascadia subduction zone

Off the coast of the American Northwest, the 1700 Cascadia earthquake generated a tsunami. It was recorded in Japan. The Indigenous peoples of the Pacific Northwest Coast carried the story on in many oral traditions, though they left no written records. There are records of several paleotsunamis hitting the southwest coast of Canada, northwest coast of the United States through northern California.

### Eastern Mediterranean

In the Eastern Mediterranean, there has been evidence found of paleotsunamis occurring.

## List of historic paleotsunamis

| Year | Location | Main article | Primary cause | Description |
| ≈1.4 Ma | Molokai, Hawaii | East Molokai Volcano | Landslide | One-third of the East Molokai volcano collapsed into the Pacific Ocean, generating a tsunami with an estimated local height of 2,000 feet (610 m). The wave traveled as far as California and Mexico. |
| ≈9.91–9.29 ka | Dor, Israel | | Unknown | A mega-tsunami had a run of at least 16 metres (52 ft) and traveled between 1.5 and 3.5 km (0.9 and 2.2 mi) inland from the ancient Eastern Mediterranean coast. |
| ≈7000–6000 BC | Lisbon, Portugal | | Unknown | A series of giant rocks and cobblestones have been found 14 metres (46 ft) above mean sea level near Guincho Beach. |
| ≈6225–6170 BC | Norwegian Sea | Storegga Slide | Landslide | The Storegga Slides, 100 kilometres (62 mi) northwest of the coast of Møre in the Norwegian Sea, triggered a large tsunami in the North Atlantic Ocean. The collapse involved around 290 kilometres (180 mi) of coastal shelf, and a total volume of 3,500 km$^3$ (840 cu mi) of debris. Based on carbon dating of plant material in the sediment deposited by the tsunami, the latest incident occurred around 6225–6170 BC. In Scotland, traces of the tsunami have been found in sediments from Montrose Basin, the Firth of Forth, up to 80 kilometres (50 mi) inland and 4 metres (13 ft) above current normal tide levels. |
| 5,500 BP | Northern Isles | Garth tsunami | Unknown | The tsunami may have been responsible for contemporary mass burials. |
| ≈1600 BC | Santorini, Greece | Minoan eruption | Volcanic eruption | The volcanic eruption in Santorini, Greece triggered tsunamis which caused damage to some Minoan sites in eastern Crete. |

## Future implications

Scientists continue to find evidence of ancient tsunamis larger than those recorded in historical records.
The tsunami caused by the 2011 Tōhoku earthquake is a prime example of the dangers of ignoring evidence of past tsunamis. It was generated by a megathrust earthquake and made tsunamis up to 40 metres (130 ft) high. It washed over sea walls and drowned over 100 designated tsunami evacuation sites. From historical records, there were three large tsunamis dating back as far as the 17th century, some producing waves dozens of meters high. However, the Japanese based many of their tsunami-defense preparations on smaller tsunamis that had previously hit Japan. In 2011, tsunamis destroyed entire cities, crippling the Fukushima Daiichi Nuclear Power Plant. Over 15,000 people were killed by the tsunami. Not long before the Tōhoku earthquake, the Japanese had set up tsunami stones, warning of tsunami danger. One reads "High dwellings are the peace and harmony of our descendants. Remember the calamity of the great tsunamis. Do not build any homes below this point."

## Megatsunami on other planets

The surface of Mars once had oceans but is now dry, and a 2019 study found a paleotsunami may have ravaged some of the surface after a cosmic impact similar to the one that created the Chicxulub crater and likely ended Earth's age of dinosaurs. The impact may have made Pohl Crater. Near where Viking I landed were many boulders, possible debris from a megatsunami, which may have struck perhaps 3.4 billion years ago. The megatsunami could have reached 930 miles (1,500 km) from the impact site, well past Viking 1's landing site. The tsunami may have been 1,640 feet (500 m) high on the ocean, and perhaps 820 feet (250 m) on land.
What happened was possible via two different scenarios, one caused by a 5.6 miles (9.0 km) asteroid meeting "strong ground resistance," releasing 13 million megatons of TNT energy, or a 1.8 miles (2.9 km) asteroid hitting the softer ground, releasing 0.5 million megatons of TNT energy.