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.

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 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 m high was registered. It had reached all coasts within a radius of 800 km, including Hawaii.



New Zealand
In New Zealand, large boulders have been found close to 1 km 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.

Current dangers
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 m 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. 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 3.4billion years ago. The megatsunami could have reached 930 mi from the impact site, well past Viking 1's landing site. The tsunami may have been 1640 ft high on the ocean, and perhaps 820 ft on land.

What happened was possible via two different scenarios, one caused by a 5.6 mi asteroid meeting "strong ground resistance," releasing 13 million megatons of TNT energy, or a 1.8 mi asteroid hitting the softer ground, releasing 0.5million megatons of TNT energy.

External links and references

 * https://paleoseismicity.org
 * The USGS on paleotsunamis
 * Paleotsunami Research—Current Debate and Controversies
 * How likely is it that a mega-tsunami strikes Hawaii?
 * Chapter 3 – Paleotsunami Research—Current Debate and Controversies
 * Age determination of paleotsunami sediments around Lombok Island, Indonesia, and identification of their possible tsunamigenic earthquakes
 * How Big Were Ancient Tsunamis? Paleotsunami Researchers Examine Olden Disasters
 * In search of Holocene trans-Pacific palaeotsunamis
 * Paleotsunami History Recorded in Holocene Coastal Lagoon Sediments, Southeastern Sri Lanka
 * Tsunami Deposits in the US Pacific Northwest
 * Paleotsunami research along the Nankai Trough and Ryukyu Trench subduction zones – Current achievements and future challenges
 * Paleotsunami history of Hachinohe, northern Japan: a multiproxy analysis and numerical modeling approach
 * USGS: Paleotsunami