Many inhabitants of the planet are aware of the San Andreas Fault, which runs through California. It is known that it can cause the strongest earthquake and cause a large tsunami. However, few people know that there is another watershed to the north of the fault, which carries a much greater threat.
In January 1700, a powerful tsunami hit the coast of Japan, which no one expected. The locals did not feel the earthquake, which could have caused a giant wave. Only more than 300 years later, scientists were able to figure out where this tsunami came from, and were even able to simulate a map of its distribution.
Scientists have conducted a thorough analysis of the historical records of Japan and the accounts of Native Americans, and also studied the sedimentary deposits of rocks in the Northwest America. It turned out that the tsunami came from the California coast and was caused by a magnitude 9.2 earthquake that shook the Cascadia subduction zone on January 26, 1700. Nine hours after the earthquake, a giant tsunami wave hit Japan.
What is Cascadia?
The Cascadia Subduction Zone is a fault that runs from Vancouver Island to Northern California. It stretches for 900 kilometers from the California Cape Mendocino to the Canadian Vancouver. At this point, tectonic plates overlap one another. The junction of these plates is the subduction zone.
Cascadia zone joints. A volcanic ridge formed after the fracture of the tectonic plate is marked in red.
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Photo: NASA
Gradually, the oceanic plate creeps onto the plate of the North American continent, squeezing it. As a result, North America shrinks by 30–40 mm each year. According to seismologists, sooner or later the pressure between the plates will accumulate to the limit, after which a strong shock will occur, leading to a mega-earthquake with a magnitude of 8.7 to 9.2.
During the movement of the plates, the edge of the North American continent will drop by almost two meters, after which it will sharply shift by 10-30 meters. Most of the shift will take place in the ocean, causing a giant wave. Part of it will go towards Japan, and part - on the coast of North America itself. According to scientists, the wave can rise to a height of 30 meters. According to the American Emergency Management Agency (FEMA), Cascadia will cause the death of more than 13 thousand residents of the United States and Canada.
Photo: hansol / flickr.com / CC BY 2.0
Tsunamis can also lead to increased soil moisture and the formation of powerful mudflows. They could flood Willamette Valley, Puget Sound, Seattle, Tacomu, Portland and other communities along the Columbia River. According to scientists, the consequences of these landslides will be much larger than the landslide in the Nepalese village of Langtang, which came down last year as a result of an earthquake.
What is the likelihood of a cataclysm?
At present, tremors are not felt in this region, so the locals do not think that this can happen. However, scientists estimate the probability of an earthquake in the next 50 years as 1: 3, and the probability of a powerful earthquake as 1:10. Moreover, 45 years ago, no one even knew about the existence of Cascadia.
Scientists have noticed the remains of dead trees in this region. The study showed that they all died at the same time. The end rings of the trees date from the summer of 1699. This coincided with an earthquake in January 1700, during which there was a sudden subsidence of the ground and a rise in waves.
Cascadia subduction zone area.
Photo: Alicia.iverson / wikipedia.org / CC BY-SA 4.0
The study of this region showed that over the past 10 thousand years, earthquakes have occurred here 41 times. That is, the average time interval between shocks is about 243 years. Considering that the last cataclysm here was in 1700, a new earthquake can occur at any moment. At the same time, researchers believe that Cascadia can bring much more trouble and destruction than the famous San Andreas fault. San Andreas can only cause a series of earthquakes, while the movement of Cascadia will entail a giant tsunami.
Meanwhile, Cascadia has been hidden from people for so long that America is not ready for the likely destructive events. For July of this year, US authorities have planned a large-scale exercise in the event of a catastrophic earthquake in the Cascadia subduction zone. Such exercises were already held in October last year, and FEMA plans to conduct them in the future.