At about 9:00 p.m. on the evening of January 26, 1700, the largest earthquake to hit the lower 48 states occurred just off the Cascadia Pacific coast. The magnitude 9 megathrust earthquake and resulting tsunami damaged not only the Northwest Pacific coastline, but villages in Japan as well. Three hundred years ago, the damage would have been minimal. Today, the result would be much different.
And that's why Oregon is preparing.
The Oregon legislature has just been presented with, and is considering, the Oregon Resilience Plan — a report containing policy and investment recommendations spanning the next fifty years in preparation for the next Big One. According to a timeline for the Cascadia subduction zone, a magnitude 9 earthquake is overdue.
It wasn't until the 1980s that scientists realized the Cascadia subduction zone was an active fault — potentially affecting Oregon, Washington, northern California and southern British Columbia. Lying 50-75 miles off the Oregon coastline, it is where the Juan de Fuca Plate subducts under the North American Plate. Many minor earthquakes and tsunamis have been recorded in this region. But it wasn't until fairly recently that scientists realized the last major earthquake occurred in 1700.
What is known about the 1700 quake is that the length of the offshore fault rupture was 620 miles, covering an area from Vancouver Island to northern California. The shaking destroyed houses on Vancouver Island and caused landslides. The shaking was so violent that people were unable to stand during it; and it was so prolonged that it made people ill. The tsunami completely destroyed the village of the Pachena Bay People on the west coast of Vancouver Island, killing all of them. It also caused parts of the coast in Washington to drop by as much as five feet.
The tsunami traveled the Pacific Ocean and damaged coastal villages in Japan. It also destroyed the rice crop that was stored in coastal warehouses. It reached a height of 12 feet. This tsunami came to be known in Japan as the "Orphan Tsunami" because its origin was unknown — there had been no earthquake in the region to cause it, and it caught people completely unaware. It wasn't until 2005 that an international group of scientists determined its cause.
Some of the history of the 1700 quake is gleaned from oral tradition of the peoples of the area who were impacted by the event, which took place 100 years before Lewis and Clark explored the area. More information came from Japan, as the Japanese kept excellent records of the event's impact.
Evidence of the 1700 earthquake has been found by looking at tree rings: groves of red cedar trees drowned by the tsunami show the last growth period being the year 1699. Geologic evidence has provided a record of earthquake/tsunami events from the Cascadia region going back 10,000 years, showing regular intervals of large magnitude quakes.
A 13-year study by researchers at Oregon State University, led by Chris Goldfinger, a professor in OSU's College of Earth, Ocean, and Atmospheric Sciences, was concluded and published by the U.S. Geological Survey in 2012. The study concluded that there is a 40% chance of a major earthquake in southern Oregon within the next 50 years. The resulting tsunami could rival the 2004 Indian Ocean tsunami.
The study also determined that large earthquakes hit the area about every 240 years. Since it's been 313 years since the last one, the area is long overdue; all the more reason for the area to be prepared.