1700 Cascadia Subduction Zone Earthquake

On January 26^th, 1700, one of the world’s largest known earthquakes occurred on the Cascadia Subduction Zone. The massive undersea Cascadia megathrust fault ruptured along much of its 1000-kilometer length, potentially spanning Vancouver Island, British Columbia to Cape Mendocino, California. This resulted in an approximately magnitude (M) 9 earthquake that likely caused strong ground shaking throughout the Pacific Northwest, and also generated a devastating tsunami that affected the coastline and areas throughout the Pacific Ocean. This event is the most recent great Cascadia Subduction Zone earthquake among the 19 events identified in the geologic record in the last 10,000 years.

Numerous studies (e.g., Satake et al.) have carefully modeled the 1700 earthquake and tsunami impacts and concluded that its magnitude was between M 8.7 and M 9.2.

In late January 1700, a tsunami mysteriously arrived along the coast of Japan unaccompanied by strong ground shaking or other evidence of a local earthquake. Over 300 years later, careful analysis of Japanese historic records, coastal geologic evidence in the Pacific Northwest, and Native American oral histories revealed that the cause of the “orphan” tsunami that struck Japan was a massive earthquake that originated in the Pacific Northwest.

The tsunami would have taken about 9 hours to cross the Pacific Ocean to reach Japan, so the earthquake must have occurred around 9 pm in Cascadia (05:00 January 27 UTC). This is supported by Japanese written records, which state that a tsunami struck Japan before midnight on January 27, 1700 (14:00 January 27 UTC). Detailed descriptions of the tsunami size and arrival time in the Japanese records allow us to accurately constrain the size and timing of this great Cascadia earthquake.

Another significant piece of evidence for the 1700 Cascadia earthquake comes from dead trees in “ghost forests” along the coast of the Pacific Northwest. These ghost forests were previously lush coastal forests that are thought to have been instantly killed by saltwater inundation when the coastline suddenly dropped 1-2 meters (3.3-6.6 feet).

By analyzing the tree rings of dead trees, Yamaguchi et al. gave the 1700 Cascadia earthquake a firm place in history, even though it predates the Pacific Northwest’s earliest written records by nearly a century. The tree ring studies showed no signs of gradual drowning, as would be expected from long-term processes (e.g., sea level rise). Instead, wide rings up until the time of death suggested that the trees were healthy and experienced an abrupt environmental change, which could be explained by a down-dropping of the Earth’s surface (i.e., subsidence) due to an earthquake. Tree ring dating suggests that the trees died sometime during the winter of 1699-1700, further supporting the January 26th, 1700 date of the Cascadia earthquake.

The impacts of the 1700 Cascadia earthquake are difficult to constrain due to the lack of local written records. However, the Indigenous Native American and First Nation tribes living on the coast of Vancouver Island, Canada have passed down oral histories from generation to generation that tell of an earthquake and tsunami on a winter’s evening that collapsed houses of the Cowichan people and caused numerous landslides. Accounts of the great earthquake recorded from Coast Salish and Nuu-chah-nulth elders describe strong shaking and a destructive tsunami (summarized in Ludwin et al., 2005). The tribes described shaking that was so violent that people could not stand, and lasted so long that it made them seasick. On the west coast of Vancouver Island, the winter village of the Pachena Bay people was completely destroyed by the tsunami that struck the area. There were no survivors in the low-lying settlements located less than 75 feet above sea level. There are likely many more stories of the earthquake-induced damage in the Pacific Northwest that will remain untold.

The 1700 Cascadia earthquake serves as a reminder that future great earthquakes have the potential to impact the people, buildings and infrastructure, and our environment in the Pacific Northwest.  Scientists at the University of Washington, U.S. Geological Survey, and elsewhere are continually working to improve our understanding of future Cascadia megathrust earthquakes that may impact the Pacific Northwest.

Individuals can also take simple steps to prepare for earthquakes and reduce the risk of injuries, damage, and loss of life. Identify hazards and secure shelves, furniture, and other movable items in your home and workplace. Develop an emergency plan and supply kit based on local guidelines. If you feel ground shaking, Drop, Cover, and Hold On. Be familiar with local tsunami evacuation routes, and if you are in an area that is susceptible to tsunamis during an earthquake, immediately make your way to high ground after shaking stops. For more information about how to prepare for an earthquake, visit here: Preparedness.

The 1700 Cascadia earthquake was a subduction zone megathrust earthquake. Along the coast of the Pacific Northwest, the Juan de Fuca plate is pushed under the continental North American plate, forming the Cascadia Subduction Zone. As the Juan de Fuca plate descends beneath North America, it bends and encounters increasing temperature and pressure. At depths shallower than 30 kilometers, the blocks of rock on either side of the CSZ fault become stuck or “locked” by friction and are unable to easily slide past each other. Strain slowly builds up until the fault's frictional strength is exceeded and the rocks slip past each other along the “megathrust” fault in an earthquake. Megathrust earthquakes will likely generate intense shaking across a large portion of the Pacific Northwest, potentially causing major damage throughout the region.

For more information about subduction zone earthquakes, visit here: Cascadia Subduction Zone Megathrust.