A 600-mile Fault in Slow Motion
Off the coast of the Pacific Northwest, the Cascadia Subduction Zone is a 600-mile-long fault extending from Northern California to British Columbia. Here, the oceanic Juan de Fuca tectonic plate slides beneath the North American plate at a rate of 3 to 4 centimeters per year. For centuries, a segment of these plates has been locked together, accumulating immense stress. When this stress is released, it triggers a "megathrust" earthquake, the most powerful type on Earth.
The last full-rupture event on this fault occurred around 9:00 PM on January 26, 1700. This quake, estimated at magnitude 8.7–9.2, caused the entire coastal region to suddenly drop by several feet. The rupture is thought to have involved a slip of about 20 meters along a 1,000-kilometer fault line. The resulting ground shaking would have lasted for several minutes, a duration capable of causing damage to structures.
Ghost Forests and an Orphan Tsunami
Scientists confirmed the precise timing of the 1700 earthquake by combining two sets of evidence from opposite sides of the Pacific Ocean. Along the coasts of Washington and Oregon, "ghost forests" of dead red cedar and Sitka spruce trees stand in tidal flats. Tree-ring dating, or dendrochronology, showed that these trees all died between the end of the 1699 growing season and the beginning of the 1700 season. The trees were killed when the earthquake caused the coastline to subside, plunging their roots into saltwater.
Across the ocean, Japanese historical records from January 27-28, 1700, describe a destructive tsunami that struck the coast without any preceding earthquake felt locally. Samurai and merchants documented the flooding, which they called an "orphan tsunami." Knowing the tsunami's travel time across the Pacific (about nine hours), researchers calculated the exact time of the parent earthquake in North America, corroborating the evidence from the ghost forests. Indigenous oral histories from the Pacific Northwest also contain accounts of a great, nighttime earthquake and subsequent flood that destroyed coastal villages.
Geological evidence from seafloor sediment cores reveals a long history of these massive quakes. Layers of sediment, called turbidites, are deposited by underwater landslides triggered by intense shaking. Analysis of these layers suggests that 19 full-margin ruptures, similar to the 1700 event, have occurred in the last 10,000 years. This indicates an average recurrence interval of roughly 500-600 years, though the time between events has been as short as a few hundred years.