Oregon is a seismically active state with a complex geological structure that exposes it to multiple distinct earthquake hazards. Its location along the Pacific coast places it on the Pacific Ring of Fire, a major belt of tectonic activity. This geographical position means Oregon faces two primary types of earthquake threats, each with different mechanisms and potential impacts.
The Threat of the Cascadia Subduction Zone
Oregon’s largest seismic threat originates offshore at the Cascadia Subduction Zone (CSZ), a massive 700-mile megathrust fault running from northern California to British Columbia. Here, the dense Juan de Fuca oceanic plate is subducting beneath the lighter, overriding North American continental plate. The plates are currently locked, causing immense strain to build up over centuries as the North American plate is slowly compressed and dragged downward.
When the locked section eventually ruptures, it will generate a megathrust earthquake, an event estimated to reach a magnitude between 8.0 and 9.0 or higher. The resulting ground shaking from this “Big One” could last for several minutes, with intense shaking potentially lasting five to seven minutes along the coast. The sudden release of stress will cause the seafloor to instantly move, displacing a massive volume of water and generating a devastating tsunami that will strike the Oregon coast within minutes of the initial shaking.
Tsunami waves generated by a full-margin CSZ rupture could reach heights of up to 100 feet in some coastal areas, inundating communities and causing catastrophic damage. Geological evidence suggests that such full-margin ruptures occur, on average, every 500 to 600 years, though the recurrence interval is irregular.
Local Faults and Shallow Crustal Quakes
Separate from the deep, offshore Cascadia megathrust are the local, shallow crustal faults that lie directly beneath Oregon’s most densely populated areas. These faults are situated entirely within the North American plate, in the upper 15 miles of the Earth’s crust. Earthquakes generated here are generally smaller in magnitude than a Cascadia event, typically ranging from magnitude 5.0 to 7.2, but their proximity makes them highly destructive to infrastructure.
The forces exerted by the subducting Juan de Fuca plate cause north-south compression, stressing these inland faults. The Portland metropolitan area is threatened by the Portland Hills fault zone, a system including the Portland Hills, East Bank, and Oatfield faults. These faults run directly beneath the city and are believed capable of generating a major earthquake in the range of magnitude 6.8 to 7.2.
Another significant crustal structure is the Mount Angel fault, located in the northern Willamette Valley near Salem. This fault was the likely source of the damaging 1993 Scotts Mills earthquake, which registered a magnitude of 5.6. Such moderate-sized, shallow quakes pose a substantial hazard because they generate intense ground shaking where people live and work. While a crustal quake would not generate a massive tsunami, it could trigger significant liquefaction and landslides in the Willamette Valley’s soft sediments.
Evidence from Oregon’s Seismic Past
The certainty of Oregon’s earthquake risk is validated by paleoseismology, a geological record where scientists examine physical evidence to reconstruct the history and timing of ancient earthquakes. This evidence confirms that great earthquakes have repeatedly struck the Oregon coast over millennia.
One of the most compelling forms of evidence is the presence of “ghost forests” along the coastline, such as those found at Neskowin. These are stands of dead trees that were rapidly submerged and killed by saltwater when the land abruptly dropped during past megathrust events. The preservation of the stumps in the anoxic muds provides a precise time capsule of when the sudden subsidence occurred.
Further corroborating the ghost forests are layers of tsunami sand deposits found interbedded with peat and marsh sediment along the coast. These distinctive sand sheets were deposited by massive waves that swept inland following the sudden lowering of the land, providing a clear stratigraphic record of multiple past tsunamis. Scientists have identified over 50 sites along the Cascadia margin containing these tsunami sand layers.
This geological sleuthing, combined with tree ring dating, allowed researchers to pinpoint the exact date of the most recent great Cascadia event: January 26, 1700. This timing was confirmed by historical Japanese records describing an “orphan tsunami” that struck their shores, generated by the massive M9.0+ rupture off the coast of North America. The paleoseismic record indicates the Cascadia Subduction Zone has experienced at least 43 major earthquakes over the last 10,000 years.