Portland, Oregon, is seismically active and subject to earthquake risk due to its location in a region of significant geological activity. Residents face potential threats from two distinct sources of seismic energy. Understanding the underlying geology is necessary to prepare for future ground shaking. This information explains the forces at work, the historical context of the risk, and the immediate steps residents can take to enhance their safety.
The Geological Forces Affecting Portland
The primary source of seismic hazard for the Pacific Northwest is the Cascadia Subduction Zone (CSZ), an oceanic plate boundary located offshore. The Juan de Fuca tectonic plate is slowly sliding, or subducting, beneath the North American continental plate. This boundary stretches for approximately 700 miles from northern California to British Columbia, lying about 70 to 100 miles off the Oregon coast.
The plates are currently locked, causing massive strain to accumulate. This strain will eventually be released in a megathrust earthquake. Estimates suggest this event could reach a magnitude between 8.0 and 9.0 or higher. Such a rupture would generate powerful ground shaking that could last for five to seven minutes in the Portland metropolitan area.
A second, more localized threat comes from shallow crustal faults situated beneath and surrounding the city. These faults, such as the Portland Hills Fault and the East Bank Fault, result from the North American plate deforming under pressure from the subducting plate. The Portland Hills Fault, estimated to be 40 to 60 kilometers long, runs through the western part of the city. While these faults generate smaller earthquakes than the CSZ, a rupture here poses a severe risk due to proximity to dense urban areas.
History of Seismic Activity and Future Projections
The most powerful seismic event in the region’s recorded history occurred approximately 300 years ago. On January 26, 1700, the Cascadia Subduction Zone ruptured along its entire length, generating an estimated magnitude 9.0 earthquake. Evidence for this colossal event comes from geological records, Native American oral histories, and tsunami records documented in Japan.
Geological data indicates that megathrust events have occurred on the CSZ roughly 43 times in the last 10,000 years. The average recurrence interval is about 350 years in the most recent cluster of events. Scientists estimate there is currently about a 37% chance of a magnitude 7.1 or greater earthquake occurring on the CSZ in the next 50 years. Local crustal faults, like the Portland Hills Fault, have much longer recurrence intervals, estimated between a few thousand and more than 10,000 years.
The impact of either a megathrust event or a large local fault earthquake is compounded by Portland’s underlying geology, particularly the risk of liquefaction. Liquefaction occurs when intense shaking causes saturated, sandy soils to temporarily lose their strength and behave like a liquid. Areas built on river sediments, such as the land along the Willamette and Columbia rivers, are highly susceptible to this phenomenon. The Northwest industrial area and the Portland International Airport face a high risk of catastrophic ground failure in a major earthquake.
Immediate Preparation Steps for Residents
Preparing for a large earthquake involves taking practical steps to secure the immediate environment and ensure self-sufficiency. Inside the home, residents should focus on anchoring tall, heavy items that could fall and cause injury or block exit routes. This includes bolting bookshelves and cabinets to wall studs and securing water heaters with metal strapping.
Every household should assemble an emergency supply kit capable of sustaining all occupants for a minimum of two weeks. The kit should contain:
- At least one gallon of water per person per day.
- Non-perishable food.
- Necessary medical supplies.
- Important documents, such as copies of insurance policies and identification, stored in a waterproof container.
All residents must know the basic actions to take during ground shaking: Drop, Cover, and Hold On. This means immediately dropping to the floor, taking cover under sturdy furniture, and holding onto it until the shaking stops. Families should establish an out-of-state contact person and a specific meeting place to reconnect, as local phone lines and cell service may be unavailable following a major disaster.