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EARTHQUAKE HAZARDS IN THE PACIFIC NORTHWEST

Earthquakes from three different sources threaten communities in the Pacific Northwest. These sources are crustal, intraplate, and subduction zone earthquakes.

Crustal Earthquakes:
•	most common
•	typically occur at relatively shallow depths of 6-12 mi (10-20 km) below the surface
•	two largest earthquakes in recent years in the Pacific Northwest-- Scotts Mills, Oregon (magnitude 5.6) Klamath Falls, Oregon main shocks (magnitude 5.9 and magnitude 6.0 of )

Intraplate Earthquakes:
•	deeper, occur within the remains of the ocean floor that is sliding beneath North America
•	could occur beneath much of the Northwest at depths of 25-37 miles (40-60 km)
•	caused damage in the Puget Sound region in 1949 and 1965

Subduction Earthquakes:
•	occur around the world where plates that make up the surface of the earth collide--one plate slides (subducts) beneath the other, where it is reabsorbed into the mantle of the earth dipping interface between the two plates is the site of some of the most powerful earthquakes ever recorded, often having magnitudes
•	1960 Chilean (magnitude 9.5)
•	1964 Great Alaskan (magnitude 9.2)

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HISTORY

Since the late 1980's there has been a significant increase in the understanding of earthquake hazards in the Pacific Northwest. The Cascadia Subduction Zone, which lies off the Oregon and Washington coasts, has been identified, even though there have been no Subduction Zone earthquakes during our 200-year historical record.

In the past several years, a variety of studies have found widespread evidence that very large earthquakes have occurred repeatedly in the past, most recently about 300 years ago in January, 1700. The best available evidence indicates that these earthquakes occur, on average, every 500 to 540 years with an interval between individual events that ranges from 100-300 years to about 1000 years. There is every reason to believe that they will continue to occur in the future.

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PLANNING

Planning to respond to earthquake disasters and strengthening homes, buildings utility lifelines and communications systems can greatly reduce the impact of an earthquake.

These measures should be based on the best possible forecast of the amount and distribution of future earthquake damage. The amount of damage sustained by a building during a strong earthquake is difficult to predict and depends on the size, type and location of the earthquake, the characteristics of the soils at the building site, and the characteristics of the building itself.

At present, it is not possible to accurately forecast the location or size of future earthquakes. It is possible however to predict the behavior of the soil (soil means the relatively loose and soft geologic materials that typically overlie solid bedrock in the Northwest) at any particular site. In fact, in many major earthquakes around the world, a large amount of the damage has been due to the behavior of the soil.

The Oregon Department of Geology and Mineral Industries has created maps that identify areas in selected Oregon communities that will suffer more damage, relative to other areas, during a damaging earthquake. The analysis is based on the behavior of the soils, and does not depict the absolute earthquake hazard at any particular site. It is quite possible that, for any given earthquake, damage in even the highest hazard areas will be light. On the other hand, during an earthquake that is stronger or much closer than our design parameters, even the lowest hazard categories could experience severe damage.

Learn more about earthquakes on thr Oregon Department of Geology web page.

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