| Earthquake hazards in the Pacific NW |
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Earthquake Information from the Pacific Northwest Seismograph Network (PNSN)
Recent earthquakes in the Pacific Northwest
Most recent notable earthquake
Notable Pacific Northwest earthquakes since 1993
The latest earthquake news worldwide
Earthquakes from three different sources threaten communities in Oregon and the Pacific Northwest. These sources are crustal, subduction zone and intraplate earthquakes. The most common are crustal earthquakes, which typically occur along faults, or breaks in the earth’s crust, at shallow depths of 6-12 miles (10-20 km) below the surface. The two largest earthquakes in recent years in Oregon, Scotts Mills, (magnitude 5.6) and the Klamath Falls, main shocks (magnitude 5.9 and magnitude 6.0) of 1993 were crustal earthquakes.
Great subduction zone earthquakes occur around the world where the tectonic plates that make up the surface of the earth collide. When these plates collide, one plate slides (subducts) beneath the other, where it is reabsorbed into the mantle of the earth. This dipping interface between the two plates is the site of some of the most powerful earthquakes ever recorded, often having magnitudes of 8 to9 or larger. The 1960 Chilean (magnitude 9.5) and the 1964 Great Alaska (magnitude 9.2) earthquakes were subduction zone earthquakes.
Deeper intraplate earthquakes occur within the remains of the ocean floor that is beingsubducted beneath North America.The magnitude 6.8 intraplate earthquake that struck the Puget Sound area on February 28th, 2001 caused $2 billion in damage but was much less destructive than a crustal earthquake of the same magnitude would have been because of its great depth (33 miles deep). For example, the magnitude 6.7 crustal earthquake that struck Northridge, California in 1994 caused $44 billion in direct physical damage. Intraplate earthquakes have also caused damage in the Puget Sound region in 1949 and again in 1965. This type of earthquake could occur beneath much of the Northwest at depths of 25-37 miles (40-60 km).
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 short 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 600 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.
We now know this region may experience damaging earthquakes much larger than any that have occurred in the historical past. Because of this threat, 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.
Earthquake hazard maps and many other publications that will help you prepare for an earthquake are available through the Nature of the Northwest Information Center. We also have a wealth of resources and information available here on our website and through our offices in Portland, Baker City, Grants Pass and Newport.
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