The strongest earthquake in Oregon’s history hit the State from an offshore epicenter. Damage from this particular earthquake was minimal.
Two of the largest earthquakes in Oregon occurred in 1910 and 1993. The 1910 earthquake was the largest historical shock within the state’s boundaries at a magnitude of 6.8, but it occurred too far offshore to cause damage, whereas the damaging 1993 earthquake was the largest historical earthquake beneath the land area of Oregon, with a magnitude of 5.9.
There have been no big earthquakes in Oregon’s brief history, and there is no question that damaging earthquakes have been far less frequent in Oregon than in California or Washington. However, geologic research tells scientists that Oregon will someday experience big earthquakes, and both the Scotts Mills earthquake of March 25, 1993, and the Klamath Falls earthquake of September 20, 1993, confirm such research. Because the Oregon is poorly prepared, the damage could be great.
Geologic research has shown that Oregon and Washington have probably been shaken by numerous subduction zone earthquakes during the last several thousand years. Subduction zone earthquakes occur when two great crystal plates slide past each other beneath the coast of Oregon and Washington. These earthquakes occur, on average, every 300–600 years, and the most recent was about three hundred years ago. The subduction zone earthquakes were probably centered just off the coast of Oregon and Washington and may have been as large as magnitude 8 to magnitude 9. Such earthquakes would cause significant shaking and damage in much of western Oregon. Scientists cannot predict whether the next such event might occur in two years or two hundred years.
Local earthquakes are most common in the Portland metropolitan area, northern Willamette Valley, and Klamath Falls area and may threaten the coast from Coos Bay south to Brookings. There is little knowledge at the present time as to the risk of local earthquakes in most other parts of western Oregon. All of Oregon west of the Cascades, as has been pointed out above, is at risk from the subduction earthquakes that will come someday as the Juan de Fuca Plate continues to move beneath the North American Plate. The amount of earthquake damage at any place will depend on its distance from the epicenter, local soil conditions, and types of construction. To date, no fault in western Oregon has been proven to be likely to move in an earthquake. Although many faults have been identified, it cannot be said yet whether being near a fault is any more hazardous than being far from one.