Fault zones in and near Washington County
Three active local faults zones, consisting of shallow crustal faults, exist in proximity to Washington County – the Portland Hills fault zone beneath the Portland metropolitan area, the Gales Creek fault zone along the east flank of the Coast Range, and the Mount Angel fault zone in the northern Willamette Valley. These local fault zones can experience earthquakes of magnitudes as high as 6.8 to 7.2. The 1993 magnitude 5.6 Scotts Mills earthquake, originating from the Mount Angel fault zone, was one of the most damaging earthquakes in Oregon's history and is an example of a crustal earthquake.
The Portland Hills fault zone
The Portland Hills fault zone lies just east of Washington County. This zone is comprised of three crustal faults that trend roughly northwest to southeast and are about two miles apart – the Oatfield fault, which runs along and just west of the spine of the Portland Hills; the Portland Hills fault, which runs directly under downtown Portland; and the East Bank fault, which runs along the east bank of the Willamette River. According to the Oregon Department of Geology and Mineral Industries (DOGAMI), several lines of indirect evidence have led to the conclusion that the Portland Hills fault zone appears to be capable of generating a large-magnitude earthquake. These faults are believed to be capable of generating an earthquake of magnitude 6.8 to 7.2 on average once every 1,000 years.
In the past 150 years, the Portland metropolitan area has experienced six earthquake of magnitude 5.0 or greater. The most recent earthquake to significantly affect our region was the March 25, 1993, Scotts Mills earthquake, a magnitude 5.6 earthquake with an epicenter about 30 miles southeast of Washington County. Impacts from this earthquake included toppled chimneys, damages to the state capitol, school closures, and the failure of walls made of unreinforced masonry – damages totaled $30 million.
A Portland Hills earthquake will cause strong shaking
A large-magnitude earthquake in the Portland Hills fault zone would cause strong ground shaking and damages in Washington County. According to an earthquake scenario created by DOGAMI, a 6.8 magnitude earthquake along the Portland Hills fault would cause ground shaking intensities on the Modified Mercalli Scale ranging from violent (intensity IX) in the Portland Hills, to severe (intensity VIII) in Beaverton and Tigard, to very strong (intensity VII) in the central areas of Washington County.