Portland Oregon Earthquake Fault Zones & Earthquake Types

Earthquake insurance in Oregon should be a consideration for all homeowners because the risk of an earthquake is likely as Oregon is designated a ‘very high seismic hazard’ and ranks third in the nation for earthquake damage estimates in the future. Projected losses in the Cascadia region alone could exceed $12 billion, with over 30,000 destroyed buildings, and 8,000 lives lost in the event of a magnitude 8.5 Cascadia Subduction Zone earthquake. 1

Based on a March 4, 2001 Oregonian article by Richard Hill, Oregon and the city of Portland faces danger from three types of earthquakes, all of which are potentially lethal:

The first are Benioff-zone earthquakes, which are also called intraplate or slab earthquakes. These occur 25 to 40 miles deep on faults within the subducting oceanic crust of the the Juan de Fuca Plate which is located off of the coast of Oregon.

The second are crustal earthquakes which are produced by shallow faults as deep as 15 miles below the surface and are usually from local faults. These are the most common earthquakes that we experience.

The third are Cascadia Subduction Zone earthquakes. These are of particular worry to scientists, because they could cause a disaster unparalleled in U.S. history. The earthquake would occur offshore, where the heavier Juan de Fuca Plate plunges, or subducts, beneath the North American Plate as it heads eastward. The plates meet in a line that is 60 to 150 miles offshore and parallel to the coast, running 750 miles from British Columbia to Northern California. Evidence shows that the plates are sticking as they pass each other, building up tremendous pressure that could be unleashed in a magnitude 8 or 9 earthquake.

Geologic evidence shows that the Cascadia Subduction Zone has generated great earthquakes, and that the most recent one was about 300 years ago. What can not be answered is when the next Cascadia Subduction Zone earthquake will be. It is not a question of it but when.

In the same article Hill also reports that scientists have identified three large faults beneath Portland: the East Bank Fault, the Portland Hills Fault and the Oatfield Fault. The faults run parallel to one another along the Willamette and are about 1-1/2 miles apart.

The East Bank Fault, on Portland's east side, underlies Central Catholic and Benson high schools, Lloyd Center, the Oregon Convention Center, the Rose Garden arena, Mocks Bottom and the University of Portland. The Portland Hills Fault runs from the northern edge of Forest Park, goes along the foot of Portland's West Hills and beneath Portland State University, crosses the Willamette River and heads southeast to Milwaukie. The Oatfield Fault runs west of Skyline Road from Sylvan Hill to Germantown Road through Bonny Slope.

Based on the City of West Linn Natural Hazards Mitigation Plan there is a direct relationship between a fault’s length and its capabilities for generating earthquakes. Oregon earthquake fault lines such as the Oatfield and Portland Hills faults are large, running for up to 30 miles. Earthquakes on these local faults can be of high magnitude, and can cause intense shaking. The duration of the event is generally short seconds rather than minutes. Smaller nearby faults produce lower magnitude events, but their ground shaking and damage can be intense because of the fault’s proximity and local soil conditions.

Earthquake fault lines that lay within Washington County include the Tualatin-Sherwood, Costco, and Gales Creek faults.

Earthquakes can trigger other types of ground failures which could contribute to damage to your home. These would include:

  • Landslides – this is an abrupt movement of soil and bedrock downhill in response to gravity.
  • Dam failures - this is the sudden, rapid, and uncontrolled release of impounded water.
  • Liquefaction - this is when ground shaking mixes groundwater and soil, liquefying and weakening the ground that supports buildings and severing utility lines. This is a significant problem in low-lying areas adjacent to rivers where the water table is shallow and the soils are subject to liquefaction.


References

  1. Wang, Yumei and J.L. Clark. 1999. Earthquake Damage in Oregon: Preliminary Estimates of Future Earthquake Losses. DOGAMI: Special Paper 29.
  2. Natural Hazard Mitigation Plan Josephine County Oregon, August 2004
  3. Hill, Richard. “A region at risk.” The Oregonian. March 4, 2001
  4. The City of West Linn Natural Hazards Mitigation Plan
  5. Washington County Mitigation Action Plan: Earthquake; April 2004