Overview

The Pacific Northwest is the second most earthquake vulnerable location in the United States. Here in
Washington County, we are vulnerable to earthquakes from two primary sources – the Cascadia Subduction
Zone and local fault zones.


Plate Tectonics

Plate tectonics teach us that the Earth's crust is made up of large fractured plates of rock that are in constant motion.
Oregon resides on the North American Plate while just offshore is the Juan de Fuca Plate, which is moving toward and
under the North American Plate at roughly the same rate that your fingernails grow. This process is known as
subduction and the boundary between the plates is known as the Cascadia Subduction Zone.
                                  Cascadia Plate Tectonics
Two Earthquake Regions Threaten Washington County

Two types of earthquakes threaten Washington County as a result of the Juan de Fuca Plate subducting beneath
the North American Plate – Cascadia Megathrust and Crustal. The Cascadia Subduction Zone is capable of rupturing
and generating a "megathrust earthquake," an earthquake of magnitude 8.0 to 9.0 or larger. 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.

 
Types of Damage that Occur During an Earthquake

Earthquakes produce damages from ground shaking and liquefaction of soils, and from landslides and slope failures.
Intense and prolonged ground shaking will cause significant damages in Washington County. In residential settings,
wooden frame structures tend to incur minimal damages provided they are bolted to their foundations. Structures in
commercial and industrial settings built before modern seismic building codes, which in Oregon were adopted in 1993,
tend to have higher levels of damage – particularly unreinforced masonry buildings and non-ductile concrete structures,
which are buildings held up by non-reinforced or poorly reinforced concrete pillars and shear walls. Examples include
department stores, schools, parking structures, office buildings, and big-box stores.


Liquefaction                                                        

Liquefaction is a process that occurs when saturated               Car front end in Flood
soil is subjected to prolonged and intense shaking
causing it to behave like a liquid. When it occurs, soils
tend to settle and shift causing roads to fracture,
buildings to tip and shift, vehicles to sink, and buried
infrastructure (like pipelines, fuel tanks, and septic
systems) to become buoyant and break. The soils
within the Tualatin Valley of Washington County are
highly susceptible to liquefaction;therefore, it is
anticipated that significant damage to infrastructure and
structures will occur during the next big earthquake.


Additional Earthquake Information and Resources:

Historic Earthquakes in Oregon
Historic Earthquake of Cascadia
Pacific Northwest Seismic Network
Map of Recent Earthquakes in Oregon
Earthquake Google Earth/KML files 
Cascadia Region Earthquake Workgroup 
Oregon Resilience Plan
Modified Mercalli Intensity Scale
Turbidite Event History - Methods and Implications for Holocene Paleoseismicity
of the Cascadia Subduction Zone

Northwest Oregon fault zones and basins (USGS)


Earthquake Hazard Maps

Portland Hills Fault, M 6.8 Earthquake Peak Horizontal Acceleration
Earthquake Scenario and Probabilistic Ground Shaking Maps for the Portland, Oregon, Metropolitan
Earthquake Faults Interactive Map