Geologic Framework and Tectonic Setting of the Pacific Northwest

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Geologic Framework and Tectonic Setting of the
Pacific Northwest
By Ben Shivers, Earth Science Major, Western
Oregon University, Monmouth, OR 97361,
bshivers05_at_wou.edu
Figure 1. Location map of Pacific Northwest
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Abstract
The Pacific Northwest is a tectonically active
region comprised of complex fault systems in
conjunction with the Cascadia subduction zone.
The tectonic setting is characterized by a long
history of oblique convergence of the Juan de
Fuca plate beneath North America. Subduction
processes are manifested in the Pacific Northwest
by an offshore trench, accretionary Coast Range
complex, Puget-Willamette lowland, and Cascade
Volcanic arc. Seismicity in the region is
driven by an assortment of stress regimes that
are propagated via faults in the underlying
bedrock. Earthquakes are triggered by movement
along these fault systems and the hazards
associated with the Northwest include ground
shaking, liquefaction, landslides, structural
damage and tsunamis. Many earthquakes in this
region stem from shallow-crustal and
deep-intraplate fault movements, both of which
commonly produce magnitudes of 5.0-7.0 with
recurrence intervals between 10-30 years.
Primary concerns now focus upon a catastrophic
9.0 magnitude earthquake centered within the 800
mile long Cascadia subduction zone. Research
places seismic recurrence intervals along the
subduction zone between 300-500 years with the
last Cascadia event occurring January 26, 1700
off the coast of Oregon.
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Tectonics of Pacific Northwest
Oregon tectonics are a complex mix of
compression, extension, and rotation. The primary
driving force stems from the Cascadia Subduction
zone which lies off shore in the Pacific Ocean.
Along with the production of magma through
partial melting of the subducting lithosphere,
the subduction zone is also associated with
seismic activity throughout the region due to the
locking and releasing of the plates. Oregon
is separated by many different regions like the
Blue and Klamath mountains, the Willamette
valley, the Coast Range, and the Western and High
Cascades. Each of these regions are characterized
and uniquely defined by their own seismic
history, and more importantly their seismic
potential. A majority of the seismic events that
occur in Oregon are crustal quakes occurring
along major fault systems. Intraplate quakes have
occurred in the past and are usually quite
damaging with magnitudes that can reach as high
as 7.5. The last intraplate quake in Oregon
occurred in Portland in 1949, and these are the
quakes that have traditionally rocked the Puget
Sound region. By far, the most prolific earth
quake type is one that occurs along a subduction
zone. These cataclysmic forces can have
magnitudes in excess of 9.0 and cause immense
casualties and destruction. A Subduction zone
earthquake would likely generate an enormous
tsunami hundreds of feet tall that would
annihilate the coastlines of the Pacific
Northwest. Ground shaking would also topple
building and cause liquefaction through many
parts of the Northwest because of the
unconsolidated alluvium on which the region is
based upon. Fires from broken gas lines would
terrorize urban areas, and flooding would likely
occur as well. Recurrence intervals have been
placed between 300-500 years for this type of
earthquake in the Northwest, so it is pertinent
that action be taken to establish an earthquake
mitigation program. People living in the
Northwest must be educated on the dangers of this
seismically active region and how to effectively
prepare for a large scale earthquake which is
likely to occur in the next 100-200 years.
Figure 3. Elevation model showing the tectonic
setting of the Pacific Northwest Image obtained
from Ralph Archuleta, University of California
Santa Barbara.
Figure 7. Map showing ground peak acceleration
across Oregon. Map and data created by state
geologist Donald A. Hull of the Oregon Department
of Geology and Mineral Industries.
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Stress Provinces In the Pacific NW
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Conclusions

• Stress provinces in the Pacific Northwest include
  the following
• Pacific Northwest stress province
• N-S orientated compressive stress
• Cascade Convergence stress province
• NNE-SSW orientated compressive stress
• Basin and Range Stress province
• E-W orientated tensional stress
• Canadian mid-plate stress province
• NE-SW orientated compressive stress
• (Geist, 1996)

The Pacific Northwest is region that
produces a lot of seismic activity due in part to
the many stress regimes present. The primary
focus of this project was to create an overall
image and conceptual understanding of the
tectonic setting in which the Pacific Northwest
is apart of. Based on the research that took
place during the course of this project it is
apparent that the Pacific Northwest is one of the
most dangerous places to live. For that reason it
is absolutely necessary to develop a knowledge of
the landscape and how we as inhabitants can
better prepare and mitigate a large scale
disaster like a Cascadia Subduction zone quake.
With the amount of hazards in the Pacific
Northwest associated with seismic events and the
lack of ability to effectively predict
earthquakes, it is essential to create new
building codes and laws requiring a thorough
inspection of the stratigraphy beneath a building
or structure and new engineering techniques that
can withstand major earthquakes. Because a large
majority of the population in the Northwest lives
in the Puget Sound and Willamette valley areas,
which are comprised of unconsolidated alluvium
and soil, liquefaction and ground shaking
acceleration are a major concern that needs to be
addressed. Tsunami warnings have already begun to
be installed and retrofitted along the
coastlines, but the same cant be said for much
of the densely populated areas in the region.
Figure 2. Cross section of Cascadia Subduction
zone. Circled section is a representation of the
locked Cascadia subduction zone. Image obtained
from Ralph Archuleta, University of California
Santa Barbara
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Introduction
Tectonics is the geologic study of crustal
lithosphere plate movement by internal forces and
mechanisms associated with Earth. Plate tectonics
are driven by geothermal energy and convection
within the ductile mantle. These processes move
the large lithospheric plates around the surface,
producing divergent, convergent, and transform
plate boundaries. Tectonics play a vital role in
the geomorphology of Earths landscapes and the
formation of fault systems around the world.
Because of its complex fault systems coupled with
the massive Cascadia subduction zone, the Pacific
Northwest has become one of the most intriguing
regions in the field of neo-tectonics. The
contemporary tectonic setting and geomorphology
of the region is a direct product of tension,
compression and rotation. Since 22 Ma, the
principal stress directions have rotated
gradually clockwise by a total of 3648. This
probably reflects the increasing shear force
applied to North America by the Pacific plate
along the lengthening San Andreas transform
system, perhaps with a final increment caused by
the formation of the transpressive left step of
the San Andreas in southern California (Bird,
2002).
Figure 4. Stress direction history of the western
United States and Mexico since 85 Ma. Image
obtained from Bird (2002).
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References
(1) Geologic framework of the Willamette
lowland aquifer system, Oregon and Washington.
Marshall W. Gannet and Rodney R. Caldwell. US
Geological Professional Survey Paper. (2)
Probable local precedent for earthquakes
magnitude 8 or 9 in the Pacific Northwest. Brian
Atwater. US Geological Survey. (3) Seismicity in
the Pacific Northwest Regional Tectonic Setting
Great Earthquakes. Ralph Archuleta, University
of California Santa Barbara. (4) Stress
Direction History of the Western United States
and Mexico since 85Ma. Bird, Peter, Department
of Earth and Space Sciences, University of
California, Los Angeles, California, USA
TECTONICS, VOL. 21, NO. 3, 10.1029/2001TC001319,
2002 (5) Earthquake Hazards Map For Oregon.
Oregon Department of Geology and Mineral
Industries. Hull, Donald A. (6) The Cascadia
Megathrust and Tectonic Stress in the Pacific
Northwest. 1996 USGS. Geist, Eric L. (7) Late
Pleistocene Stratigraphy in the South-Central
Puget Sound Lowland, Pierce county, Washington.
Figure 6. Mapped faults in Oregon based on recent
fault movement. Map and data created by state
geologist Donald A. Hull, of the Oregon
Department of Geology and Mineral Industries.
Figure 5. Map showing current plate boundaries,
movement, volcanic eruptions and seismic activity
(studentatlasoforegon.pdx.edu/PDFs/ Map18.pdf)