Download Geologic Framework of Western Oregon and Washington

Geologic Framework and Tectonic Setting of the Pacific Northwest
Figure 1. Location
map of Pacific
Northwest
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By Ben Shivers, Earth Science Major, Western Oregon University, Monmouth, OR 97361, [email protected]
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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Figure 3. Elevation model showing the tectonic setting of the Pacific Northwest:
Image obtained from Ralph Archuleta, University of California Santa Barbara.
3 Stress Provinces In the Pacific NW
Stress provinces in the Pacific Northwest include the
following:
1. Pacific Northwest stress province
N-S orientated compressive stress
2. Cascade Convergence stress province
NNE-SSW orientated compressive stress
3. Basin and Range Stress province
E-W orientated tensional stress
4. Canadian mid-plate stress province
NE-SW orientated compressive stress
(Geist, 1996)
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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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 300500 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.
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
Earth’s 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 36–48. 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 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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Conclusions
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 can’t be said for much of the
densely populated areas in the region.
Figure 4. Stress direction history of the western United States
and Mexico since 85 Ma. Image obtained from Bird (2002).
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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)
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.