Geologic Framework and Tectonic Setting of the Pacific Northwest

By Ben Shivers, Earth Science Major, Western Oregon University, Monmouth, OR 97361, bshivers05@wou.edu

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.

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.

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.

(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.

Abstract1Tectonics of Pacific Northwest4a

Conclusions5

References6

Figure 1. Location map of Pacific Northwest

Figure 3. Elevation model showing the tectonic setting of the Pacific Northwest: Image obtained from Ralph Archuleta, University of California Santa Barbara.

Figure 5. Map showing current plate boundaries, movement, volcanic eruptions and seismic activity (studentatlasoforegon.pdx.edu/PDFs/ Map18.pdf)

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

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).

Introduction2

Stress provinces in the Pacific Northwest include the following:

1.Pacific Northwest stress provinceN-S orientated compressive stress2.Cascade Convergence stress provinceNNE-SSW orientated compressive stress3.Basin and Range Stress provinceE-W orientated tensional stress4.Canadian mid-plate stress provinceNE-SW orientated compressive stress(Geist, 1996)

Stress Provinces In the Pacific NW3

Figure 4. Stress direction history of the western United Statesand Mexico since 85 Ma. Image obtained from 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.

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.