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5.6.12: Summary

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    In estimating the seismic hazard from crustal earthquakes, we study three lines of evidence: geology, seismology, and geodetic evidence using GPS. In the Puget Sound region, we have all three: Holocene active faults and folds, high instrumental seismicity, and GPS evidence of shortening. In northern California, we also have geological and seismological evidence of earthquake hazard, including damaging historical earthquakes that have caused fatalities. The two Oregon earthquakes come close: the Scotts Mills earthquake probably took place on the Mt. Angel fault, and the Klamath Falls earthquakes were the result of motion on normal faults bounding the Klamath Falls graben.


    In other places, the evidence is less complete. The largest crustal earthquakes in the Pacific Northwest on Vancouver Island and near Entiat in northern Washington took place in areas with little or no geological evidence of young faulting. The active Portland Hills Fault is in an area of moderate seismicity, but many of the earthquakes around Portland cannot be correlated to that fault. The Milton-Freewater Earthquake was not assigned to a specific fault, but it may be part of an active fault system following the Olympic-Wallowa Lineament (OWL).


    Some areas have geological evidence for young faulting but have not experienced large earthquakes. These areas include the Oregon Basin and Range east and north of Klamath Falls and the folded basalt ridges of the Pasco Basin in Washington. The faults around La Grande and Baker City, Oregon, show geological evidence of activity, but they have not been the source of large earthquakes. The southeastern end of the OWL has moderate seismicity, but as yet, this area has not been damaged by a large earthquake.


    What about the rest of the Northwest? The Oregon Coast Range and the Klamath-Siskiyou regions of Oregon have no clear evidence of active faulting and also have very few earthquakes. Similarly, the Coast Mountains of British Columbia, the Columbia Plateau of Washington, and much of the Blue Mountains of Oregon have low seismicity and little evidence of active faulting. At present, these areas are placed in a lower-risk category, but the next earthquake could prove this assessment wrong.

    Suggestions for Further Reading

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    Atwater, B. F., and A. L. Moore. A tsunami about 1000 years ago in Puget Sound, Washington. Science, v. 258, p. 1614-17.

    Bakun, W. H., R. A. Haugerud, M. G. Hopper, and R. S. Ludwin., 2002. The December 1872 Washington State earthquake. Bulletin of the Seismological Society of America, v.     92, p. 3239-58.

    Blakely, R. J., R. E. Wells, T. L. Tolan, M. H. Beeson, A.M. Tréhu, and L. M. Liberty. 2000. New aeromagnetic data reveal large strike-slip faults in the northern Willamette     Valley, Oregon. Geological Society of America Bulletin, v. 112, p. 1225-33.

    Blakely, R. J., R. E. Wells, C. S. Weaver, and S. Y. Johnson. 2002. Location, structure, and seismicity of the Seattle fault zone, Washington: Evidence from aeromagnetic anomalies, geologic mapping, and seismic reflection data. Geological Society of America Bulletin, v. 114, p. 169-77.

    Bott, J. D. J., and I. G. Wong. 1993. Historical earthquakes in and around Portland, Oregon. Oregon Geology, v. 55, no. 6, p. 116-22.

    Bourgeois, J., and S. Y. Johnson. 2001. Geologic evidence of earthquakes at the Snohomish Delta, Washington, in the past 1200 yr: Geological Society of America Bulletin, v.     113, p. 482-94.

    Brocher, T. M., T. Parsons, R. J. Blakely, N. I. Christensen, M. A. Fisher, R. E. Wells, and the SHIPS Working Group. 2001. Upper crustal structure in Puget Lowland,     Washington: Results from the 1998 Seismic Hazards Investigations in Puget Sound. Journal of Geophysical Research, v. 106, p. 13,541-13,564.

    Bucknam, R. C., E. Hemphill-Haley, and E. B. Leopold. 1992. Abrupt uplift within the past 1700 years at southern Puget Sound, Washington. Science, v. 256, p. 1611-14.

    Campbell, N. P., and R. D. Bentley., 1981. Late Quaternary deformation of the Toppenish Ridge uplift in southcentral Washington. Geology, v. 9, p. 519-24.

    Cassidy, J. F., R. M. Ellis, and G. C. Rogers. 1988. The 1918 and 1957 Vancouver Island earthquakes. Bulletin of the Seismological Society of America, v. 78, p. 617-35.

    Clarke, S. H., and G. A. Carver. 1992. Late Holocene tectonics and paleoseismicity, southern Cascadia subduction zone. Science, v. 255, p. 188-92.

    D’Antonio, M. 1993. Atomic harvest: Hanford and the lethal toll of America’s nuclear arsenal. New York: Crown Publishers, 304 p.

    Dehlinger, P., R. G. Bowen, E. F. Chiburis, and W. H. Westphal. 1963. Investigations of the earthquake of November 5, 1962, north of Portland. The Ore Bin, v. 25, no. 4, p.     53-68

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    Dengler, L., K. Moley, R. McPherson, M. Pasyanos, J. W. Dewey, and M. Murray. 1995. The September 1, 1994 Mendocino Fault earthquake. California Geology, v. 48, p. 43-53.

    Dragovich, J.D., J. E. Zollweg, A. L. Qamar, and D. K. Norman., 1997. The Macaulay Creek Thrust, the 1990 5.2-magnitude Deming Earthquake, and Quaternary geologic     anomalies in the Deming area, western Whatcom County, Washington—Cause and effects? Washington Geology, v. 25, no. 2, p. 15-27.

    Geomatrix Consultants. 1995. Seismic design mapping, State of Oregon. Final report prepared for the Oregon Department of Transportation, Project 2442, Salem, OR.

    Gore, R., and J. Richardson. 1998. Cascadia: Living on Fire. National Geographic, v. 193, no. 5, p. 6-37.

    Gower, H. D., J. C. Yount, and R. S. Crosson. 1985. Seismotectonic map of the Puget Sound region, Washington. USGS Map I-1613, scale 1:250,000, booklet, 15 p.

    Haugerud, R. A., D. J. Harding, S. Y. Johnson,J. L. Harless, C. S. Weaver, and B. L. Sherrod. 2003. High-resolution lidar topography of the Puget Lowland, Washington. GSA     Today, v. 13, no. 6, p. 4-10.

    Hemphill-Haley, M. A., W. D. Page, G. A. Carver, and R. M. Burke. 2000. Paleoseismology of the Alvord Fault, Steens Mountain, southeastern Oregon. American Geophysical Union Reference Shelf 4, p. 537-40.

    Humboldt Earthquake Information Center. How to survive earthquakes and tsunamis on the north coast. Humboldt State University, Arcata CA 95532-8299, 23 p. A simple     guide to protection against earthquakes and tsunamis. Free. Web site:

    Hyndman, R. D., S. Mazzotti, D. Weichert, and G. C. Rogers. 2003. Frequency of large crustal earthquakes in Puget Sound-southern Georgia Strait predicted from geodetic and     geological deformation rates. Journal of Geophysical Research, v. 108, doi:10.1029/2001JB001710.

    Hyndman, R. D., G. C. Rogers, H. Dragert, K. Wang, J. J. Clague, J. Adams, and P. T. Bobrowski. 1996. Giant earthquakes beneath Canada’s west coast. Geoscience Canada, v.     23, no. 2, p. 63-72.

    Jacoby, G. C., P. L. Williams, and B. M. Buckley. 1992. Tree ring correlation between prehistoric landslides and abrupt tectonic events in Seattle, Washington. Science, v. 258,     p. 1621-23.

    Johnson, S. Y., C. J. Potter, J. M. Armentrout, J. J. Miller, C. Finn, and C. S. Weaver. 1996. The southern Whidbey Island fault: An active structure in the Puget Lowland,     Washington. Geological Society of America Bulletin, v. 108, p. 334-354.

    Johnson, S. Y., S. V. Dadisman, J. R. Childs, and W. D. Stanley. 1999. Active tectonics of the Seattle fault and central Puget Sound, Washington: Implications for earthquake     hazards. Geological Society of America Bulletin, v. 111, p. 1042-1053.

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    This page titled 5.6.12: Summary is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Robert S. Yeats (Open Oregon State) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.