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5.6.12: Summary

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    Overview

    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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    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.

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    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: http://sorrel.humboldt.edu/~geodept/.../eqk_info.html

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