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Despite its low seismicity, the Cascadia Subduction Zone has been revealed as a major seismic source, capable of an earthquake of magnitude 9. The last earthquake was a 9, based on modeling of a tsunami that struck Japan in January 1700. The Native Americans who were living here in A.D. 1700 reported the earthquake in their oral traditions handed down from generation to generation. Some of the earlier earthquakes may have been smaller since the subsidence accompanying them was less than the subsidence in 1700. However, the consistent number of earthquake-generated turbidites identified in submarine channels on the Juan de Fuca Plate from Washington to southern Oregon argue for a magnitude 9 for most of these earlier events. The theoretical models of Roy Hyndman and Kelin Wang also point to a magnitude 9. Goldfinger has shown that at least two of the earlier earthquakes had magnitudes larger than the 1700 earthquake.
In Del Norte and Humboldt counties in northern California, the situation is different. This region not only has the highest seismicity in the Pacific Northwest, it also has the highest seismicity in California, higher than that along the San Andreas Fault. The Cascadia Subduction Zone in this region was struck by a moderate-size earthquake in 1992 that, because it was closer to the trench, it uplifted the coastline rather than downdropped it as happened farther north. In addition, the earthquakes return more frequently in that area, as documented by uplifted coastal marine platforms and the more-frequent appearance of turbidites in submarine channels.
The paleoseismological record from Cascadia is the most fully documented in the world, yet the record does not allow us to forecast closely the arrival time of the next subduction-zone earthquake, nor does it allow a forecast of whether the next earthquake will be a 9 or larger or smaller. The recognition of episodic tremor and slip demonstrates that the subduction zone is active, but its current activity consists of slow slip events accompanied by earthquake tremors. This allows for the recognition of the landward boundary of that part of the subduction zone, and also suggests that the next great earthquake might be preceded by a slow slip event or ETS.
However, we still have much to learn.
Suggestions for Further Reading
Atwater, B. F., and E. Hemphill-Haley. 1997. Recurrence intervals for great earthquakes of the past 3500 yr at northeastern Willapa Bay, Washington. USGS Professional Paper 1576, 108 p.
Clague, J. J. 1997. Evidence for large earthquakes at the Cascadia Subduction Zone. Reviews of Geophysics, v. 35, p. 439-60.
Clague, J., B. Atwater, K. Wang, M. Wang, and I. Wong. 2000. Great Cascadia earthquake tricentennial. Proceedings of the Geological Society of America Penrose Conference, printed by the Oregon Department of Geology and Mineral Industries, 131 p.
Clarke, S. H., and G. A. Carver. 1992. Late Holocene tectonics and paleoseismicity, southern Cascadia subduction zone. Science, v. 255, p. 188-92.
Dengler, L., G. Carver, and R. McPherson. 1992. Sources of North Coast seismicity. California Geology, v. 45, p. 40-53.
Dengler, L., and K. Moley. 1999. Living on shaky ground: how to survive earthquakes & tsunamis on the North Coast. Humboldt Earthquake Education Center, Humboldt State University, 3rd edition, 24 p. (A simple guide to protection against earthquakes and tsunamis). Free. Web site: http://sorrel.humboldt.edu/~geodept/ earthquakes/eqk_info.html
Goldfinger, C., C. H. Nelson, and J. E. Johnson. 2003. Holocene earthquake records from the Cascadia Subduction Zone and northern San Andreas Fault based on precise dating of offshore turbidites. Annual Reviews of Earth and Planetary Sciences, v. 31, p. 555-77.
Goldfinger, C., Nelson, C.H., Johnson, J.E., et al., 2012, Turbidite event history: Methods and implications for Holocene paleoseismology of the Cascadia subduction zone: U.S. Geol. Survey Prof. Paper 1661F, http://pubs.usgs.gov/pp/pp1661f/
Goldfinger, C., Ikeda, Y., and Yeats, R.S., 2013, Superquakes, supercycles, and global earthquake clustering: Earth, v. 58, no. 1, p. 34-43.
Gomberg, J., and the Cascadia 2007 and Beyond Working Group, 2010, Slow-slip phenomena in Cascadia from 2007 and beyond: A review: Geological Society of America Bulletin, v. 122, p. 963-978.
Gore, R., and J. Richardson. 1998. Cascadia: Living on Fire. National Geographic, v. 193, no. 5, p. 6-37
Henderson, B., 2014, The next tsunami: living on a restless coast: OSU Press, Corvallis, 322 p .
Houtman, N., Ashford, S., Mason, B., Barbosa, A., Bethel, J., and Olsen, M., 2013, Terra, Oregon 9.0, available at http://oregonstate.edu/terra/2013/05/oregon-9-0/.
Hyndman, R. D. 1995. Great earthquakes of the Pacific Northwest. Scientific American, v. 273, no. 6, p. 50-57. Written for the lay reader.
Kelsey, H. M., R. C. Witter, and E. Hemphill-Haley. 2002. Plate-boundary earthquakes and tsunamis of the past 5500 yr, Sixes River estuary, southern Oregon. Geological Society of America Bulletin, v. 114, p. 298-314.
Lienau, M., for Global Net Industries. 2003. Cascadia: the Hidden Fire. One hour educational documentary video, www.globalnetproductions.com
McCrory, P. A. 1996. Evaluation of fault hazards, northern coastal California. USGS Open-File Report 96-656, 87 p.
Miller, M. M., T. Melbourne, D. J. Johnson, and W. Q. Sumner. 2002. Periodic slow earthquakes from the Cascadia Subduction Zone. Science, v. 295, p. 2423.
Mitchell, C. E., P. Vincent, R. J. Weldon, and M. A. Richards. 1994. Present-day vertical deformation of the Cascadia margin, Pacific Northwest, United States. Journal of Geophysical Research, v. 99, p. 12, 257-12, 277.
Oppenheimer, D., and others. 1993. The Cape Mendocino, California, earthquakes of April 1992: subduction at the triple junction. Science, v261, p. 433-38.
Rogers, G., and H. Dragert., 2003. Episodic tremor and slip on the Cascadia Subduction Zone: the chatter of silent slip. Science, v. 300, p. 1942-43.
Satake, K., K. Shimazaki, Y. Tsuji, and K. Ueda. 1996. Time and size of a great earthquake in Cascadia inferred from Japanese tsunami records of January, 1700. Nature, v. 379, p. 246-49.
Schulz, K., 2015, The really big one: New Yorker, July 20, 2015, p. 52-59.
Wang, K., R. Wells, S. Mazzotti, R. D. Hyndman., and T. Sagiya. 2003. A revised dislocation model of interseismic deformation of the Cascadia subduction zone. Journal of Geophysical Research v. 261, p. 433-38.