Great Cascadia Earthquake Penrose Conference

Geological Society of America Penrose Conference on "Great Cascadia Earthquake Tricentennial"

The proceedings and abstract volume of the Penrose Conference "Great Cascadia Earthquake Tricentennial" is now available for U.S. $15 from the DOGAMI. The citation is:
"Penrose Conference 2000, Great Cascadia Earthquake Tricentennial, Program Summary and Abstracts". Oregon Department of Geology and Mineral Industries, Special Paper 33.
The volume is also available as a Geological Survey of Canada Open File Report (no. 3938).
Some information is available online at Oregon Department of Geology and Mineral Industries

A brief summary of the proceedings is online.

The Oregonian newspaper article
The Seattle Times newspaper article

The Great Cascadia Earthquake of 26 January AD 1700

Coastal field work over the past ten years by many scientists from universities and federal, state, and provincial geological surveys in northern California, Oregon, Washington, and British Columbia has uncovered overwhelming evidence of a great (moment magnitude larger than 8) earthquake or series of earthquakes about 300 years ago. The earthquake (or earthquakes) was caused by the sudden slip of the Pacific plate beneath the North America plate along the Cascadia subduction zone, a 1000-km(600-mi)-long fault that marks the landward-dipping boundary between the two tectonic plates off the coast of western North America.

sand over peat photo

Some of the most convincing and best-preserved evidence of the earthquake(s) are sand layers that cover the peaty soils of coastal lowlands. A large tsunami that was generated by sudden movements of the ocean floor during the earthquake(s) deposited the layers when it inundated the coasts bordering the fault zone. The above photo shows such a sand layer in an exposure near the mouth of the Salmon River along the central Oregon coast about 8 km (5 mi) north of Lincoln City. One of the series of tsunami surges that probably followed the earthquake by 20 minutes to several hours picked up sand from the beach or dunes as it came ashore and deposited the sand as it moved up the river valley. At the site of the photo, the sand bed covers the remains of two fire pits dug by Native Americans, perhaps not long before the tsunami. The layers are well preserved partly because much of this part of the Oregon coast permanently subsided about 0.5-1.0 m (2-3 ft.) during the earthquake. The rise in sea level produced by the subsidence allowed tidal sediments to quickly bury the sand layers, protecting them from later erosion.

An exciting recent discovery about this earthquake is that its tsunami was probably recorded in Japan on 27 January AD 1700. Extensive, unusually precise radiocarbon dating of geologic evidence of the earthquake(s) at estuaries in northern California, Oregon, Washington, and British Columbia suggested that the earthquake(s) occurred during a twenty-year period centered at about AD 1710. Careful study of written records from this period in Japan by Japanese historians and seismologists shows that a large tsunami was recorded in Japan on 27 January AD 1700. Because written records from other coasts that experience great earthquakes, such as western South America, suggest that no earthquake large enough to produce this type of tsunami in Japan occurred during this period, the Japanese researchers concluded that the tsunami was probably produced at the Cascadia subduction zone. By making reasonable assumptions about the size and speed of the tsunami from the records of tsunami inundation along the Japanese coast the researchers concluded that the tsunami was produced by an earthquake larger than magnitude 8.5, probably about 9 PM on 26 January AD 1700. This conclusion agrees not only with the geologic evidence, but with remaining fragments of Native American legends of a large earthquake and/or tsunami that occurred on a winter night hundreds of years ago.

The following sources provide more information about the great earthquake of AD 1700 at the Salmon River and elsewhere, and about earthquake and tsunami hazards in western central North America:

Research Papers and Reports

Asquith, A. C., 1996, A biostratigraphic test of the earthquake deformation cycle at the Salmon River estuary, Oregon, USA: unpub B.Sc. thesis, Department of Geography, University of Southampton, Southampton, England, 87 p.

Atwater, B.F., Nelson, A.R., Clague, J.T., Carver, G.A., Yamaguchi, D.K., Bobrowsky, P.T., Bourgeois, Joanne, Darienzo, M.E., Grant, W.C., Hemphill-Haley, Eileen, Kelsey, H.M., Jacoby, G.C., Nishenko, S.P., Palmer, S.P., Peterson, C.D., and Reinhart, Mary Ann, 1995, Summary of coastal geologic evidence for past great earthquakes at the Cascadia subduction zone: Earthquake Spectra, v. 11, p. 1-18.

Atwater, B. F., and Hemphill-Haley, E., 1996, Recurrence intervals for great earthquakes of the past 3500 years at northeastern Willapa Bay, Washington: U.S. Geological Survey Bulletin 96-001, p.

Clague, J.J., and Bobrowsky, P.J., 1994a, Tsunami deposits beneath tidal marshes on Vancouver Island, British Columbia: Geological Society of America Bulletin, v. 106. p. 1293-1303.

Clague, J.J., and Bobrowsky, P.J., 1994b, Evidence for a large earthquake and tsunami 100-400 years ago on western Vancouver Island, British Columbia: Quaternary Research, v. 41, p. 176-184.

Darienzo, M. E., and Peterson, C. D., 1995, Magnitude and frequency of subduction-zone earthquakes along the northern Oregon coast in the past 3,000 years: Oregon Geology, v. 57, p. 3-12. Hemphill-Haley, Eileen, 1995, Diatom evidence for earthquake-induced subsidence and tsunami 300 yr ago in southern Washington: Geological Society of America Bulletin, v. 107, p. 367-378.

Grant, W. C., 1994, Paleoseismic evidence for late Holocene episodic subsidence on the northern Oregon coast: unpub. report completed as part of M.S. degree, Department of Geological Sciences, University of Washington, Seattle, WA, 42 p.

Nelson, A. R., Atwater, B. F., Bobrowsky, P. T., Bradley, L.-A., Clague, J. J., Carver, G. A., Darienzo, M. E., Grant, W. C., Krueger, H. W., Sparks, R., Stafford, T. W., Jr., and Stuiver, M., 1995, Radiocarbon evidence for extensive plate-boundary rupture about 300 years ago at the Cascadia subduction zone: Nature, v. 378, p. 371-374.

Nelson, A. R., Shennan, I., and Long, A. J., 1996a, Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America: Journal of Geophysical Research, v. B3, 101, p. 6115-6135.

Nelson, A. R., Jennings, A. E., and Kashima, K., 1996b, An earthquake history derived from stratigraphic and microfossil evidence of relative sea-level change at Coos Bay, southern coastal Oregon: Geological Society of America Bulletin, v. 108, p. 141-154.

Satake, K., Shimazaki, K., Tsuji, Y., and Ueda, K., 1996, Time and size of a giant earthquake in Cascadia inferred from Japanese tsunami records of January 1700: Nature, v. 379, p. 246-249.

Shennan, Ian, Long, A.J., Rutherford, M.M., Green, F.M., Innes, J.B., Lloyd, J.M., Zong, Y., and Walker, K.J., 1996, A 5000-year record of large earthquakes and sea-level rise in Washington, USA, revealed by biostratigraphic analysis of intertidal sediments: Quaternary Science Reviews, in press.

For help in obtaining any of the above references email Alan Nelson

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