USGS Project No. 7460-12080
Brian F. Atwater
US Geological Survey
Department of Geological Sciences
University of Washington Box 351310
Seattle WA 98195
Program elements I, II, III, IV
Key words: recurrence interval, tsunami, age dating
RESULTS FROM TREE-RING DATING
Two complementary reports, Yamaguchi and others (1997) and Jacoby and others (1997), establish 1700 as the year of a huge earthquake in the Pacific Northwest. The reports present tree-ring dates for an earthquake and tsunami that had been previously inferred from geology in the northwestern United States and adjacent Canada. The reports’ authors compare these dates with the time of a tsunami known from village records in Japan. The trans-Pacific agreement is so remarkable that the Japanese records become written proof that the earthquake really happened.
At issue is certainty about an earthquake threat at the Cascadia subduction zone, which extends along the coast from southern British Columbia to northern California. Cascadia is defined by the boundary between the subducting Juan de Fuca plate and the overriding North America plate. This huge active fault lacked a recognized history of great (magnitude 8 or larger) earthquakes until the late 1980’s, when scientists began to find geologic evidence that the fault has repeatedly produced great earthquakes in the past several thousand years (Atwater and others, 1995).
The most recent of these events, initially viewed as either a single huge earthquake or a swift series of smaller ones, was soon dated by radiocarbon methods to the decades between 1680 and 1720 (Atwater and others, 1991; Nelson and others, 1995). This finding caught the attention of Satake and others (1996), who checked Japanese village records for signs of an orphan tsunami of that era. They found just one such tsunami. After ruling out other potential sources (Japan, Kamchatka, Alaska, South America), they used the tsunami’s size and date to calculate that the Pacific Northwest had an earthquake close to magnitude 9 in January of 1700.
Spurred by these findings from Japan, tree-ring scientists in the United States set out to learn whether or not a huge Pacific Northwest earthquake could have occurred in January 1700. One team, led by David Yamaguchi of the University of Washington, studied trees killed by an earthquake. Another team, led by Gordon Jacoby of Lamont-Doherty Earth Observatory in Palisades, New York, focused on trees that barely survived it. Atwater’s USGS project contributed to both efforts, particularly the dating of the earthquake-killed trees. All the trees are located along a 100-km stretch of the Cascadia subduction zone, between the mouth of the Columbia River and the Copalis River.
Each tree-ring team determined that a huge Pacific Northwest earthquake had indeed occurred in a narrow window that includes January 1700. Yamaguchi and others (1997) showed that six trees killed by the earthquake died sometime after 1699 growing season ended but before the 1700 growing season began. Jacoby and others (1997), who were responsible for much of the dating of one of these trees, found signs of trauma that begin with the 1700 ring in some of the trees that survived the earthquake.
RESULTS ABOUT EARTHQUAKE RECURRENCE
Seven great earthquakes, or earthquake series, probably ruptured the southern Washington part of the Cascadia subduction zone in the past 3500 years. The earthquakes left estuarine evidence for abrupt lowering of land, accompanied at some sites by evidence for tsunamis or shaking. Much of this evidence has been brought together in a monograph slated for publication in January of 1998 (Atwater and Hemphill-Haley, in press).
The inferred history contains six recurrence intervals that average about 500 years. Two of these intervals are centuries longer than any of the others. The longest interval, about 800-1100 years, was followed by one of the shortest, 250-420 years. The other long interval, 550-900 years, ended 300 years ago with the January 1700 earthquake.
Neither of the two long intervals is easy to shorten by inserting an additional, unrecognized earthquake. During both long intervals, tidal marshes were widely changed into forests, probably because of prolonged interseismic uplift. The forests are marked by growth-position tree stumps, diatom assemblages typical of land above all but extreme high tides, and weathering of underlying tidal deposits. Some of the stumps contain hundreds of annual rings.
Atwater, B.F., and Hemphill-Haley, E., Recurrence intervals for great earthquakes of the past 3,500 years at northeastern Willapa Bay, Washington: U.S. Geological Survey Professional Paper 1576, 108 p. (in press).
Yamaguchi, D.K., Atwater, B.F., Bunker, D.E., Benson, B.E., and Reid, M.S., 1997, Tree-ring dating the 1700 Cascadia earthquake: Nature, v. 389, p. 922-923.
17-day exhibit, about tree-ring dating of the 1700 Cascadia earthquake, at the Western Washington Fair in Puyallup, Washington, September, 1997 (with Pat Pringle of Washington Division of Geology and Earth Resources)
Diagrams about this tree-ring dating are available by anonymous ftp. The files, created in Coreldraw 7.0 (*.cdr), are also available in GIF (*.gif) and Adobe Illustrator (*.ai) formats. At the location prompt on your web browser, type ftp://ftp.geophys.washington.edu/pub/out/tree/
ADDITIONAL REFERENCES CITED
Atwater, B.F., Nelson, A.R., Clague, J.J., Carver, G.A., Yamaguchi, D.K., Bobrowsky, P.T., Bourgeois, J., Darienzo, M.E., Grant, W.C., Hemphill-Haley, E., Kelsey, H.M., Jacoby, G.C., Nishenko, S.P., Palmer, S.P., Peterson, C.D., and Reinhart, M.A., 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., Stuiver, M., and Yamaguchi, D.K., 1991, Radiocarbon test of earthquake magnitude at the Cascadia subduction zone: Nature, v. 353, p. 156-158.
Jacoby, G.C., Bunker, D.E., and Benson, B.E., 1997, Tree-ring evidence for an A.D. 1700 Cascadia earthquake in Washington and northern Oregon: Geology, v. 25, p. 999-1002.
McCaffrey, R., and Goldfinger, C., 1995, Forearc deformation and great subduction earthquakes: implications for Cascadia offshore earthquake potential: Science, v. 267, p. 856-859.
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., and Stuiver, Minze, 1995, Radiocarbon evidence for extensive plate-boundary rupture about 300 years ago at the Cascadia subduction zone: Nature, v. 378, p. 371-374.
Satake, K., Shimazaki, K., Tsuji, Y., and Ueda, K., 1996, Time and size of a giant earthquake in Cascadia inferred from Japanese tsunami record of January 1700: Nature, v. 379, p. 246-249.
New tree-ring dates provide exceptional certainty that the Pacific Northwest is subject to earthquakes of magnitude 8 or larger. The dates were determined for an earthquake and tsunami recorded by geology in the northwestern United States and adjacent Canada. The tsunami had previously been linked, by Japanese scientists, with a tsunami that struck Japan in January 1700. The tree-ring dates show that the earthquake and tsunami occurred in the months between August 1699 and May 1700. The dates make village records in Japan into written proof that a huge earthquake struck the Pacific Northwest in the year 1700.