(Quat. Int. 15/16, special issue, 1992), p. 611, aseismic uplift of Restoration Point would have required 100 years. Even at the highest known rates of uplift of about 200 to 800 mmtyr, which have only been observed on the flanks of active volcanoes [K. R. Lajoie, in Active Tectonics: Im- pact on Society (National Academy Press, Wash- ington, DC, 1986), pp. 95-1241, 7 m of uplift would have required at least a decade. 16. This age is likely much younger than the time of uplift due to continual addition of young organic material to the soil profile [J. A. Matthews. Geogr. Ann. 62A, 185 (1 980)l. 17. Ages in parentheses are conventional radiocar- bon ages in 14C years before A.D. 1950 . correct- ed for the measured 13C/12C ratio, with 1 standard deviation in the age quoted by the laboratory. These ages were converted to la tree-ring cali- brated age ranges [M. Stuiver and P. J. Reimer. Radiocarbon 28, 1022 (1986)l with the use of an estimated laborat0~ error multi~lier of 2 and are reported in the text as "years agou relativeto A.D. 1990. The humus concentratewas calibrated with a range of 300 years for the carbon in the sample. For marine shells we used a reservoir correction of 800 2 25 years [S. W. Robinson and G. Thompson. Syesis 14, 45 (1 981)l. 18. F. Weinmann, M. Boule. K. Brunner, J. Malek. V. Yoshino. Wetland Plants of the Pacific Northwest (Final Report, U.S. Army Corps of Engineers, Seattle, 1984). 19. J. C. Yount and M. L. Holmes, Geol. Soc. Am. Abstr. Progr. 24, 93 (1992). 20. J. C. Yount and H. D. Gower, U.S. Geol. SUN. Open-File Rep. 91 - 147 (1 991 ). 21. L. Mansinha and D. E. Smylie. Seismol. Soc. Am. Bull. 61. 1433 (1977). 22. Over most of the area at Lynch Cove a layer of very fine-grained, well-sorted sand, commonly 10 to 20 cm thick, caps the section of tidal flat mud that underlies the peat. Locally, the sand wedges out and the peat lies directly on mud. Both the sand and the mud contain diatoms characteristic of tidal flats. 23. Laboratory numbers Beta-46730 and Beta-491 76. respectively. 24. J. R. Wilson. M. J. Bartholomew, R. J. Carson. Geology 7, 235 (1 979). 25. J. C. Ruegg, M. Kasser,A. Tarantola, J. C. Lepine. B. Chouikrat. Seismol. Soc. Am. Bull. 72. 2227 (1982). The geodetic data also showed 0.8 m of subsidence of the downthrown block. Ruegg etal. found that the observed displacements imply that there was 8 m of slip on a fault about 35 km long that extended from the surface to a depth of 12 km at dips between 54' and 70". 26. M. Eronen. T. Kankainen. M. Tsukada. Quat. Res. 27. 147 (1987). 27. E. Hemphill-Haley, thesis, University of California. Santa Cruz. CA (1992); H. Germain. Flore des Diatomkes Eaux Douces et Saumdtres (Boubee. Paris. 1981). p. 72; N. Foged. Bibliotheca Phyco- logica (Cramer, Vaduz, 1981). Band 3, p. 123. 28. We thank B. Atwater, T. Barnhard, B. Benson, and J. Suhy for their help with this study, and property owners who generously provided access to criti- cal sites on their land. P. Bierman reported the Alki Point excavation. The manuscript was im- proved by reviews by B. Atwater. A. Nelson. K. Berryman, and C. Weaver. 24 July 1992; accepted 22 October 1992 A Tsunami About 1000 Years Ago in Puget Sound, Washington Brian F. Atwater and Andrew L. Moore Water surged from Puget Sound sometime between 1000 and 1100 years ago, overrunning tidal marshes and mantling them with centimeters of sand. One overrun site is 10 kilometers northwest of downtown Seattle; another is on Whidbey Island, some 30 kilometers farther north. Neither site has been widely mantled with sand at any other time in the past 2000 years. Depositionof the sand coincided-to the year or less--with abrupt, probablytectonic subsidence at the Seattle site and with landsliding into nearby Lake Washington. These findings show that a tsunami was generated in Puget Sound, and they tend to confirm that a large shallow earthquake occurred in the Seattle area about 1000 years ago. A large earthquake probably happened between 500 and 1700 years ago on the Seattle fault (I), which has been inferred to extend westward across Puget Sound from downtown Seattle (2). The main evidence for the earthquake consists of terraces that record meters of abrupt uplift at Puget Sound (I). If abrupt enough to have accompanied an earthquake, such uplift should have generated a tsunami in Puget Sound. In this report, we show that a tsunami originated in Puget Sound be- tween 1000 and 1100 vears aeo (3) and quake on the Seattle fault. Tsunamis can deposit sand on coastal lowlands. Modem examples have been re- ported from Chile (4, 5), Japan (6), and British Columbia (7), and ancient exam- ples have been inferred for Chile (5), Japan (6), Scotland (8), Alaska (9), and the Pacific coast of Washington and Oregon (10, 11). In most of these examples, an onshore sheet of marine or estuarine sand dates to the time of an event known or inferred to have generated a tsunami. We found tsunami de~osits at two sites The tsunami deposit at Cultus Bay forms a sheet of sand mostly 5 to 15 cm thick in an area at least 100 by 200 m (Figs. 1 and 2). There, wetland peat has built upward and bayward since a tidal marsh began to supplant a tidal flat about 2000 years ago. This Deat contains the sand sheet. which we found in scores of auger borings and followed as a continuous bed along more than 100 m of a drainage ditch. Neither the auger borings nor the ditch revealed any other sand bed in the peat. The surface covered by the sand shows 2 m of relief: 1.5 m where the sand mantled a sloping marsh (12) and another 0.5 m where the sand covered colluvium of an adiacent hillside (Fig. 2). The median grain size, mostly about 0.1 mm, decreases landward and stratigraphically upward (1 3). The sand contains microscopic marine fossils (1 4). Deposition of the sand sheet at Cultus Bay occurred sometime between 850 and 1250 years ago, and it happened while the site probably underwent little or no subsid- ence. We dated the sand sheet by obtaining radiocarbon ages on plant remains in growth position in the sand (Fig. 2, in ditch). The dated remains are rhizomes (below-ground stems) and attached leaf bases of arrowgrass (Triglochin maritimum), which at modem Cultus Bay thrives only in a 1-m range high in the intertidal zone. Because additional arrowgrass rhizomes lie both below and above thi sand, we suspect that the dated rhizomes grew upward through the sand sheet within years of its de~osition. Such maintenance of arrow- grass would mean that deposition of the sand attended little or no subsidence of the Cultus Bay marsh (15). The sand sheet at Cultus Bay is better explained by a tsunami than by a flood or storm. The landward fining and salt water fossils of the sand implicate a surge from ., ., that it probably was generated by an earth- north of the Seattle fauit [see figure 1 in ,. maps of Cultus Bay, M, marsh (')I. One of these sites borders Cultus surveyed for vertical ranges of plants plotted on B. F. Atwater, U.S. Geological Survey at Department of which opens southward from Whidbey Is- right of F ~ ~ , 2, inset shows extent of sand Geological Sciences, University of Washington, AJ-20. Seattle, WA 98195. land, 40 km north of the fault- The other sheet in peat or peaty mud, as seen in auger A. L. Moore, Department of Geological Sciences, site is West Point, which Juts into Puget borings (0, sand present; x, absent) and in ditch University of Washington. AJ-20, Seattle. WA 98195. Sound 7 km north of the fault. (filled, present; open, absent). 1614 SCIENCE VOL. 258 4 DECEMBER 1992