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Evidence for a Large Prehistoric Seismically Induced Landslide into Lake Sammamish.
Local residents and boaters on Lake Sammamish east of Seattle have long been
aware of the many large snags that protrude from the lake near Geenwood
Point. In 1897, in his discussion of geologic features of the Puget Sound
Evidence for a large seismic event or events occurring about 1,000 years ago is widespread throughout the Puget Sound region (Buckman and others, 1992; Atwater and Moore, 1992; Karlin and Abella, 1992; Schuster and others, 1992; Jacoby and others, 1992). Radiocarbon dates on snags from the lake support this possibility. The Lake Sammamish sunken forest could extend the area affected by seismic activity eastward along the projection of the Seattle fault if all of these potentially seismogenic features are synchronous. Greenwood Point is underlain by alluvium attributed to the partial redistribution of glacial deposits from the slopes of Cougar Mountain to the southwest (Booth and Minard, 1992). As the continental ice withdrew from the Sammamish trough, deposition of Greenwood Point alluvium must have been fairly rapid, leaving a steep and relatively unstable delta front, a prime locus for failure during seismic loading. Wood samples from the Lake Sammamish sunken forest were collected by Jim Doolittle of Issaquah and forwarded to us by Brian Atwater of the USGS. In the spring of 1995, we obtained radiocarbon dates on these samples that confirm the landslide's antiquity and are close enough to the ages of other seismogenic features in the Puget Sound Lowland to suggest a seismic trigger from the same earthquake source. Our investigations have been limited to about ten minutes of photography and note-taking during a quick boat ride to the eastern end of the snag field in May of 1995 and about 3 hours of snag mapping during another short boat trip in July 1995. We mapped the snag locations using global positioning system (GPS) technology. A GPS uses satellite signals to precisely locate objects on the Earth's surface. The GPS data were differentially corrected to give a horizontal precision of about 3 feet. Our mapping reveals that the sunken forest is composed of two clusters of snags. This distribution indicates that either two separate landslides moved the trees into the lake or some snags have been removed in the gap between the clusters. The adjacent shoreline morphology consists of two embayments that could be headwall scarps of landslides that carried the ancient trees into the lake off the tip of Greenwood Point. We took measurements on about a third of the approximately 70 snags. A good portion of those snags point shorewood, as if they had been rotated into that position on slump blocks. Although we have precise GPS locations on many of the snags in the lake, we do not know from exactly which snags the radiocarbon-dated wood came or whether the wood was from inner or outer rings of the snag. One of the analyzed samples, Beta 80713, was saturated with pitch and had annual growth rings as wide as 4mm. These characteristics suggest that this wood is from near the heart of the tree and that the tree may have died several hundred years later (than the age of the heart wood) when it rode the landslide into the lake. The date of 1,450 +/- 40 yr B.P. on this heartwood is consistent with the possibility that the tree was drowned about 1,000 to 1,100 years ago. The other sample, Beta 80917, had narrower rings, up to 1.5mm, and was not laden with pitch, indicating that it may have come from farther away from the heart of the tree. The radiocarbon age of 1,330 +/- 50 yr B.P. that was obtained for this sample is also consistent with that supposition. We know from experience with other ancient drowned forests in western Washington that the outermost (that is, the youngest) wood is typically removed or eroded away by exposure to oxidation and (or) biological agents and (or) abrasion. Neither Sammamish sample is likely to represent the true tree age of the forest when it was drowned. Indeed, some trees have been reported to be at least 6 ft in diameter at depth in Lake Sammamish. The snags probably represent the narrowing tips of large trees, and it is likely that some erosion of the trunks has produced the smaller diameter of snags observed at the lake surface. So, what is the true age of the landslide, and how did it happen? Answers to these and other questions are pending investigation of the site when time and resources permit. Additional sampling for precision and radiocarbon dating of wood and coring of the lake floor for possible stratigraphic dating of the landslide should help us better understand the prehistoric seismicity of our region and thereby help us plan for the future. For more info on this ancient underwater forest, click here. References Cited:
Atwater, B. F.; Moore, A. L., 1992, A tsunami about 1000 years ago in Puget
Sound, Washington: Science, v. 258, no. 5088, p. 1614-1617.
Booth, D. B.; Minard, J. P., 1992, Geologic map of the Issaquah 7.5'
quadrangle, King County, Washington: U.S. Geological Survey Miscellaneous
Field Studies Map MF-2206, 1 sheet, scale 1:24,000.
Bucknam, R. C.; Hemphill-Haley, Elieen; Leopold, E. B., 1992, Abrupt uplift
with the past 1700 years at southern Puget Sound, Washington: Science, v.
258, no. 5088, p. 1611-1614.
Jacoby, G. C.; Williams, P. L.; Buckley, B. M., 1992, Tree ring correlation
between prehistoric landslides and abrupt tectonic events in Seattle,
Washington: Science, v. 258, no. 5088, p. 1621-1623.
Karlin, R. E.; Abella, S. E. B., 1992, Paleoearthquakes in the Puget Sound
region recorded in sediments from Lake Washington: Science, v. 258, no.
5088, p. 1617-1620.
Kimball, J. P., 1897, Physiographic geology of the Puget Sound basin: The
Geological Publishing Company [Minneapolis], The American Geologist, v. XIX,
no. 5, p. 304-321.
Schuster, R. L.; Logan, R. L.; Pringle, P. T., 1992, Prehistoric rock
avalanches in the Olympic Mountains, Washington: Science, v. 258, p. 1620-1621.
Yount, J. C.; Holmes, M. L., 1992, The Seattle Fault - A possible
Quadternary reverse fault beneath Seattle, Washington (abstract): Geological
Society of America Abstracts with Programs, v. 24, no. 5, p. 93.
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area, Kimball reported the location of the snags as a landslide. The snags
are indeed the remnants of a prehistoric forest that may have been drowned
when the southwest shore of Lake Sammamish collapsed in the lake. This
may have happened during a large earthquake focused along what was
recognized as a late Quarternary fault by Gower and others (1985) and is
commonly referred to as the Seattle fault (Yount and Holmes, 1992).
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