ML20151P858

Paleoseismicity in Puget Sound Area as Recorded in Sediments from Lake Washington, Presented at 880506-08 Meeting in Seattle,Wa

ML20151P858
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Issue date:	05/06/1988
From:	Abell S, Karlin R, Leopold E WASHINGTON, UNIV. OF, SEATTLE, WA
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References
NUDOCS 8808100208
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Paleoseismicity in the Puget Sound Area as recorded in sediments from Lake Washington Robert Karlin School of Oceanography, University of Washington WB 10, Seattle, WA 98195 Sally Abella, Department of Zoology, University of Washington NJ-15, Seattle, WA 98195 Estella Leopold, Quaternary Research Center, University of Washington KB-15, Seattle, WA 98195 Patrick Williams, Lamo.nt Doherty Geological Observatory. Palisades NY 10964 l

In tectonically active regions, lacustrine sediments potentially can provide a proxy record l of palcoseismicity, because of disturbances in mass flux and sediment texture due to mass wasting and gravity flow processes during and after strong ground motion. In 1929, a moderate earthquake on the Grand Banks caused major submarine slumping and large  ;

scale turbidity currents on the Atlantic continental margin (Heezen and Ewing,1952). In  :

Kenai Lake, Alaska, the great earthquake of 1964 initiated seiching and delta l

destabilization, resulting in massive slides, debris flows, and turbidites (McCulloch,1966). l Similarly, major earthquake activity in western Washington might be expected to cause slides and slumping on the steep sides of fiords, estuaries and lakes of the Puget l Lowlands. Major increases in sedirnent supply to lake bottoms might also occur due to seismically-induced landslides in the catchment basins as well as seiching and delta destabilization along lake margins. The effects of these disruptions would be to introduce l

changes in sediment type, concentration, and texture which are in disequilibrium with  !

arnbient sedimentation conditions. '

In this research, we have been evaluating the history of seismic activity in the Puget Soand region as recorded by turbidites and other rapid changes in mass flux in sediments i

from Lake Washington. Forming the western boundary of Seattle, the Lake Washington '

basin is underlain by a thick sequence of blue glacial clay of indeterminate thickness, 1

overlain by 7 17 m of Holocene limnic peat or gyttja less than 13,400 yrs in age (Gould and Budinger,1958; Leopold et al.,1982). The limnic section contains the Mazama ash (6850 yrs BP) anc' aistinctive varved layers which serve as key marker teds. The lake contains three sunken forests with standing trees that were presumably ernplaced by massive block slides. Two of the subrnerged forests, lying at the north and southwest ends of the lake, have been radiocarbon dated at 1160 yrs BP and 2750 yrs BP, respectively. These events are synchronous with postulated seismic activity along the Washington coast which caused rapid subtnergence of marsh peats (Atwater,1987). The younger event is also coeval with faultir g reported on the Olympic Peninsula (Wilson, et al.,1979).

X-radiographs and whole core magnetic susceptibility (K) profiles of 10 piston cores and 14 gravity cores from the lake have been used to delineate the spatial and temporal variations in sedunentation. Since the sediments' magnetic properties are very sensitive to changes in the concentration and grain size of magnetic minerals, susceptibility measurement i are an extremely useful remote sensing technique for intercore correlation and rapidly identifying lithologic and textural changes. The K proff' s show remarkably consistent downcore patterns which can be readily correlated throughout the lake. A characteristic series of magnetic peaks is observed that correspond enctly with ash horizons and X-ray opaque intervals containing terrigenous lutites, presumably turbidites.

From the size and lithologies of the turbidites and their areal distribution throughout the I lake, we conclude that these deposits are not due to flooding or local slides, but must I 8808100200 880615 PDR 7

P ADOCK 05000344 PDR

. a represent basinwide disruptions, perhaps related to seismic activity. Preliminary results indicate that one such layer :nay have been deposited in response to the 1160 yr BP slide ,

that produced the sunken forest in the lake's north end. In post Mazama time (<6850 l

y:s BP), more than fourteen such events are observed, tentatively suggesting recurrance intervals of less than 500 years. The significance of these sedimentary disturbances and j their possible relationship to palcoseismicity and paleoclimate will be discussed.

Gould, H.R. and Budinger, T.F.,1958. Control of sedimentation and bottom configuration by convection currents, Lake Washington, Washington, Journ. Mar. Res., 17:183 197.

Heezen, B.C. and Ewing, M. Turbidity currents and submarine slumps and the 1929 Grand Banks earthquake, Amer. Journ. Sci.,250:849 873.

Leopold, E.B., Nickman, R., Hedges, J.I., and Ertel, J.R.,1982. Pollen and lignin records of Late Quaternary vegetation, Science,218:13051307.

i McCulloch, D.S.,1966. Slide induced waves, seiching and ground fracturing caused by the Earthquake of March 27,1964 at Kenai Lake, Alaska, U.S.G.S. Prof. Paper 543B. l i

An abstract for the Symposium on "Holocene Subduction in the Pacific Northwest"  !

May 6 8,1988, sponsored by the Quaternary Research Center, University of Washington l

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