Abstract

Lake Washington, which forms the eastern boundary of Seattle, is located in a tectonically active area containing several strands of the Seattle fault. High-resolution seismic reflection profiling, sidescan swath imagery, and sediment coring were used to define deformation of Holocene lake sediments and the distribution, geometry, age, and causes of submarine landslides. Numerous large block slides, sediment slumps, and debris flows are present throughout the lake, and large landslides obscure the surficial structure of Seattle fault strands as they pass through the lake. In addition, most bays along the lake margin are the headwalls of large submarine slides. Submerged forests show evidence of deep- seated block failures that have exposed glacial sediments and Tertiary rocks. The massive submarine block slides, and retrogressive submarine slope failures were triggered most likely by large (mb>7) earthquakes on the Seattle fault and/or large to great (mb>8) temblors occurring elsewhere in Cascadia. Buried landslides suggest that submarine slope failures and mass wasting occurred more than once in the last 11,000 yr.

Sediments in Lake Washington preserve a record of episodic deposition of turbidites possibly caused by seismically induced submarine landslides. Magnetic susceptibility profiles on 36 gravity cores show a characteristic series of magnetic peaks that can be traced throughout the lake. X- radiography and grain size analyses suggest that the magnetic peaks represent anomalous detrital layers that in some cases are turbidites. The areal extent and magnetic signatures of many of the deposits suggest multiple sources, which is consistent with numerous local landslides caused by large earthquakes. Radiocarbon dating and correlation of the downcore magnetic profiles establish a sediment record in which episodic sedimentary disturbances occurred seven times in the last 3500 yr. If these deposits are seismically induced turbidites (seismites), then large earthquakes have occurred about every 300–500 yr in the Puget Sound region.

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