Seismic landslides in Puget Sound (SLIPS) IV:
Holocene neotectonics and mass wasting in Lake
Sammamish related to the Seattle Fault

C.F. Prunier, R.E. Karlin (Department of Geological
Sciences, University of Nevada, Reno /172, Reno, NV
89557; 702-784-6723; e-mail: prunier@seismo.unr.edu)
M. Holmes (School of Oceanography, University of
Washington, Seattle, WA 98195)
T. Pratt (U.S. Geological Survey, Seattle, WA 98195)
Sponsor: Robert E. Karlin

Lake Sammamish, a N-trending glacially formed lake, lies on
the projection of the Seattle Fault, a major high angle
reverse crustal fault zone in central Puget Sound. High
resolution seismic reflection profiling, sidescan swath
imaging, and piston coring operations were conducted in the
lake to examine Holocene deformation history, identify and
map possible seismically-induced submarine landslides, and
determine the timing and recurrence intervals of sediment
disruptions perhaps related to paleoearthquakes.  The
Seattle fault appears to be a blind thrust as it passes
through the lake. The surface trace of the main fault is a
westward plunging anticline with faulting on the northerly
downthrown limbs.  The main surface rupture appears to be
a growth fault with 3-5 m offset, suggesting that the Seattle
Fault has been active several times in the last 11 ka. At
least 4 major submarine slides are found in the lake, one
of which with exposed trees was dated at <1300 yBP by
other workers.  At least 3 slides are partially buried or
show multiple slumps suggesting several episodes of
landsliding.  The sediments in 20 3-m piston cores contain
at least six anomalous turbidite layers for which
particular slumps can be identified as sources based
on magnetic susceptibility profiles.  The ages of these
events will be determined by radiocarbon dating and
compared to similar records from Lake Washington.  The
similarity of sediment records from these two lakes
suggests that major seismic shaking has have occurred
repeatedly in the Puget Sound area in the last few
thousand years.