Seismic landslides in Puget Sound (SLIPS) I:
Distribution of coastal landslides

M.J. Grass, R.J. Watters, and R.E. Karlin
(Department of Geological Sciences, University of
Nevada, Reno /172, Reno, NV 89557; 702-784-6134;
e-mail: grass@mines.unr.edu)
M.L. Holmes (School of Oceanography, University of
Washington, Seattle, WA 98195)
J.C. Yount (U.S. Geological Survey, Reno, NV 89557)
Sponsor: Robert E. Karlin

A map of coastal landslides in Puget Sound has been
compiled from available sources and new field
investigations on land and sea.  Coastal landslides are a
frequent occurrence, with hundreds of slides ranging from a
few meters to hundreds of meters in length.  The
probability of slope failure is mainly controlled by local
topography, lithology, bluff erosion rates, and degree of
soil saturation.  Most slides in the Seattle-Tacoma area
are strongly affected changes in water table elevation in
slopes containing Lawton Clay overlain by Esperance Sand.
Historically these failures have influenced patterns of
urban development in the Seattle-Tacoma area.  Due to
seasonal high pore pressures, relatively low levels of
ground acceleration could trigger larger numbers of slides
in the event of an earthquake. The area around the South
Whidbey Island fault zone contains large slides which may
be associated with episodic seismic loading.  One series of
landslides in the Possession Point slide complex occurred
within a year of large earthquakes in 1910 and 1949.
Geostatistical analyses of coastal landslide distributions
will be used to examine the relation between landslides
and geologic units, tectonic structure and slope
geometries.