Seismic landslides in Puget Sound (SLIPS) II: Quaternary faulting and 
submarine mass-wasting

R. E. Karlin, R.J. Watters, M.J. Grass,  C.F. Prunier (Department of 
Geological Sciences, University of Nevada, Reno /172, Reno, NV 89557; 
702-784-1770; e-mail: karlin@mines.unr.edu)
M. Holmes (School of Oceanography, University of Washington, Seattle, WA 
98195)

Quaternary structure, stratigraphy, distribution, and
geometry of submarine landslides in central Puget Sound
have been mapped from Everett to Tacoma with high
resolution seismic reflection profiling and sidescan swath
imaging.  Major regional crustal features such as the
Tacoma, Seattle and South Whidbey Island fault zones are
observed to cause deformation of surface sediments,
attesting to activity continuing to the present day.
However, no evidence is found that the Tacoma fault
passes through Commencement Bay as previously reported.
Submarine landslides are relatively common in the eastern
passages of Puget Sound and tend to be better preserved
than on land. Despite numerous coastal slides, the
western passages generally do not do have submarine
expression of mass wasting because of intense tidal
scouring.  Older Pleistocene units north of Seattle tend to
fail as coherent blocks, whereas the younger Esperance
Sand/Lawton Clay units south of Seattle generally fail as
incoherent slides, flows, and spreads.  Massive submarine
landslide complexes (>1 km wide) are found in several
locations including Maury Island near Tacoma, Skiff
Point on Bainbridge Island, Possession Point on Whidbey
Island, Mukilteo, Darlington, and Gedney Island near
Everett.  Many of the large slides appear to have been
activated several times during the Holocene, and occur
near or in major fault zones. The ages of these slides are
being determined by dating turbidite layers in piston cores
of nearby hemipelagic sediments.  Extensive Holocene
thrust faulting and very large landslides associated with
the South Whidbey Island fault zone suggests that this
area is very active tectonically and may constitute a
major seismic risk to the Everett-Seattle area.