Figure 64. -- Evidence from radiocarbon dating of landslide-formed dams (square symbols from data summary from Logan et. al., 1998), Lake Washington landslides (Karlin and Abella, 1992) coupled with dating of abrupt uplift and subsidence in estuaries(triangles from Bucknam et al., 1992), strongly suggests regional effects of a ~1100 yr B. P. earthquake. It has been generally assumed that this event was largely a crustal earthquake with magnitude of ~M7.5. The distribution of landslides and subsidence/uplift features suggests that the earthquake involved a main key block of Crescent Fm. (dashed outline) that we identify from velocity models. This block is in faulted contact at ZFB and other places (Glassley, 1974) with the shallow, thin separate unit of Crescent Fm. that rims the Olympic Peninsula. Our subduction zone model, based upon tomographic images and stress analysis, suggests that the key Crescent Fm. block outlined in the figure above is locked into the mantle wedge below and to the underlying Juan de Fuca plate against the southeast nose of the arch in this subducting plate. This combined structure may represent a deep asperity in the subduction thrust zone. A large earthquake here at 1100 yr B. P. may have been a complex event involving the key Crescent Fm. block responding to release of coupling stresses below it, generating large crustal motions, especially along the Seattle fault zone. A similar process occurred in the 1964 Alaskan subduction zone event, where the Patton Bay fault on Montague Island was the focus of the largest crustal uplift, but in a relatively slowly propagating manner (Plafker, 1969). The energy release was charactistic of the subduction thrust slip, and not of a simple crustal faulting event along the Patton Bay fault (Plafker, 1969). CRF=Canyon R. fault, DMF=Dow Mt. fault, SMWF=Saddle Mt. West fault, SMEF=Saddle Mt. East fault, ZFB=zone of fractured basalt of Glassley, 1974.

Bucknam, R. C., Hemphill-Haley, Eileen, Leopold, E. B., 1992, Abrupt uplift withing the past 1700 years at southern Puget Sound, Washington: Science, 258, 1611-1614.

Glassley, William, 1974, Geochemistry and tectonics of the Crescent volcanic rocks, Olympic Peninsual, Washington: Geological Society of America Bulletin, 85, 785-794.

Karlin, R. E. and Abella, S. E. B., 1992, Paleoearthquakes in the Puget Sound region recorded in the Puget Sound region recorded in sediments from Lake Washington, U.S.A.: Science, 258, 1617-1620.

Logan, R. L., Schuster, R. L., Pringle, P. T., Walsh, T. J., and Palmer, S. P., 1998, Radiocarbon ages of probable coseismic features from the Olympic Peninsula and Lake Sammamish, Washington: Washington Geology, 26, 59-67.

Plafker, George, 1969, Tectonics of the March 27, 1964 Alaska earthquake: U. S. Geological Survey Professional Paper 543-I, 74 p.