STUDY OF ENIGMATIC TSUNAMI DEPOSITS AT LYNCH COVE, WASHINGTON, U.S.A

Puget Sound, Washington, is a unique area carved by Quaternary glacial activity, which left behind deeply cut fjords that are currently filled in by the Pacific Ocean. Movement along the Seattle fault within the past 1700 years in Southern Puget Sound has caused great uplift (as much as 7 m) and subsidence in this area. Two tsunami deposits (dated 1000-1100 y.a.) located north of the Seattle fault have been linked to a possible large shallow earthquake occurring in this area. Sediment records in nearby Lake Washington, tree ring correlation between area prehistoric landslides, and diatom studies at varying localities all indicate that seismic activity has been prevalent and are thought to result in large wave run-up on areas of Puget Sound.

Lynch Cove is located approximately 40 km to the south west of Seattle and the Seattle fault. A sand sheet at Lynch Cove has been described as a tsunami deposit by the identification of marine diatoms. The likelihood of this deposit being linked with events occurring on the Seattle fault is improbable due to the great ground distance and elevation separating the fault from Lynch Cove (assuming flow from north to south). Alternately, the tsunami could enter via Hood Canal, which is to west of Lynch Cove. This is also improbable as this long narrow canal turns at a right angle as it enters the cove. This deposit may instead be linked to another fault to the west of Lynch Cove, which is currently being investigated.

During Summer 2002 field season, sediment samples were collected using a 2cm diameter hand-operated push corer and analyzed for median grain size using sieves at 1/4 Phi intervals along three transects located on the eastern shore of Lynch Cove. Preliminary grain-size results indicated a lateral sediment deposit that coarsens both toward the bay and landward, while being separated by a much finer deposit in the middle region. Although more sediment analysis is planned for the future, preliminary results continue to question the possibility of Lynch Cove having an approximate 1000 year old tsunami deposit.