A 2500-YEAR RECORD OF LATE-HOLOCENE TSUNAMIS AT DISCOVERY BAY, WASHINGTON STATE, USA

A combination of trenching, gouge coring and vibra-coring reveal nine anomalous muddy sand beds in a 2500-yr-old sequence of peat deposits beneath a tidal marsh at Discovery Bay, on the south shore of the Strait of Juan de Fuca, Washington. We infer a tsunami origin for the muddy sand beds from stratigraphic characteristics, 14C ages, and diatom assemblages. The four youngest beds display many characteristics common to tsunami-laid sediments in the Pacific Northwest. The next two youngest beds have some tsunami-related characteristics and are probably tsunami deposits. The three oldest beds may also record tsunamis, but their areal extent is too limited to confirm this origin. Four of the muddy sand beds correlate in age with well dated, late-Holocene plate-boundary earthquakes at the Cascadia subduction zone and probably record plate-boundary tsunamis. Diatom analysis indicates that deposition of another tsunami bed, about 1000 cal. yr BP, coincided with a few decimeters of submergence of the marsh. If submergence was caused by local crustal deformation, the bed probably records a tsunami generated by an earthquake on a nearby upper-plate fault. Sources for some inferred tsunami beds could include plate-boundary earthquakes, distant upper-plate earthquakes, or local submarine landslides, which may have been triggered by earthquake shaking. Thus, tsunamis from the Cascadia subduction zone can impact coastlines well inland from the Pacific coast, and tsunamis from both subduction zone and upper-plate earthquakes pose a significant hazard to shoreline areas in this region.