Investigation of Lake Quinault in western Washington, including a reflection seismic survey, analysis of piston cores, and preliminary mapping in the steep, landslide-prone Quinault River catchment upstream of the lake, reveals evidence for three episodes of earthquake disturbance in the past 3000 yr. These earthquakes triggered failures on the lake’s underwater slopes and delta front, as well as subaerial landsliding, partial channel blockage, and forced fluvial sediment aggradation. The ages of the three Lake Quinault disturbance events overlap with those of coseismically subsided, coastal marsh soils nearby in southwest Washington that are interpreted to record ruptures of the Cascadia megathrust. Absent from Lake Quinault, however, are signals of obvious disturbance from five additional subduction earthquakes inferred to have occurred during the period of record. The lack of evidence for these events may reflect the limitations of the data set derived from the detrital, river-dominated lake stratigraphy but may also have bearing on debates about segmentation and the distribution of slip along the Cascadia subduction zone during prior earthquakes.

With few exceptions, today's tidal trees near Washington's Pacific coast postdate an earthquake that lowered the region by 1 m or more. The earthquake, which occurred in A.D. 1700, is the most recent to have ruptured much of the plate boundary at this central part of the Cascadia subduction zone. Because of the coseismic subsidence, lowland forests became tidal flats where thousands of trees died. Most of the trees killed were Sitka spruce (Picea sitchensis). In the centuries since the earthquake, tidal deposits have built new land that has been colonized by new Sitka spruce. All but several tens of the region's tidal spruce consequently postdate 1700, as shown by counts of annual rings in 121 of the largest spruce in tidal forests at Copalis River, Grays Harbor, and Willapa Bay. Forests began to return to each of these estuaries in the early 1700s and spread seaward in the late 1700s and 1800s. Annual rings in the oldest of the trees thus record a large fraction of the earthquake-recurrence interval that began with the 1700 earthquake.