HOLOCENE PALEOSEISMICITY AND SEDIMENT DEGASSING, LAKE QUINAULT, WASHINGTON

Lake Quinault is a small, deep, glacial-moraine-dammed lake located at the foot of the Olympic Mountains in western Washington. Since its formation 20,000-29,000 years ago, the lake has served as a trap for sediment delivered from the steep, landslide-prone terrain of the Upper Quinault River catchment. The lacustrine sedimentary deposits record environmental changes since that time, including the in-lake and upland response to large earthquakes known to have impacted the region.

The sedimentary infill of Lake Quinault is dominated by deposition during river floods. Minor episodes of soft-sediment deformation at the lake margins are also recorded, and based on a preliminary age model, may be related to known earthquakes, including the well documented 1700 AD Cascadia megathrust event. By far the most dramatic event in the middle-late Holocene record of Lake Quinault, however, is the lateral spreading and degassing of sediments on its gentle western slopes during an event ca. 1300 years ago. High-resolution seismic reflection data reveal that strata in that part of the lake are pervasively cross-cut by sub-vertical zones of acoustic turbidity interpreted as gas chimneys. Several of these gas chimneys extend from the limit of seismic penetration at 15-20 m depth in the lake bed upward to the lake bottom where they terminate at mounds with evidence for active gas venting. Most of the gas chimneys, however, end abruptly around 2.5 m beneath the lake floor and are overlain by parallel, continuous reflectors. Piston cores reveal soft-sediment deformation at this level, and abrupt shifts in density, magnetic susceptibility, particle size, color, and inorganic geochemistry. We interpret these shifts to mark the contact between sediments that experienced shaking and degassing during a strong earthquake event and overlying sediments that have not experienced comparable seismicity. The earthquake evidently strongly affected the Upper Quinault River catchment, causing increased sediment input to the lake and stepwise progradation of the delta at its eastern end. Our results suggest that similar events may have occurred previously in the lake, at a frequency of several thousands of years, and may reflect a response to large earthquakes generated on nearby crustal faults rather than at the subduction interface.