LACUSTRINE PALEOSEISMOLOGY FROM LAKE CRESCENT CONFIRMS MULTIPLE HOLOCENE RUPTURES OF THE LAKE CREEK – BOUNDARY CREEK FAULT ZONE IN RESPONSE TO NORTHWARD CONVERGENCE AND CLOCKWISE ROTATION OF THE NORTHERN OLYMPIC PENINSULA, WASHINGTON

The Lake Creek–Boundary Creek fault zone (LCBCFZ) is similar to a number of steeply dipping crustal faults in northwest Washington State exhibiting evidence for post-glacial surface rupture that accommodates N-S contraction and clockwise rotation of the Cascadia forearc. The fault zone trends directly beneath Lake Crescent (LC). There is direct physical evidence for surface ground rupture along at least 57 km of the fault zone both beneath the lake as well as to the west and east. Large Holocene landslide scarps and deposits surround the lake, and swath bathymetric data demonstrate that they deposited large masses of material into the lake. High-resolution seismic reflection data and a suite of piston cores reveal that these landslides produced four lake tsunami/seiche events during the past 8500 years, resulting in lakebed scour and the deposition of meter-thick “megaturbidite” layers that are traceable over the entire 20 km² lake basin. The megaturbidites consist of normally graded, faintly laminated sand overlain by thick, massive silt with dispersed plant debris, and clay caps. These megaturbidites yield calibrated ¹⁴C ages of about 3.1, 4.0, 4.9, and 7.2 ka. The 3.1 ka deposit has been undisturbed by subsequent events, but progressive deformation of the three earlier megaturbidite layers visible in seismic reflection profiles along the trace of the LCBCFZ indicates that their deposition is related to surface rupturing earthquakes.

Based on the ages of the four dated earthquakes at LC, the probability of a similar earthquake on the LC segment of the fault zone in the next 50 years is about 5%. An additional three candidate megaturbidite layers are visible in seismic reflection imagery from beneath the maximum depth of sediment core recovery, and if confirmed, would extend the paleoseismic record to at least seven earthquakes on the LCBCFZ at LC since deglaciation (c. 14 ka). Previous USGS paleoseismic trenching of the fault c. 30 km to the east of LC identified earthquake events at about 1.3, 2.9, 5-8, and 8-12 ka. Interestingly, the 1.3 ka event is not recorded at LC, nor is the 4.0 LC megaturbidite event observed in the fault trench record, suggesting that the fault zone is characterized by partial (e.g. 1.3 and 4.0 ka) and full-length (e.g. 3.1 ka) ruptures with the ability to produce up to M_w 7.5 earthquakes.