GEOMORPHIC EVIDENCE FOR HOLOCENE TECTONISM IN WESTERN WASHINGTON: A STATUS REPORT

High-resolution lidar mapping, begun in 1999, has found evidence for Holocene activity on as many as ten faults, many of which were previously unrecognized:

Boulder Creek fault—Tertiary normal fault reactivated as a thrust to produce the S-side up Kendall scarp NE of Bellingham.

Unknown fault—Offset shorelines NW of Bellingham are yet to be tied to a structure.

Utsalady Point fault—Rocky Point scarp coincides with this strand of the Darrington-Devils Mtn fault zone.

N margin Olympic Mtns—Several scarps decorate the Little River fault; also the River Road scarp and a NE-down warp in Sequim.

South Whidbey Island fault—Scarps near Woodinville show that post-glacial slip on the SWIF extended east of Puget Sound.

Seattle fault zone—Four N-side-up scarps west of Seattle are on S-verging shallow thrusts that sole into a roof thrust above a N-verging wedge; a widespread uplifted beach reflects N vergence on the wedge-floor fault.

Unnamed fault or landslide?—The Sunset Beach scarp records NW-down slip near Belfair.

Tacoma fault zone—Catfish Lake scarp is N side up, in agreement with shoreline uplift at the head of Case Inlet and farther west at Lynch Cove, at the head of Hood Canal south of Belfair.

SE margin Olympic Mtns—A third SE-up scarp in addition to the two previously identified near Price Lake, NE of Cushman Dam. SE-down normal slip on Frigid Creek scarps S of Cushman Dam.

Unnamed fault—An E-side up scarp dams the lower Skokomish valley and contributes to frequent flooding.

Offsets appear to be mostly late Holocene. Good preservation of glacial landforms suggests that earlier offsets have not been lost to diffusion, thus deformation appears to be episodic. Aggregate offset represents about half of the 4.4 mm/yr N-S shortening that McCaffrey et al. (2006) infer from GPS and fault-slip data.

Raised late-glacial marine shorelines north of Seattle may be datums with which we can limit the extent of local tectonic uplift, given lidar data of highest quality.