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Information from marine high-resolution and conventional seismic-reflection surveys, acromagnetic mapping, coastal exposures of Pleistocene strata, and lithologic logs of water wells is used to assess the active tectonicsof the northern Puget Lowland and eastern Strait of Juan de Fuca region of the Pacific Northwest. These data indicate that the Devils Mountain fault and the newly recognized Strawberry Point and Utsalady Point faults are active structures and represent potential earthquake sources. The north-dipping (45°-75°) Devils Mountain fault extends westward for more than 125 kilometers from the Cascade Range foothills to Vancouver Island. The Devils Mountain fault is hounded by northwest-trending en-echelon folds and faults, a pattern that strongly suggests it is a left-lateral, oblique-slip. transpressional "master fault." Aeromagnetic anomalies coincide with both the trace of the Devils Mountain fault and en-echelon structures. Quaternary strata are deformed on nearly all crossing seismic-reflection profiles, and onshore subsurface data indicate offset of upper Pleistocene strata. The west-northwest-trending, subvertical Strawberry Point fault cuts across northern Whidbey Island and has a minimum length of about 25 kilometers. On the west coast of Whidbey Island and in the Strait of Juan de Fuca, the fault has south-side-up offset and forms the northern boundary of an uplift of pre-Tertiary basement rock. Exposures and subsurface logs of upper Pleistocene strata from Strawberry Point on the east coast of Whidbey Island indicate that the fault bifurcates into a 2-kilometer-wide zone as it crosses Whidbey Island. Each of the four fault splays within this zone has apparent north-side-up offset. and upper Pleistocene strata between the faults exhibit considerable shortening (dips as steep as 45°). The vertical fault trace, reversal of offset along strike, and evidence for associated contractional deformation suggest that the Strawberry Point fault is an oblique-slip, transpressional fault. The northwest-trending, subvertical Utsalady Point fault similarly cuts across northern Whidbey Island and has a minimum length of 25 kilometers. It forms the southern margin of the pre-Tertiary basement horst block on the west coast of Whidbey Island, where it has north-side-up offset. Offshore seismic-reflection data from east of Whidbey Island indicate that it also bifurcates eastward into a broad (1.5 kilometer) zone of several splays. Onshore outcrops and subsurface logs from Camano Island indicate a probable reversal of offset (to south side up) along the zone and display both faulting and folding (dips as steep as 24°) in upper Pleistocene strata. As with the Strawberry Point fault, the vertical fault trace(s), reversal of offset, and evidence for associated contractional deformation suggest that the Utsalady Point fault is an oblique-slip, transpressional fault. Collectively, the Devils Mountain, Strawberry Point, and Utsalady Point faults represent a complex, distributed, transpressional deformation zone. The cumulative slip rate on three main faults of this zone probably exceeds 0.5 millimeter/year and could be much larger. This new information on fault location, length, and slip rate should he incorporated in regional seismic hazard assessments.

u.s. geological survey professional papers/u.s. geological survey professional paper

Active tectonics of the Devils Mountain Fault and related structures, northern Puget Lowland and eastern Strait of Juan de Fuca region, Pacific Northwest