Geologic map of the Suquamish 7.5' quadrangle and part of the Seattle North 7.5' x 15' quadrangle, Kitsap County, Washington

Haugerud, Ralph A.; Troost, Kathy Goetz

doi:10.3133/sim3181

Cited by 1 publication

(1 citation statement)

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“…The base of the Quaternary deposits, and top of consolidated Tertiary strata, is interpreted at the downward change to more reflective and more-continuous strata (Johnson et al, 1999). Shoreline exposures within the Seattle fault zone (Fulmer, 1975;McLean, 1977;Haugerud, 2005;Kelsey et al, 2008) and borings north of the Seattle fault zone show the Tertiary strata to be Miocene Blakely Harbor Formation, Oligocene Blakeley Formation, and Eocene marine strata above Crescent Formation basaltic bedrock (Johnson et al, 1994(Johnson et al, , 1999ten Brink et al, 2002;Haugerud and Troost, 2011).…”

Section: Stratigraphymentioning

confidence: 99%

Kinematics of shallow backthrusts in the Seattle fault zone, Washington State

et al. 2015

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Near-surface thrust fault splays and antithetic backthrusts at the tips of major thrust fault systems can distribute slip across multiple shallow fault strands, complicating earthquake hazard analyses based on studies of surface faulting. The shallow expression of the fault strands forming the Seattle fault zone of Washington State shows the structural relationships and interactions between such fault strands. Paleoseismic studies document an ~7000 yr history of earthquakes on multiple faults within the Seattle fault zone, with some backthrusts inferred to rupture in small (M ~5.5-6.0) earthquakes at times other than during earthquakes on the main thrust faults. We interpret seismic-reflection profiles to show three main thrust faults, one of which is a blind thrust fault directly beneath downtown Seattle, and four small backthrusts within the Seattle fault zone. We then model fault slip, constrained by shallow deformation, to show that the Seattle fault forms a fault propagation fold rather than the alternatively proposed roof thrust system. Fault slip modeling shows that back-thrust ruptures driven by moderate (M ~6.5-6.7) earthquakes on the main thrust faults are consistent with the paleoseismic data. The results indicate that paleoseismic data from the back-thrust ruptures reveal the times of moderate earthquakes on the main fault system, rather than indicating smaller (M ~5.5-6.0) earthquakes involving only the backthrusts. Estimates of cumulative shortening during known Seattle fault zone earthquakes support the inference that the Seattle fault has been the major seismic hazard in the northern Cascadia forearc in the late Holocene. by guest seismic profiles acquired in Puget Sound are shown by black lines with 1 km marks shown as white dots, with a label on 10 km marks. Black lines on land in West Seattle, south Seattle, Mercer Island, and Bellevue are locations of land seismic profiles. Black triangles mark surveyed terrace elevations, with the amount of uplift or subsidence labeled (ten Brink et al., 2006). Focal mechanism shows the location of the 1997 Point White (Bremerton) M 4.9 earthquake at a depth of ~9 km. EH-Eagle Harbor; RP-Restoration Point; AP-Alki Point; BI-Blake Island; MI-Mercer Island; VI-Vashon Island; BR-Bremerton; TJH-Toe Jam Hill; VP-Vasa Park; RP-Rich Passage; WS-West Seattle; WP-Waterman Point; PG-Point Glover; IW-Island-Wood; BH-Beacon Hill. Bedrock geology is from Booth (2007, written commun.) and Haugerud (2009). (B-C) Proposed thrustfault and roof-thrust (wedge) models for the Seattle fault zone structure. The thrust-fault model is a fault-propagation fold with trishear behavior and several shallow fault splays including backthrusts (Suppe and Medwedeff, 1990; Shaw et al., 2005). The roof-thrust model has a wedge being thrust into Seattle basin strata, and backthrusts formed by bedding-plane slip caused by folding above the wedge tip (Brocher et al., 2004; Kelsey et al., 2008).

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