Seattle Fault Zone

Paleoseismic History of the Seattle Fault

Active Tectonics of the Seattle Fault and Central Puget Sound, Washington: Implications for earthquake hazards

We use an extensive network of marine high-resolution and conventional industry seismic-reflection data to constrain the location, shallow structure, and displacement rates of the Seattle fault zone and crosscutting high-angle faults in the Puget Lowland of western Washington. Analysis of seismic profiles extending 50 km across the Puget Lowland from Lake Washington to Hood Canal indicates that the west-trending Seattle fault comprises a broad (4-6 km) zone of three or more south-dipping reverse faults. Quaternary sediment has been folded and faulted along all faults in the zone but is clearly most pronounced along fault "A", the northernmost fault, which forms the boundary between the Seattle uplift and Seattle basin. Analysis of growth strata deposited across fault "A" indicate minimum Quaternary slip rates of about 0.6 mm/yr. Slip rates across the entire zone are estimated to be 0.7 to 1.1 mm/yr.

The Seattle fault is cut into two main segments by an active, north-trending, high-angle, strike-slip fault zone with cumulative dextral displacement of about 2.4 km. Faults in this zone truncate and warp reflections in Tertiary and Quaternary strata and locally coincide with bathymetric lineaments. Cumulative slip rates on these faults may exceed 0.2 mm/yr. Assuming no other crosscutting faults, this north-trending fault zone divides the Seattle fault into 30-40 km long western and eastern segments. Although this geometry could limit the area ruptured in some Seattle fault earthquakes, a large event about 900 A.D. ago appears to have involved both segments. Regional seismic-hazard assessments must (1) incorporate new information on fault length, geometry, and displacement rates on the Seattle fault, and (2) consider the hazard presented by the previously unrecognized, north-trending fault zone.

Danes and others (1965) first suggested the presence of a significant west-trending fault in the Puget Lowland through Seattle on the basis of gravity data. They inferred a steeply north-dipping zone consisting of two parallel normal faults with about 11 km of vertical slip. Gower and others (1985) briefly outlined geologic relationships across the Seattle fault, and Yount and Holmes (1992) suggested the fault dipped to the south and had reverse displacement. Johnson and others (1994) used industry seismic-reflection data from Puget Sound to show that the Seattle fault is a broad zone comprising south-dipping thrust or reverse faults. Johnson and others also inferred that the Seattle fault has been active from about 40 Ma to the present, and linked north-vergent thrust faulting to flexural subsidence in the adjacent Seattle basin (Fig. 1). They suggested the Seattle fault represents a restraining transfer zone between right-lateral shear zones near Hood Canal and the southwest Washington Cascade foothills (Fig. 1; Gower et al., 1985). Pratt and others (1997) subsequently proposed a complementary model in which the Seattle fault is one structural component of a north-directed thrust sheet that underlies the central Puget Lowland from the Black Hills on the southwest to the southern Whidbey Island fault (Johnson et al., 1996) on the north.

Gower and others (1985) suggested that the large thickness of Quaternary strata in the Seattle basin indicate possible large Quaternary offsets, and noted an uplifted Holocene marine terrace within the Seattle fault zone at Restoration Point (Fig. 2A). Bucknam and others (1992) documented as much as 7 m of uplift at Restoration Point and inferred that it occurred during a large (M > 7) earthquake on the Seattle fault about 900 A.D. This earthquake was accompanied by a tsunami in Puget Sound (Atwater and Moore, 1992), landslides in Lake Washington (Fig. 2A, B; Jacoby et al., 1992; Karlin and Abella, 1992, 1996) and rock avalanches in the Olympic Mountains (Fig. 1; Schuster et al., 1992).

Rates of displacement and earthquake recurrence intervals for the Seattle fault are essentially unknown. Thorson (1993) used elevations of glacial deltas to infer about 9 m of uplift along the Seattle fault in the last 16,000 yrs, which suggests that most postglacial uplift occurred on the ~900 A.D. event and that recurrence intervals for such large events must be on the order of several thousand years. However, Thorson (1996) also speculated that motion on the Seattle fault over the last 15,000 years may be anomalous because of deglaciation and that relevant recurrence intervals could be shorter or longer. After assuming a thrust-sheet model of deformation, Pratt and others (1997) used fault-segment lengths and fold geometries to deduce an average slip rate of 0.25 mm/yr for the Seattle fault over the last 40 million years. However, other models of Puget Sound structure are possible and there is no basis for assuming that this inferred long-term Tertiary rate applies to the Quaternary. Pratt and others also calculated the total surface area of the Seattle fault from their model and concluded that earthquakes of magnitude 7.6 to 7.7 were possible.

Seismogenic depths below Puget Sound are typically about 15 to 25 km (Ludwin et al., 1991). Since 1970 when the regional seismic network became operational, the largest two earthquakes associated with the Seattle fault include a M 5.0 event that occurred at a depth of about 17 km beneath Point Robinson on 29 January 1995 (Dewberry and Crosson, 1996), and a M 4.9 event that occurred at a depth of 7 km beneath Point White on southwestern Bainbridge Island on 23 June 1997.

Marine, high-resolution, seismic-reflection data reveal that the Seattle fault forms a 4- to 6-km-wide, west-trending zone of three or more south-dipping reverse faults. The fault zone can be mapped in waterways across the Puget Lowland for at least 40 km from Dyes Inlet to Lake Washington. The fault zone was not recognized in Hood Canal. Quaternary sediments have been folded and faulted along all faults in the zone. Deformation is clearly most pronounced along the northernmost fault (fault "A"), which forms the boundary between the Seattle uplift and Seattle basin. Analysis of growth strata deposited across this structure indicate minimum Quaternary slip rates of about 0.6 mm/yr. Slip rates for the entire Seattle fault zone are estimated to be 0.7 to 1.1 mm/yr.

The Seattle fault is cut into two main segments by an active, north-trending, high-angle, strike-slip fault zone with cumulative dextral displacement of about 2.4 km. This strike-slip zone truncates and warps reflections in Tertiary and Quaternary strata and has a minimum cumulative slip rate of about 0.2 to 0.3 mm/yr. Although segmentation could limit the ruptured area in some Seattle fault earthquakes, a large event about 900 A.D. appears to have involved both segments. Future regional and local earthquake hazard assessments must incorporate the new information reported here on fault location, length, geometry, and slip rate.

This work was funded jointly by Earthquake Hazards Reduction Program and the Coastal and Marine Geology Program of the U.S. Geological Survey. We thank Guy Cochrane, Kevin O’Toole, Larry Kooker, Fred Payne, Curtis Lind, and the captain and crew of the MV Robert Gray for help in data collection, John Miller and William Stephenson for help in data processing; and Susan Rhea for GIS support. We thank Mobil, Chevron, and Western Geophysical for providing industry seismic-reflection data. We benefited from many discussions with John Armentrout, Brian Atwater, Rick Blakely, Derek Booth, Joanne Bourgeois, Tom Brocher, David Dethier, Don Easterbrook, Judith Boughner, Robert Bucknam, Carol Finn, Arthur Frankel, Ralph Haugerud, Mark Holmes, Robert Karlin, William Lingley, David Perkins, Christoper Potter, Thomas Pratt, Brian Sherrod, William Stephenson, Timothy Walsh, Craig Weaver, Ray Wells, Thomas Yelin, and James Yount. Michael Maler, Thomas Pratt, John Stamatakos, and William Stephenson provided stimulating reviews.