ACTIVE TECTONICS OF THE SEATTLE FAULT AND CENTRAL PUGET SOUND, WASHINGTON — IMPLICATIONS FOR EARTHQUAKE HAZARDS

Seattle fault zone

Stratigraphic and structural relationships indicate that recent deformation in the Seattle fault zone is greatest on fault "A", the northernmost splay. This fault forms the boundary between the Seattle uplift and the Eocene to Quaternary Seattle basin (Figs. 1, 5, 6). The northern part of the Seattle uplift (hanging wall of fault "A") locally includes steeply north-dipping (70°-80°) alluvial-fan and fluvial deposits of the Miocene Blakely Harbor Formation (Fulmer, 1975; Yount and Gower, 1991). Interpretation of industry seismic-reflection data indicate this unit is 2 to 3 km thick in the Seattle basin (Johnson et al., 1994; Pratt et al., 1997). B. Sherrod and J.A. Vance (1996, oral commun.) obtained a 13.4 Ma fission-track date on a tuff from the apparent base of the Blakely Harbor Formation. Assuming even extremely high sediment accumulation rates for foreland basins (600 m/my; Johnson, 1985, Table 1), the top of the Blakely Harbor Formation should be no older than ~10 Ma. This maximum inferred age for the top of the Blakely Harbor Formation provides a maximum age for initiation of fault "A". Bucknam and others (1992) showed that significant uplift occurred during a large earthquake along fault "A" of the Seattle fault about 900 A.D. Thus, fault "A" is still active.

Seismic-reflection data can be used to estimate rates of slip on fault "A" by determining the dimensions and geometry of the folded basal Quaternary reflections at the southern margin of the Seattle basin. This information can then be used to calculate the horizontal (shortening) and vertical (relative subsidence) components of structural growth (e.g., Schneider et al., 1996, their Fig. 10), and the amount of displacement required to produce this growth. This method is best applied in central Puget Sound where growth elements are buried by younger strata. Assuming mean velocities in the Pleistocene section of 2,000 m/s for line P346 (Fig. 5D), the vertical and horizontal components of growth are 640 m and 550 m, respectively, and inferred Quaternary fault displacement is 850 m on a plane dipping 49°. Assuming an age of 1.9 Ma for the base of the Quaternary section yields a slip rate of about 0.4 to 0.5 mm/yr.

This method can be applied more loosely in eastern and western Puget Sound (Figs. 5C and 5E) where the amount of structural relief across fault "A" (900 and 825 m, respectively) provides a minimum estimate of Quaternary relative subsidence. Assuming a fault dip similar to that for central Puget Sound yields Quaternary displacements of about 1190 m and 1095 m, and minimum Quaternary slip rates (assumes base of Quaternary is 1.9 Ma) of about 0.5 to 0.7 mm/yr.

Three problems with this method are discussed below, all of which would lead to an underestimation of slip rates:

(1) On the Puget Sound seismic-reflection profiles (Figs. 5C, D, E), there is an erosional unconformity within the Quaternary section at the crest of the anticline, which leads to underestimation of the amount of relative subsidence of growth strata. However, the amount of material eroded off the fold crest is probably less than a few hundred meters.

(2) The rate estimate only considers slip on fault "A" within the fault zone, but seismic-reflection data (Fig. 5) suggest faults "B" and "C" were also active (although lesser amounts) in the Quaternary.

(3) The assumption that the base of the Quaternary section is 1.9 Ma and has the same age across Puget Sound is probably not valid. For example, the bases of the Pleistocene and latest Pleistocene to Holocene sections in central Puget Sound are at depths of about 1,100 and 600 m, respectively. The depth to the base of the latest Pleistocene to Holocene section reflects the amount of subglacial erosion (Booth, 1994) associated with the most recent of at least 6 Quaternary glaciations in Puget Sound (Fig. 3; Easterbrook, 1994a, b). Assuming comparable amounts of erosion in central Puget Sound during the earlier glaciations and uniform subsidence/sediment accumulation rates, a Quaternary section would not begin to be preserved in central Puget Sound until about 1 Ma. This hypothesis cannot be tested without drilling, but depends on geologically realistic criteria and we regard it as more likely than a model in which the lowest Quaternary strata in the central Puget Sound trough are ~1.9 Ma. Assuming an age of 1 Ma for the base of Quaternary section in central Puget Sound yields slip rates of about 0.8 to 0.9 mm/yr on fault "A". The differences in structure and structural relief of Quaternary strata adjacent to fault "A" across the Puget Lowland may thus primarily reflect the age of involved strata.

Fault "A" of the Seattle fault zone therefore has a minimum Quaternary slip rate of about 0.6 mm/yr. Other geologic factors suggest the actual rate for the entire zone is higher, probably about 0.7-1.1 mm/yr, significantly higher than that inferred by Pratt and others (1997; 0.25 mm/yr) for the 40 million year history of the fault. These estimates are not contradictory but rather indicate variable slip rates through time. The inferred Quaternary rate suggests that there should be 11.9 to 18.7 m of displacement on fault "A" in the last ~17,000 years, which, assuming a 49° dip, would yield 9.0 to 14.1 m of uplift. This amount of uplift is consistent with Thorson (1996; his Fig. 6), which uses the contact between the Lawton Clay and Esperance Sand (Fig. 3) as a horizontal datum for estimating 8 to 14 m of post-17 ka uplift across the Seattle fault at Alki Point (Fig. 2A). We regard the resolution on this datum as far more reliable than the other potential datums described by Thorson (1996) that are based on strandlines of widely spaced proglacial lakes.

North-trending fault zone

The north-trending strike-slip zone cuts Quaternary deposits and displaces the youngest splay of the Seattle fault (fault "A"), and must therefore be considered potentially active. It must also be considered younger than 10 Ma, described above as the maximum age for initiation of fault "A". Treated as a whole, cumulative displacement across this zone using fault "A" as a piercing point is about 2.4 km. If fault "A" and the north-trending strike-slip zone were each initiated at 10 Ma, then the cumulative slip on the north-trending zone is about 0.2 to 0.3 mm/yr. However, the similarity in fault "A" style and geometry from Lake Washington to Dyes Inlet (Fig. 5) suggest that fault "A" may have originated and evolved as an unbroken structure for much of its history. Thus, the cumulative rate of slip on the strike-slip zone could be 0.5 to 1.0 mm/yr (for initiation at 5 and 2.5 Ma, respectively) or even higher.