Marine seismic refraction (sonobuoy and OBS) and gravity data obtained from the Puget Sound main basin and Lake Washington show a major discontinuity in both seismic velocities and rock densities across a steep (15 mgal/km) gravity gradient striking generally westward through Seattle from the Cascade Range foothills to Hood Canal. North of this gradient is a -129 mgal Bouguer gravity minimum centered over Lake Washington. A least squares inversion analysis of the residual Bouguer gravity field was combined with the refraction velocities to model the subsurface density distribution beneath the central Puget Sound lowland.

The data suggest the existence of a 7 to 8-km deep sedimentary basin beneath the gravity minimum north of the steep gradient. The basin is filled with probable Tertiary and Quaternary rocks having densities ranging from 2.0 to 2.6 g/cc. Modeled rock densities beneath the basin (2.7 to 2.8 g/cc) may indicate the presence of volcanic basement rocks. South of the gravity gradient, Tertiary volcanic and intrusive rocks are overlain by Tertiary and Quaternary sedimentary rocks up to 2 km thick.

The gravity gradient appears to mark a steep fault or faulted flexure forming the southern boundary of the Tertiary basin lying beneath Lake Washington and Seattle. The gravity model suggests that much of the steepness in the gradient across this feature is due to a near-surface density contrast between a west-trending belt of high density Tertiary igneous rocks and low density tuffs or pyroclastic volcanic and sedimentary rocks along the southern margin of the basin.

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