inferred on the basis of differences in the basement rock to the west and east of Puget Sound (the Crescent Formation—Cascadia core contact), and arbitrarily mapped at various locations including Lake Washington; north of the OWL this is now generally identified, with the Southern Whidbey Island Fault. Where it might run south of Seattle is not known; an argument has been made that it runs beneath Seattle but this is still conjectural. Study of surface deformation suggests possible unmapped faults near Federal Way, running between Sumner and Steilacoom, and south of Renton. See also Geology of the Pacific Northwest Notes Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The basis of . . . . . . . . . . . . . . . . . . . . . . . . External links Preliminary Atlas of Active Shallow Tectonic Deformation in the Puget Lowland, Washington (USGS Open-File Report 2010-1149) Maps of the region's faults, with an overview. USGS Quaternary fault and fold database Technical descriptions and bibliographies. The Pacific Northwest Seismic Network All about earthquakes and geologic hazards of the Pacific Northwest. The Origin of Puget Sound Short, but good. Earthquake locations. Geologic map of northwestern Washington (GM-50). Geologic map of southwestern Washington (GM-34). Various maps from Washington DNR. Aeromagnetic anomaly maps (USGS OFR 99-514). Seismic faults of Washington (state)