strata has been uplifted by the Olympics) and a linear aeromagnetic anomaly. The Canyon River Fault is a major fault in itself, associated with a 40 km long lineament and distinct late Holocene scarps of up to 3 meters. Although these faults are west of the Hood Canal Fault (previously presumed to be the western boundary of the Puget Lowland), new studies are revealing that the Saddle Mountain and related faults connect with the Seattle fault zone. Trench studies indicate major earthquakes (in the range of M 6. to 7.8) on the Saddle Mountain faults at nearly the same time (give or take a century) as the great quake on the Seattle Fault about 1100 years ago (900–930 AD). Such quakes pose a serious threat to the City of Tacoma's dams at Lake Cushman, located in the fault zone, and to everyone downstream on the Skokomish River. The Canyon River Fault is believed to have caused a similar-sized earthquake less than 2,000 years ago; this is a particular hazard to the Wynoochee Dam (to the west). The history and capabilities of the Frigid Creek Fault are not known. USGS QFFDB Fault #575, Saddle Mountain Faults Olympia Structure The Olympia structure – also known as the Legislature fault – is an 80 km long gravitational and aeromagnetic anomaly that separates the sedimentary deposits of the Tacoma Basin from the basalt of the Black Hills Uplift (between lines A and B on the map). It is not known to be seismic – indeed, there is very little seismicity south of the Tacoma Basin as far as Chehalis – and not even conclusively established to be a fault. This structure is shown in the gravitational mapping of 1965, but without comment. , labelling it "structure L", mapped it from Shelton (near the Olympic foothills) southeast to Olympia (pretty nearly right under the state Legislature), directly under the town of Rainier, to a point due east of the Doty Fault, and apparently marking the northeastern limit of a band of southeast striking faults in the Centralia-Chehalis area. They interpreted it as "simple folds in Eocene bedrock", though saw sufficient similarity with the Seattle Fault to speculate that this is a thrust fault. , while observing the "remarkable straight boundaries that we interpret as evidence of structural control", refrained from calling this structure a fault. (Their model of the Black Hills Uplift is analogous with