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frequency of large earthquakes on this fault is poorly determined, but paleoseismic data show that the most recent large earthquake occurred about 1100 years ago and was associated with as much as 7 meters of uplift, major landslides, and tsunamis in Puget Sound (45). Use of laser altimetry (e.g., light detection and ranging [LIDAR]) for precise mapping of topography has led to the identification of splays of the Seattle fault. Geologists who have trenched these splays find that earthquakes of M 6.5 or greater recur as often as about 1000 years.
Earthquakes damaging to Seattle, Tacoma, and Olympia in 1949 (M 7.1), 1965 (M 6.5), and 2001 (M 6.8) were caused by rupture at depths of 50 to 60 kilometers within the subducting slab of the Juan de Fuca plate. Owing to this greater depth, the shaking intensity is lower than that of comparable shallow events. For example, the PGA recorded for the February 28, 2001, Nisqually earthquake was only about 30 percent of gravity, compared with values more than 100 percent of gravity observed for the 1994 Northridge event (M 6.7). It nevertheless caused significant damage over a broad region.
The seismic hazards in the intermontane regions of the western United States are dominated by high but relatively diffuse seismicity accommodating oblique crustal extension. The total extension rate between stable North America and the Sierra Nevada-Great Valley block is estimated by Global Positioning System (GPS) geodesy to be 1 ± 0.1 meter per century (46). Most is concentrated in the Basin and Range, a geologic province characterized by dozens of tilted, 10- to 30-kilometer-wide crustal blocks that form high mountain ranges alternating with deep basins. Earthquakes occur both on the normal faults bounding the mountain ranges and on the strike-slip faults that cut across the province.
The late Cenozoic normal faults are distributed relatively uniformly in the Basin and Range, but the historic and instrumental seismicity is concentrated in the central Nevada seismic belt, along the western margin of the province in eastern California and western Nevada, and the intermountain seismic zone, along the eastern edge of the province, from southern Nevada across central Utah to southwestern Montana and central Idaho. From 1915 to 1954, a sequence of five large earthquakes (M 6.8 to 7.7) ruptured adjacent segments of the central Nevada seismic belt. Another historically active area is the intermountain seismic zone centered on Yellowstone National Park in northwestern Wyoming, where a mantle hot spot is causing uplift and volcanism (47). The 1959 Hebgen Lake earthquake ruptured a normal fault adjacent to the Yellowstone area, and the 1983 Borah Peak earthquake (M 7.5) occurred on a range-