Seismicity of the Intermountain Seismic Belt

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Abstract
In this chapter we present an overview of the Intermountain seismic belt (ISB), a first-order feature of the Seismicity Map of North America (Engdahl and Rinehart, 1988). The ISB is a prominent northerly-trending zone of mostly shallow (<20 km) earthquakes, about 100 to 200 km wide, that extends in a curvilinear, branching pattern at least 1500 km from southern Nevada and northern Arizona to northwestern Montana (Fig. 1). Our study area, defined by the bounds of Figure 1, covers a sizable part of the western United States encompassing the ISB and is informally referred to herein as the Intermountain region. Contemporary deformation in the ISB is dominated by intraplate extension. Forty-nine moderate to large earthquakes (5.5 ≤ Ms ≤ 7.5) since 1900 and spectacular late Quaternary faulting with a predominance of normal to oblique-normal slip make the Intermountain region a classic study area for intraplate extensional tectonics. Information from the Intermountain region, relating for example to paleoseismology (Schwartz, 1987), seismotectonic framework (Smith and others, 1989), contemporary deformation from geodetic measurements and seismic moments of earthquakes (Savage and others, 1985; Eddington and others, 1987), and strong ground motion in normal-faulting earthquakes (Westaway and Smith, 1989a) has added significantly to understanding extensional seismotectonics worldwide. Particularly valuable contributions have come from field and seismological observations of two large normal-faulting earthquakes in the Intermountain region—the 1959 Hebgen Lake, Montana, earthquake (Ms = 7.5) and the 1983 Borah Peak, Idaho, earthquake (Ms = 7.3)—both described herein. Our basic intent in this chapter is to provide