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

Detailed earthquake studies throughout the transition zone between the Basin and Range (BR) and Colorado Plateaus (CP) provinces in central and southwestern Utah provide key observations relevant to (1) the subsurface geometry of seismically active faults, (2) the correlation of diffuse seismicity with geologic structure, and (3) the nature of a transitional stress state between the BR and CP provinces. Important new data in the form of three-dimensional earthquake distributions and numerous fault-plane solutions come from six field experiments in which temporary arrays of up to 13 portable seismographs were deployed to supplement a regional seismic network. Seismic slip predominates on fault segments of moderate (>30°) to steep dip—at least for small to moderate-sized earthquakes (magnitude < 5)—based on both fault-plane solutions and hypocentral distributions. Mean and median dips of seismic slip planes for normal- to oblique-slip fault-plane solutions in the study area and vicinity range from 49° to 57°. No convincing evidence has yet been found for seismic slip on either a downward-flattening or a low-angle normal fault in this region though such faults are known to be present. Low-angle structural discontinuities in the study area appear to play a fundamental role in separating locally intense upper-crustal seismicity above 6–8 km depth from less frequent background earthquakes at greater depth, down to about 15 km. Diffuse epicentral patterns result from block-interior microseismic slip and from superposed patterns of shallow upper-crustal seismicity and subjacent seismicity. If large surface-faulting earthquakes (magnitude 6½ to 7¾) nucleate at about 15 km depth in the study area, as observed elsewhere in the Intermountain region, then rupture pathways remain to be identified between deep nucleation points and existing surface fault scarps. Effective seismic surveillance will require precise resolution of focal depths to discriminate depth-varying seismicity. Fifty-three fault-plane solutions provide significant detail for mapping changes in upper-crustal stress orientation across the BR-CP transition. Important observations include (1) the alignment of horizontal principal stresses perpendicular and parallel to the BR-CP boundary, (2) average regional orientation of minimum principal stress within the transition in the 102°–282° direction, and (3) an eastward change through the transition from normal faulting to strike-slip faulting to mixed faulting, including compressional reverse faulting. Intermediate principal stress throughout the western and central part of the transition must be close in value to the maximum principal stress, and these two principal stresses interchange in orientation between vertical and a north-northeast-south-southwest horizontal direction.