This report consolidates and synthesizes information on Quaternary faulting, folding, and volcanism in Utah and characterizes recent tectonic activity throughout the state. The primary purpose is to provide a comprehensive reference on faultspecific seismic sources and surface rupture to facilitate the evaluation of earthquake hazards in Utah. Two 1:500,000-scale maps show Quaternary tectonic features categorized according to probable ages of most recent surface deformation (plate 1) and ages of volcanic rocks (plate 2). Two appendix tables summarize significant data on the activity of mapped features, including ages of surface displacements and volcanism, slip rates, recurrence intervals, displacement amounts, and lengths of surface ruptures. Good age control and quantitative activity data are available for relatively few tectonic features in Utah and detailed work is needed in many areas of the state.

Existing information is adequate to demonstrate that Quaternary crustal deformation, principally normal faulting, is concentrated within a broad, north-trending zone coincident with the Intermountain seismic belt and the transitional tectonic boundary between the Basin and Range and Middle Rocky MountainsColorado Plateau physiographic provinces. Large, regionally significant structures with evidence of relatively high lateQuaternary slip rates include the Wasatch fault zone in the northern half of the state and, to the south, the Hurricane and Sevier faults. Tectonic activity during the Holocene has been concentrated on the Wasatch fault zone and, to a lesser extent, on other faults and folds in the broader Wasatch Front region and in west-central Utah. The average regional recurrence interval for large-magnitude earthquakes in the Wasatch Front region during the Holocene appears to be 125 to 300 years or less, although events have been non-uniformly distributed in time. Most notably, the composite recurrence interval for the particularly active, segmented Wasatch fault zone is roughly 400 years for the middle to late Holocene, but only 220 years for the past 1,500 years. Other patterns of spatially and temporally clustered tectonic activity in northern and west-central Utah appear to be related to persistent structural controls or to tectonic perturbations, such as crustal loading from deep-lake cycles.

Most normal faults with evidence of geologically young surface displacements (or faults which may be expressed at the surface as large-scale folds) are inferred to be moderate- to high-angle structures extending down to mid-crustal levels and capable of producing large (magnitude -6.5 to 7 .5) earthquakes. However, the seismogenic potential of faults associated with shallow, low-angle geometries (identified mainly in west -central Utah, the High Plateaus, and the Middle Rocky Mountains) is poorly understood. Some faults and folds in the state may be associated with relatively aseismic processes, such as magmatism, salt diapirism, or shallow, secondary deformation arising from activity on major structures. It is hoped that the broad scope of compiled information and text discussion will yield new insights and help direct future research into Quaternary tectonics and earthquake hazards in Utah.