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Three segments of the Grand Valley fault (from south to north the Star Valley, Grand Valley, and Swan Valley segments) in southeastern Idaho and western Wyoming were identified by combining evidence from (1) the location and extent of fault scarps on late Quaternary alluvial fans and an early Quaternary basalt flow, (2) the height of the footwall escarpment, (3) the stratigraphy of deposits on the hanging wall adjacent to the fault, and (4) the depth of dissection of these hanging-wall deposits. Multiple latest Quaternary (≤15 ka) surface ruptures are displayed on the southernmost Star Valley segment by fault scarps of different heights preserved on alluvial fans with ages estimated between 11 and 15 ka. Assuming that these scarps were formed by two to four surface ruptures, the inferred average return period for large-magnitude paleoearth-quakes on this segment of the fault is 2,500 to 7,500 years. A latest Quaternary displacement rate of 0.6 mm/yr to 1.1 mm/yr is also inferred from the fault scarps. The lack of fault scarps on surfaces of similar age or older elsewhere along the Grand Valley fault readily distinguishes this segment from the other two. Height of the footwall escarpment, linearity of the range front, and shallow incision by the Salt River into deposits on the hanging wall suggest that a relatively high displacement rate persisted on the Star Valley segment throughout the Quaternary (≤2Ma).

The lack of fault scarps on undated surfaces of colluvial, alluvial, and eolian deposits along the trace of the Grand Valley fault or on a fluvial terrace with an estimated age of 15 to 30 ka that crosses the fault suggests that no latest Quaternary surface ruptures have occurred on the central, Grand Valley segment. Marked erosion of the footwall escarpment and deep incision by the Snake River and its tributaries into lower Quaternary and older deposits on the hanging wall contrast with the youthful morphology along the Star Valley segment. Lack of tectonic tilt of sediments that could be as old as 2 Ma suggests that no major Quaternary displacement has occurred on this segment. An average displacement rate on the northernmost Swan Valley segment of only 0.019 mm/yr is estimated from an inferred fault scarp on a basalt flow isotopically dated at 1.5 Ma. No other fault scarps were observed along this segment of the fault, but the only deposits that directly overlie the fault are loess deposits that could be as young as 11 to 30 ka. Assuming single-event surface ruptures of 2 to 6 m, the average return period for large-magnitude paleoearthquakes during the Quaternary is ≥100,000 years. Tectonic tilt of late Tertiary and Quaternary volcanic units in Swan Valley indicates that the displacement rate on the Swan Valley segment has decreased since 2 Ma from a maximum rate of about 1.8 mm/yr between 2 and 4.3 Ma. This change in displacement rate on the Swan Valley segment of the Grand Valley fault, along with the timing of high displacement rates on other late Cenozoic normal faults in the region, suggests that displacement on faults immediately south of the eastern Snake River Plain is probably related to the locations of calderas on the eastern Snake River Plain. These data suggest that large-magnitude paleoearthquakes were common on the Swan Valley segment of the Grand Valley fault between about 2 and 4.3 Ma but have been infrequent since about 2 Ma.

The differences in displacement rates combined with inferred differences in the average return periods between large-magnitude paleoearthquakes during intervals of the Quaternary and the contrast in the age of the most-recent surface displacements for the various fault segments are potentially important considerations in assessing the seismic hazard posed by the Grand Valley fault.

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