Scarps on the Warm Spring and northern part of the Thousand Springs segments of the Lost River fault zone, Idaho, were surveyed and analyzed to determine fault orientation and near-surface fault roughness. Scarps formed along these two segments during the 1983 Borah Peak earthquake, and the evolution of the rupture appears to have been related to the large-scale geometry of the fault segments, and to fault roughness. The Warm Spring and Thousand Springs segments are divided into approximately planar sections that, in detail, show roughness at varying scales. Individual Tault sections within these main segments have lengths of 2 to 5 km, and average dips of about 30-degrees to 50-degrees to the WSW to SSW. The boundary between the Warm Spring and Thousand Springs segments is marked by the Willow Creek Hills, a large ridge of bedrock that extends across the hanging-wall basin. No surface faulting occurred along the two main segments within this boundary during the 1983 Borah Peak earthquake, although the presence of discontinous, IIolocene scarps indicates that the boundary has ruptured during previous earthquakes. The Willow Creek Hills lic above a large, SW-plunging ridge in the southern part of the Warm Spring segment. A smaller ridge is located within the northern end of the Thousand Springs segment, within a region of rupture branching between this segment and the Arentson Gulch fault. The scarps and aftershock distributions indicate secondary faulting within the segment boundary as the result of the Borah Peak earthquake. Scarps along both flanks of the Willow Creek Hills formed as the hanging walls of the Thousand Springs and Warm Spring segments moved downward relative to the boundary, which is structurally perched upon the underlying ridge within the Lost River fault zone. Local surface faulting in the Willow Creek Hills and aftershock activity beneath them reflect continuing deformation as the result of the heterogenous strain field in the segment boundary. Fault roughness, measured as the mean deviation of fault surfaces from a best-fit plane, continuously increases with distance along a profile, at scales from 50 to 500 m. Mean deviation in amplitude is related to incremental distance along the surface using a power law function with an exponent between 0.6 and 0.8. Detailed surveying and three-dimensional analysis of fault scarps along the strike of the fault zone augments common palcoscismological methods of scarp profiling, trenching and morphological analysis of mountain fronts. The procedures developed in this study may easily applied to fault zones in many parts of world, where scarps cut across suitable topography. The results have important implications for better defining the structure of fault zones and the influence this structure may have on fault displacement, seismicity and strong ground motion.
