The 1983 Borah Peak earthquake (Ms 7.3) initiated within the southern part of the Thousand Springs segment in the Lost River fault zone, east-central Idaho. The earthquake rupture propagated unilaterally to the northwest over a distance of 36 km and was accompanied by sinistral-normal slip within the fault zone. At the surface, the southern-most part of the rupture zone is marked by a bend in the Lost River fault zone at the intersection between the Thousand Springs segment to the north and the Mackay segment of the fault zone to the southeast. The intersection between the two fault segments is a lens-shaped area that contains numerous NW- and NE-striking faults that cut the Paleozoic bedrock. Several of the faults within the intersection zone ruptured at the surface during the 1983 earthquake.
A three-dimensional geometrical model of the intersection zone between the Thousand Springs and Mackay fault segments was constructed from geological mapping in conjunction with previously published interpretations of geodetic and seismological data. The longitudinal axis of the intersection zone plunges to the southwest, where its projected position in the subsurface roughly coincides with the location of the 1983 earthquake's hypocenter at a depth of 15 to 16 km. The intersection zone between the two fault segments may have played a dual role during the Borah Peak earthquake, both marking the site of rupture nucleation, but also acting to arrest spread of the rupture to the southeast onto the adjacent Mackay segment.