Rupture characteristics of normal faults: an example from the Wasatch fault zone, Utah

Abstract

The Salt Lake fault segment is one of several independent rupture segments in the Wasatch normal fault zone, Utah. The segment is about 35 km long and consists of several, approximately linear fault sections that intersect in geometrical barriers defined by bends and branching of the fault trace. The dips of these fault sections vary based on a preliminary model of the fault zone, with estimated dips ranging from 45 to 90°. The palaeo-stress tensor was characterized by sub-vertical maximum principal compressive stress, a minimum principal stress axis trending about 050° and φ ≈ 0.2−0.5. The average slip direction across the fault segment was ≈240°. Three large, non-conservative barriers occur within the fault segment—at the northern and southern ends of the segment, and a branch point in the centre of the segment. Bifurcation of the rupture zone into two primary fault systems has presumably increased the fracture toughness of the northern part of the segment due to the overlap of secondary faulting adjacent to the primary fault planes. Two large historic earthquakes in the eastern Basin and Range and Rocky Mountain transition region have ruptured bifurcated fault zones similar in structure to the Salt Lake segment; the Hebgen Lake, Montana earthquake (M = 7.5) in 1959 and the Borah Peak, Idaho earthquake (M = 7.3) in 1983. The earthquake foci were located at depths of 12–18 km and the ruptures propagated towards the regions of fault-zone bifurcation in both cases. A characteristic earthquake (M ≈ 7−7.5) may initiate at either the centre or the southern end of the Salt Lake segment. In the first case, the rupture would propagate bilaterally, but in the latter case the rupture would propagate unilaterally to the N. A unilateral propagation direction is preferred based on inferences drawn from the structural geology of the segment and direct comparison with the rupture traces of the Borah Peak and Hebgen Lake earthquakes.