Structure of an earthquake rupture segment boundary in the Lost River fault zone, Idaho : implications for rupture propagation during the 1983 Borah Peak Earthquake

The division of normal fault zones into rupture segments that extend for tens of kilometers is a fundamental feature of extensional terranes. These segments are commonly linked by jogs or "geometrical barriers" in the fault trace marked by changes in fault zone attitude, branching of the zone into multiple strands and changes in slip vectors. The 1983 Borah Peak earthquake (Ms 7.3) initiated at the geometrical barrier between the Thousand Springs and Mackay segments of the Lost River fault zone. The barrier is partly exposed in the footwall of the fault zone. Here, a set of subparallel normal faults dissect the footwall. The normal fault set asymtomatically merges with adjoining segments, linking the two principal fault segments. The barrier plunges west to southwest into the subsurface, where its projection coincides with the hypocentral zone of the 1983 Borah Peak earthquake at depths of 12 to 16 km. A geometrical model of the barrier and adjacent segments based on structural geology and geodetic data correlates with the focal mechanism and hypocentral data from the Borah Peak earthquake. The internal structure of the barrier and segmentation of the Lost River fault Is modeled by fracture mechanics theory, and by differential crustal block rotations and by preexisting crustal structures. The orientations of the normal faults in the barrier are very similiar to the orientations of intermediate principal stresses calculated by fracture mechanics theory between the Thousand Springs and Mackay fault segments. The rotation of crustal blocks can explain the barrier structures and segmentation observed in the Thousand Springs and Mackay segments. Prexisting crustal structures can also control barrier structures and fault zone segmentation. prexlsting structures are possibly long-lived features whereas seqment a t Lon and structures in the fracture mechanics and crustal block rotation models would exist for shorter periods.