Abstract

The idea that all late Quaternary surface faults in the Intermountain Seismic Belt (ISB) are steeply dipping and penetrate from the surface to depths of 12 to 15 km may be an over-simplification of the actual tectonic-seismogenic process in the ISB/eastern Basin and Range transition zone. Studies of late Quaternary faulting in southwestern Wyoming and northcentral Utah, 125 km east of the Wasatch fault zone, indicate evolving tectonic/structural relationships exist with time and location. The Hebgen Lake and Borah Peak earthquakes represent a geologically mature stage of tectonic/seismogenic development manifested by fault-bounded mountain blocks and recurrent late Quaternary fault movements. Normally reactivated thrust faults and listric normal faults represent an early to intermediate stage of deformation in preexisting thrust-faulted terrains. The degree to which preexisting structures unrelated to the modern stress field affect tectonic development is complex, and a variety of structural relations may exist with time and location. An incipient seismogenic fault in the early stages of extension may exist only as a subdecollement, “blind” structure. As extension continues, “blind” seismogenic faults propagate upward eventually rupturing the surface as 45° to 60° planar structures. Normally reactivated thrust faults and listric normal faults, characteristic of early to intermediate extension, are tectonically beheaded and become inactive with propagation of planar seismogenic faults to the surface. The seismic potential of normally reactivated thrust faults and listric normal faults is open to debate. Whether the seismic potential of low-angle faults can be assessed from analysis of surface rupture parameters alone is a vitally important problem in seismic hazard assessment.

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