Focal mechanisms for large (M >6) earthquakes in extensional terranes suggest that seismogenic normal faults have dips that range from ∼30° to ∼70°. Geologic relations suggest that low-angle faults have accommodated large-scale upper-crustal extension. These disparate observations are often reconciled by arguments that low-angle faults move aseismically or rotate to low angles from initially high angles. Seismic reflection data from the Tucson basin in southeast Arizona image a low-angle normal fault (the Santa Rita fault) that crops out along the trend of late Quaternary fault scarps caused by large- magnitude (M ∼6.7-7.6) earthquakes.Velocity-independent dip analysis from shot records of the Santa Rita fault indicates that it has a true dip of ∼20° to a depth of at least 6 km. This observation suggests that low-angle extensional faults indeed may be seismogenic and that actual mechanisms for accommodation of upper-crustal extension depend on local conditions of stress and preexisting geologic structure.

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