Seismic reflection evidence for seismogenic low-angle faulting in southeastern Arizona

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. 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 may be seismogenic and that actual mechanisms for accommodation of upper-crustal extension depend on local conditions of stress and preexisting geologic structure. -from Authors