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

Abstract

Focal mechanisms for large (M > 6) earthquakes in extensional terranes suggest that seismogenic normal faults have dips that range from {approximately}30{degree} to {approximately}70{degree}. 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 {approximately} 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 {approximately}20{degree} to a depth of at least 6km. 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.