Active detachment faulting in the San Francisco Bay area?

Measurements of crustal deformation may help determine if mid-crustal reflectors imaged at ∼15 km depth below the San Francisco Bay area, California, are active detachment horizons that accommodate strike-slip shear and/or convergence across the region. Elastic dislocation models predict distinguishable secular surface displacements for fault geometries involving vertical shear zones or horizontal detachments only where faults step, bend, or branch along their trace. In the southern San Francisco Bay area, the San Andreas, Hayward, and Calaveras fault zones splay from a single trace of the creeping central segment of the San Andreas fault. Whereas measured horizontal surface displacements in the central and northern bay area are equally well predicted by models of strike-slip faults below the major faults and by models involving a detachment, a zone of fault-normal motion observed in the southern bay area is not well explained by models involving only vertical strike-slip faults. A model involving detachment slip parallel to the San Andreas fault appears more consistent with the observations. We find no evidence for significant San Andreas fault-normal regional contraction or detachment faulting. However, additional data and better models are required to more confidently constrain the three-dimensional active fault architecture.