Seismogenic deformation between the Sierran microplate and Oregon Coast block, California, USA

The Sierran microplate is a northwest-translating block entrained in distributed motion east of the Pacific plate. To the north, the Oregon coast block (OCB) moves northward within the hanging wall of the Cascadia subduction zone, above the obliquely converging Juan de Fuca plate. Analysis of GPS velocity data indicates that relative motion between the rigid Sierran and OCB microplates is characterized by several millimeters per year of dextral shear directed ∼N70°W, which is distinct from and counterclockwise to macroscopic dextral shear in the Walker Lane east of the Sierran microplate. We present a new analysis of focal mechanisms from small earthquakes in an 80-km-wide zone that spans the geodetically defined Sierran-OCB boundary to evaluate patterns of distributed deformation. We find that the direction of macroscopic dextral shear in this region is parallel to Sierran-OCB motion derived from GPS data. The seismogenic deformation is consistent with postulated dextral shear within an incompletely studied west-northwest–trending zone of faults and lineaments that traverses the northern Sierra Nevada; the faults and lineaments terminate westward against Quaternary folds in the northern Sacramento Valley. Active deformation at the Sierran-OCB boundary accommodates the relative motion of the bounding microplates and probably does not represent discrete transfer of Walker Lane motion to the Cascadia subduction zone in a restraining left step across the northern Sierra and Sacramento Valley.