SEISMIC HAZARDS OF THE NORTHERN CALIFORNIA SHEAR ZONE TO WATER STORAGE, CONVEYANCE AND FLOOD PROTECTION INFRASTRUCTURE, NORTHERN CENTRAL VALLEY, CALIFORNIA

Seismic hazards posed by the recently identified Northern California shear zone (Sawyer, this volume) have important implications for critical facilities in northern California, in particular water storage, conveyance and flood protection infrastructure in the northern Central Valley region. The westerly striking shear zone is comprised of Quaternary faults and fold-fault structures forming the highly complex structural transition between the Sierra Nevada-Great Valley (SN-GV) microplate on the south and the Oregon and Oregon Coast blocks on the north and northwest. The ≥225 km-long shear zone forms a large-scale left stepover within the evolving Pacific-North American transform margin that appears to accommodate the transpressional transfer of ≤6-8 mm/yr of shear strain from the northern Walker Lane westward across the northern Central Valley region to the primary plate margin in northwest California.

The southern extent of the Northern California shear zone splays from the Mohawk Valley fault zone as the WNW-striking Sierra Nevada-Cascade Range boundary zone (Sawyer, 2009), or eastern ‘leg’ of a 90 km-wide restraining left stepover across the Central Valley that appears to drive contractional deformation of the Inks Creek fold belt in the Red Bluff-Redding region. Partitioning of transpressional strain along the boundary zone provides a possible explanation for the orthogonal orientation of the fold belt to the Chico microcline-reverse-fault structure, which prominently delineates the linear eastern edge of the valley from Red Bluff to Oroville. The northward-bowing northern extent splays from or near the Honey Lake fault zone as an incipient shear zone largely superimposed on extensional structures of the Cascadia backarc rift. This zone transitions into a fold-and-thrust belt that cross-cut the active Cascade Range, in the Mt. Shasta-Pit River region, and widen westward apparently forming the distributed tectonic boundary between the Central Valley and the Klamath Mountains on the north.

The Northern California shear zone appears to accommodate up to 6-8 mm/yr of plate-boundary shear strain, involving regional contractional faults beneath and bordering the northern Central Valley, and thus, has important seismic hazard implications for critical water infrastructure in the region.