Characterization of deformation perpendicular to relative plate motion and major faults of the northern San Andreas system using geodetic data

The northern San Andreas fault system NW of Clear Lake, CA is comprised of the subparallel San Andreas, Maacama, and Bartlett Springs faults. The dominant geodetic signal across the region is right lateral shear strain largely accommodated by infrequent earthquakes on these three faults and creep on the upper 5 km of the latter two. Here we use a newly densified Global Positioning System (GPS) velocity field (Murray et al., 2014) to assess the existence of regional contraction/extension. If present, the degree to which this deformation contributes to oblique slip in future earthquakes or, conversely, is partitioned into off-fault strain has implications for anticipated ground motions and inferred on-fault and off-fault moment deficit rates used in seismic hazard assessment.

We inspect the observed horizontal GPS velocity field in a Sierra Nevada-Great Valley (SNGV) fixed frame. To first order the maximum contractile strain rate axis is well-aligned with the maximum compressive stress orientation (Provost and Houston, 2003). Although observed velocities at coastal sites compared to those on the SNGV block show little net strain perpendicular to the direction of Pacific-SNGV relative motion, velocity profiles transecting the three faults show both contraction and extension perpendicular to relative plate motion within the zone between the Great Valley and the coast.

To evaluate this signal in the context of fault slip, we consider the residual fault-perpendicular velocity after removing predicted velocities due to the strike-slip component of estimated creep and interseismic locking (Murray et al., 2014). The slip rate model accounts for the fact that the orientations of the three major faults vary along their lengths, the faults are neither uniformly parallel to each other nor to the relative plate motion direction, and strike-slip creep rates vary along strike. In many locations near-fault residual strain rates are small, suggesting geometry and non-uniform creep rates can account for much of the observed fault normal motion. Localized residual strain near faults and broader patterns in off-fault regions persist. Ongoing work is focused on comparison of geodetic strain rates, focal mechanisms, and topography to better characterize possible partitioning of shear and contraction/extension within the region.