GEODETIC AND GEOLOGIC DEFORMATION RATES FROM THE SAN ANDREAS FAULT SYSTEM, CENTRAL CALIFORNIA

The San Andreas fault system in central California, which includes the aseismic creeping segment, provides an excellent opportunity to compare deformation rates derived over different time scales using geodetic and geologic datasets. The geometry of the fault system in this region is relatively simple, with three major sub-parallel fault strands separating internally deforming blocks. The obliquely convergent relative plate motion suggests that plate boundary deformation is transpressional.

The creeping segment allows us to evaluate the partitioning of transpression across the San Andreas fault system using geodetic methods. Using continuous GPS stations, we determined a modern creep rate of 28 mm/yr on the central creeping segment with increasing rates at greater distances from the fault. Comparison of differential GPS surveys of alignment arrays with previous studies suggests that the creep rate has been constant for the past 35 years. Moreover, this rate is significantly slower than the inferred long-term geologic slip rate (34 mm/yr) and the transcurrent component of plate motion (39 mm/yr). Detailed geologic study of folded rocks adjacent to the Rinconada fault provides insight about long-term off-fault deformation. We quantified the magnitude and style of this distributed deformation using fold hinge orientations, fault populations, and kinematic modeling. Our data indicate that a significant component of transcurrent motion was accommodated by folding in these regions since the Pliocene. This result is corroborated by vertical axis rotations derived from paleomagnetic analyses.

The geodetic and geological studies are consistent despite operating over different time scales. The geodetic deformation rates suggest that less transcurrrent motion than expected is accommodated by creep on the San Andreas fault. The geologic deformation rates suggest that more transcurrent motion than expected is accommodated by distributed off-fault deformation. These two datasets demonstrate that a fully strike-slip partitioned model of deformation is not appropriate for understanding the San Andreas fault system in central California. Instead, models with moderate partitioning better match our observations of significant off-fault deformation on both modern and geologic time scales.