Slip Rate of The Northern San Jacinto Fault From GPS Measurements

Slip rate is a key determinant of the repeat time of earthquakes rupturing a fault and a critical input into assessments of seismic hazard. To determine slip rates, both geologic and geodetic methods are used. Geologic methods measure historical slip and include the use of LiDAR data or field mapping to measure offsets, and radiocarbon dating to determine the ages of those offsets. Geodetic methods can give accurate estimates of current slip rates by measuring crustal strain accumulation at various distances from a fault. In southern California, there is a discrepancy in slip rates between the two methods; the geologic slip rate estimate for the San Andreas Fault in Cajon Pass is 25 mm/year while the geodetic estimate is 5 mm/year. We conducted our GPS surveys along the San Jacinto and San Andreas Faults.  We constructed time series to obtain velocities for our sites.  We then conducted two-dimensional elastic modeling to find the ranges of fault slip rates that are consistent with our GPS velocities.  Our preliminary results from testing ad hoc models suggest the San Andreas Fault to have a slip rate up to 15 mm/yr with most of the clustering at the 5-11 mm/yr range. The San Jacinto Fault slip rate is up to 17 mm/yr with most of the clustering in the 8-12 mm/yr range. These results will be refined as we conduct more systematic testing of a wide range of possible slip rates.