VITAL SIGNS OF THE PLANET: SOUTHERN CALIFORNIA EDUCATORS CONTRIBUTE TO CRUSTAL DEFORMATION STUDIES WITHIN SAN BERNARDINO AND RIVERSIDE COUNTIES

In conjunction with California State University, San Bernardino, Inland Empire middle school and high school teachers used GPS to monitor movement along the San Andreas and San Jacinto faults within the Inland Empire, San Bernardino Mountains, and high desert regions of Southern California since 2002.  Stations observed in 2015 were selected from those that previously had relatively poorly constrained time series, so as to contribute useful new velocity constraints for use by the SCEC community and others. Procedures for the study included setting up a tripod (or spike mount), antenna and receiver over existing survey monuments for a 10 hour period each day for 3 days. GPS data was processed at the University of Arizona and benchmark positions were compared to those in previous years. Time series graphs were used to estimate the north, east and vertical velocities of each site.

 

Velocities for our sites were combined with velocities from SCEC’s Crustal Motion Model version 4 (Shen et al., 2011) and with velocities from continuous GPS stations archived at the Plate Boundary Observatory. One-dimensional elastic modeling of the combined data set was used to infer fault slip rates within a transect across the plate boundary through the San Bernardino Mountains.

 

Results indicate that the combined slip rate of the 15 faults within our transect is 45.5-46.15 mm/yr. The San Andreas (SAF) and San Jacinto (SJF) faults have the highest rate of movement with a combined slip rate of 17-18.25 mm/yr. This is substantially less than the published 35 mm/yr slip rate of the SAF alone in central California. Nonetheless our inferred slip rates for the SAF and SJF are consistent with previously published slip rates for these faults in southern California over late Quaternary time scales. The two faults are so close together within our transect that their slip rates are strongly inversely related. Our best-fitting model apportions the SJF slipping 9.75 mm/yr and the SAF slipping 8.5 mm/yr, but models with SJF slipping 15 mm/yr and SAF slipping 2 mm/yr, or SJF slipping 3.25 mm/yr and the SAF slipping 14.5 mm/yr also fit the observed site velocities relatively well.