Modeling GPS Data Across a Four Fault Transect in Southern California, USA

The southern San Andreas and San Jacinto faults (Anza and Coachella segments) are generally interpreted to have rates of 25 (+5, -5) mm/yr and 12 (+7, -5) mm/yr respectively (e.g., Working Group on California Earthquakes, 1995). New geologic data suggest that the slip rate of the San Jacinto Anza fault may be somewhat faster, perhaps as high as 20 mm/yr, and the southern San Andreas fault correspondingly slower. GPS data can also constrain the present day slip rates of these faults, provided that we apply an appropriate rheological model to convert short term geodetic velocities to longer term fault slip rates. We used SCEC GPS velocity data and a simple viscoelastic coupling model to investigate this problem. We find that the data support a faster rate interpretation for the San Jacinto fault, but results are sensitive to the assumed rheology and also to the earthquake history (recurrence interval and date of last earthquake) for the two faults, which are imperfectly known.