Creep on the Sargent Fault Over the Past 50 Years from Alignment Arrays and Implications for Slip Transfer Between the Calaveras and San Andreas Faults

The 55-km-long Sargent Fault connects the Calaveras Fault (creeping at 4-12 mm/yr) with the locked San Andreas Fault through the Santa Cruz mountains west of Gilroy, California. The position of the Sargent Fault between these two faults may have implications for slip transfer or strain accumulation between a creeping fault and a locked fault. The slip rate of the presumably dextral Sargent Fault is difficult to constrain using large-scale geodetic techniques due to the close spacing between several parallel faults in this region. There have been few scientific investigations of this fault; the lone paleoseismic study suggests that earthquakes have occurred on the southern section, but influence of aseismic creep on the seismic potential is not well constrained. In 1969, two alignment arrays separated by 3.7 km were installed across the central section of the Sargent Fault to investigate potential creep. The arrays extend ~180 m and ~140 m across the fault zone, respectively. These arrays were measured in 1970 and 1975 and comparison of these measurements yielded a creep rate of ~3 mm/yr across two fault strands in the northern array; results from the southern array were never published. As these arrays had not been re-measured since, it was unknown whether that creep rate was representative in the longer term; creep rates on other faults are known to vary through time. In 2019, we re-surveyed both arrays using a total station and analyzed the results using least-squares-fit lines to deflected monuments within the array to assess accumulated fault creep. Our preliminary results show that between 1970 and 2019, a period of 48.9 years, the northern array was dextrally offset ~180 mm across the same two fault strands that were active in the 1970s, yielding a creep rate of 3-4 mm/yr. Thus, it appears that the 5- and 50-year creep rates at this site are similar. The southern array was dextrally offset ~80 mm across two fault strands between 1975 and 2019, yielding a creep rate of ~2 mm/yr over 44.4 years. Based on the positions of linear valleys and a line of springs, the southern array may not span the entire fault zone, which may explain the lower creep rate at that site. Moment release by creep on seismogenic faults may lower hazard, thus these surveys may help clarify understanding of earthquake hazards on the Sargent Fault and other creeping faults.