Fault Creep at the Juncture of the San Andreas and Calaveras Faults
We investigated how slip at depth is being partitioned at the juncture of the San Andreas, Calaveras faults, and Quien Sabe fault zones. To do this, we determined creep rates at depth using characteristically repeating earthquakes (CREs) and the empirical formula of Nadeau and McEvilly (1999). Essentially, we are using each CRE sequence as a "creepmeter" at depth, based on the direct relationship we propose between an event's moment and rupture area.
In this area, we identified 133 CRE sequences. Using a hypoDD relocated earthquake catalog of northern California (Ellsworth et al., 2000), we were able to see where on the fault zone CREs were occurring. On the San Andreas and Calaveras faults, CRE sequences occured on horizontal linear streaks of seismicity. These streaks were more pronounced on the San Andreas fault than on the Calaveras fault in this area. The Quien Sabe fault zone was much more complex and lacked any evidence of streaks. However, CREs were also identified on this smaller structure. Additionally, the highest amounts of cummulative slip at sequence locations on the Quien Sabe fault zone exceded cummulative slip at all sequence locations on the Calaveras fault.
On the Calaveras and Quien Sabe fault zones, CREs repeated fewer times than CREs on the San Andreas fault. We observed a decrease in the time between repeats in sequences on the San Andreas Fault after the 1989 Loma Prieta earthquake. There was no obvious similar effect on the Calaveras and Quien Sabe fault zones. Aside from sequences clearly effected by nearby large earthquakes, CRE recurrence intervals on the Calaveras and Quien Sabe fault zone tended to be larger than those on the San Andreas fault. This is due to the fact that the San Andreas fault is slipping at a much greater rate and hence would have a greater number of repeats over the same time interval. Interestingly, the smaller Quien Sabe fault zone seems to be creeping about the same amount as the more mature Calaveras fault in this location.
Nadeau, R. M. and T.V. McEvilly, Fault Slip Rates at Depth from Recurrence Intervals of Repeating Microearthquakes, Science, 285, 718-721, 1999.
PapersTempleton, D., R. Nadeau, and R. Bürgmann, Fault Creep From Characteristically Repeating Earthquakes at the Juncture of the San Andreas and Calaveras Faults, in prep.
Templeton, D., R. Nadeau, and R. Burgmann, Distribution of Aseismic Slip Along the San Andreas and Calaveras Faults from Repeating Earthquakes, Seism. Res. Lett., Annual Meeting, Abstract 820, 2006.
Templeton, D, M. Traer, R. Nadeau, and R. Bürgmann, Fault Loading Variations From Repeating Earthquakes Along the San Andreas and Calaveras Faults, Eos Trans. AGU, 86(52), Fall Meet. Suppl., Abstract T21B-0467, 2005.
Templeton, D. C, R. M. Nadeau, R. Bürgmann, Structure and Kinematics at the Juncture Between the San Andreas and Southern Calaveras Faults, Eos Trans. AGU, 82(47), Fall Meet. Suppl., 881, 2001.
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Active Tectonics Research
Group at UC Berkeley
UC Berkeley Seismological Laboratory
Department of Earth and Planetary Sciences
University of California, Berkeley