We investigate fault creep at depth at the juncture of the San Andreas and Calaveras faults using repeating earthquake data from 1984 to 2005. We are interested in studying how creep is partitioned as a fault system transitions from having a single fault accommodate slip to having multiple, sub-parallel fault strands accommodate slip. In the San Francisco Bay Area, the juncture between the San Andreas fault and the Calaveras fault illustrates such a transition. This juncture region is highly complex as the seismically active portions of the Calaveras fault do not illuminate a straight, contiguous fault plane. Additionally, a shorter secondary fault structure, the Quien Sabe fault zone, is more seismically active than the neighboring Calaveras fault. If repeating earthquakes also occur on the Quien Sabe fault zone, this could suggest that this fault is accommodating, at least at depth, some of the total slip associated with the plate boundary.
We identify repeating earthquake sequences by cross-correlating the first 5 seconds of local event waveforms recorded by the Northern California Seismic Network at the surface. Pairs of events which have a cross-correlation coefficient greater than 0.90 are then visually inspected for quality assurance and identified as belonging to the same repeating earthquake sequence. By using this methodology in our study area, we identified 99 repeating earthquake sequences. Repeating sequences were found on all three faults within the study area.
Assuming that these sequences represent a stuck patch in an otherwise creeping fault, we employ the empirical formula of Nadeau and McEvilly (1999) to determine the total amount of fault slip at each sequence location by using the number of times a sequence repeats and the average moment magnitude of the repeating sequence as inputs (Figure 8.1). Filled circles indicate the hypoDD location and amount of total slip in centimeters associated with the repeating sequence. Open circles indicate the catalog location of a repeating earthquake sequence not included in the hypoDD relocated dataset. The amount of total slip at a small number of repeating earthquake locations along the southern portion of the Quien Sabe fault zone was greater than 25 cm over the observation period, comparable to total slip amounts along portions of the San Andreas fault. The number of times an earthquake repeats in a sequence depends on a number of factors, including the background loading rate of the fault. This explains why, in general, sequences on the San Andreas fault repeat more often or have greater moment magnitudes, and thus greater amounts of total slip, than sequences on the Calaveras or Quien Sabe fault zone. Preliminary results show that creep is as heterogeneous at depth as it appears on the surface. Locations of repeating earthquake sequences of significantly different total slip amounts can be located close to each other on the fault plane.
Special thank you to USGS-NEHRP for funding this project.
Nadeau, R. M. and T.V. McEvilly, Fault Slip Rates at Depth from Recurrence Intervals of Repeating Microearthquakes, Science, 285, 718-721, 1999.
Berkeley Seismological Laboratory
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