Time-Dependent Fault Slip Rates within the Southern California Fault Network

Mismatches between fault slip rates and geodetic rates in southern California capture a snapshot of unsteady fault system behavior. In the Mojave Desert, cumulative geologic (late-Pleistocene to present) dextral displacement rate across the eastern California shear zone is approximately half the geodetic rate. Conversely, slip rates inferred from geodesy underestimate the well-established long-term slip rate of the Garlock fault. This pattern has been suggested to indicate that these fault zones alternate in their loading behavior. Another area of slip-rate discrepancy is between the San Andreas and San Jacinto faults. Present-day geodetic rates are faster across the San Jacinto fault, yet late Pleistocene to present slip rates suggest the opposite. These mismatches appear to be related to unsteady rates of strain release: The fast-slipping Garlock, San Andreas, and San Jacinto faults all show clustered earthquake activity in paleoseismic records, and mid-Holocene slip rates on two parallel strands of the San Jacinto fault are faster than their late Pleistocene time-averaged rates, suggesting a recent cluster of activity. The slow-slipping dextral faults of the Mojave Desert do not express clustered earthquake behavior individually, but as a system earthquakes here are clustered. Potential causes of the observed geodetic versus geologic mismatches are (1) steady accumulation but unsteady release of elastic strain or (2) time-dependent partitioning of loading between different fault sets. Release of stored elastic strain over earthquake clusters does not explain patterns such as alternation between fault sets and coherent acceleration of slip rate on parallel strands of the San Jacinto fault. Thus we hypothesize that the partitioning of loading across the southern California fault network varies over time, and that earthquake activity is modulated by the variation in fault-zone loading. The pattern of earthquake clustering and fault slip rates suggests that the time scale of loading variation is of order ~1 kyr.