Characterizing the rupture extent of creep events along the Central San Andreas Fault.

The San Andreas Fault has been observed to creep at the surface along the creeping section between San Juan Bautista and Cholame. Slip along this creeping section accumulates at a slow background rate that is punctuated by creep events: few-mm bursts of slip that occur every few weeks to months. Despite observations of these events dating back to the 1960s, we still do not know the rupture extent of these events or the forces that drive them, as previous estimations are confined to short observation periods or one location. So in this study, we systematically characterize creep events in terms of their along-strike rupture extent and determine the depth at which these events occur.

We detect and analyze creep event rupture extent using 18 USGS creepmeters and PBO strainmeters along the San Andreas fault. Using a cross-correlation approach, we systematically detect 2120 creep events in the creepmeter record spanning 1985 - 2020. Comparing the start times of these events, we identify 306 potential multi-creepmeter events and determine their potential along-strike rupture extent. Through both visual inspection and statistical analysis, we identify five creep event types, including single-creepmeter events, small (<2 km) events, medium-sized (3-6 km) events, large (>10 km) events and events that rupture multiple fault strands. We also repeated this analysis after removing events that may be driven by rainfall, and we find that only the correlation of the very largest creep events diminishes. This suggests that these kilometer-long events are not small rainfall-associated perturbations; they are likely to be driven by complex or heterogeneous frictional weakening at depth.

We are exploring more of the properties of creep events to understand better the driving physics, primarily depth, duration, slip and slip evolution. By determining these properties, we may be able to better discriminate between the driving models of creep events and provide a window into the dynamics of larger-scale slip on the San Andreas Fault.