Are Creep Events Big: Estimating Along Strike Lengths?

Segments of many faults are observed to slip aseismically at the surface. On the central segment of the San Andreas Fault, aseismic slip accumulates largely in creep events: few-mm bursts of slip which occur every few weeks to months. But even though we have observed creep events worldwide since the 1960s, we still do not know how big most events are or which forces drive them. To address this uncertainty, we systematically identify creep events along the central San Andreas Fault and determine their along-strike rupture extents. We first use cross-correlation and visual inspection to identify events at individual creepmeters. With data from 18 USGS creepmeters, we identify 2120 records of creep events between 1985 and 2020. We then search for slip that is closely timed across multiple creepmeters. We identify 306 instances of closely timed slip, which could indicate 306 creep events that rupture multiple creepmeter locations. Through visual inspection and statistical analysis of timing, we identify a variety of 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. The existence of many large (> few-km) events suggests that creep events are not produced by small, rainfall-associated perturbations; they are more likely driven by complex or heterogeneous frictional weakening, and they may provide a window into the dynamics of larger-scale slip on the San Andreas Fault.