A unified perspective of seismicity and fault coupling along the San Andreas Fault

The San Andreas Fault (SAF) showcases the breadth of possible earthquake sizes and occurrence behavior, from repeating earthquakes to total quiescence, to large damaging earthquakes. In particular, the central SAF is a microcosm of such diversity. This section also exhibits the spectrum of fault coupling from locked to creeping. Here, we show that these varied observations are in fact tightly connected. Specifically, the creep rate along the central SAF is shown to be directly proportional to the fraction of Poisson-distributed in time (background) earthquakes for the period 19842020. This relationship provides a unified perspective of earthquake phenomenology along the SAF, where lower coupling manifests in weaker temporal clustering, with repeating earthquakes as an end-member. We compute a metric called the fraction of background events that measures the average temporal clustering of earthquakes along a fault segment. We show that this quantity is highly correlated (at a 93% level) with the rate of creep as measured from geodesy, and thus may act as a proxy for the relative strength of aseismic and seismic slip behavior along a fault segment. The degree of fault coupling thus has a first-order effect on the long-term seismicity dynamics of the entire central San Andreas. Regions exhibit lower fraction of background seismicity are interpreted of having higher likelihood of triggering mainshocks followed by Omori-type aftershock sequences by taking up an increasingly larger area of the fault surface via seismic slip. Under this unified paradigm, the northwest ~75 km of the creeping segment may be more aptly described as a transition zone, with potentially increased likelihood of a rupture propagating through the entire ~150 km-long creeping segment. Our findings may provide a way to more easily assess the degree of aseismic/seismic coupling of fault systems globally.