Poster #089, San Andreas Fault System (SAFS)

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

Yuan-Kai Liu, Zachary E. Ross, Elizabeth S. Cochran, & Nadia Lapusta
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Poster Presentation

2021 SCEC Annual Meeting, Poster #089, SCEC Contribution #11496 VIEW PDF
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 non-clustered earthquakes for the period 1984–2020. This relationship provides a unified perspective of earthquake phenomenology along the SAF, where lower coupling manifests in weaker t...emporal clustering, with repeating earthquakes as an end-member.

We compute a metric called the "fraction of background events", to describe relative dominance between aseismic and seismic processes. We show that this quantity is highly correlated (at a 93% level) with the rate of creep as measured from geodesy. The degree of fault coupling thus has a first-order effect on the long-term seismicity dynamics of the entire central San Andreas. Regions that exhibit a lower fraction of non-clustered background seismicity are interpreted as having a higher likelihood of triggering large mainshock-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 an increased likelihood of a rupture propagating through the entire ~150 km-long creeping segment. Our findings would be directly relevant to other fault systems globally.