Widespread Fault Creep in the Northern San Francisco Bay Area Revealed by Multistation Cluster Detection of Repeating Earthquakes

We search for repeating earthquakes (REs) in the northern San Francisco Bay Area in 1984-2016. By comparing over 670,000 waveforms from similar to 75,000 events, we identify candidate clusters of events whose waveforms have high cross-correlation coefficients at multiple stations. A key difference with our approach is that these multistation clusters do not require each event in a family be recorded at multiple common stations. We validate these candidate REs by estimating precise relative relocations for the events in each cluster. We identify 59 RE families whose relocated hypocenters are separated by less than one source radius. These are distributed throughout the Maacama fault zone and along the northern Rodgers Creek and central Bartlett Springs faults, implying that widespread, pervasive creep occurs on those faults, at rates of 1-6mm/year. At either end of the Maacama fault, the RE pattern highlights structural complexity, suggesting that multiple subparallel strands may be active and creeping. Plain Language Summary Repeating earthquakes (REs) are small earthquakes that repeat in the same places on faults at regular intervals. The data that REs produce look identical from earthquake to earthquake, and we can use this high similarity in the data to identify REs. Most REs occur on parts of faults that are creeping; that is, the rocks on either side of the fault slide slowly past each other, and do not cause large and damaging earthquakes. By knowing which parts of faults have REs and are therefore creeping, we can better forecast which parts of faults are more and less likely to have damaging earthquakes. We focus our study on the northern San Francisco Bay Area, where large earthquakes could potentially affect a large regional population. Using a new detection strategy, we find REs on three major faults (the Maacama, Rodgers Creek, and Bartlett Springs faults), allowing us to map out where these faults are creeping and how fast. This information should lead to more accurate future earthquake forecasts.