A Comprehensive Search for Repeating Earthquakes in Northern California: Implications for Fault Creep, Slip Rates, Slip Partitioning, and Transient Stress

Repeating earthquakes, earthquakes that rupture the same fault patch more than once, are playing an important role in the study of source processes and fault behavior, and have the potential to improve hazard assessment and earthquake forecast. They are believed to represent brittle failure of a stuck patch (asperity) on an otherwise creeping fault, and as such may offer insight into slip behavior at depth. We carried out a systematic analysis in search for repeating earthquakes in northern California between 1984 and 2014 by identifying events that are co-located, are of similar size, and share similar waveforms. We find 27,675 events in 7,713 repeating earthquake sequences (RES) that include between 2 and 32 events with magnitudes between -0.5 and 6.5. Temporal behavior of the events within each RES ranges between random and quasi-periodic over the 30-year observation period, and includes temporal clustering (bursts). The RESs occur throughout northern California, with some of the largest and most periodic sequences predominately, but not exclusively, along faults within the San Andreas Fault system that exhibit surface creep. Piecewise periodic RESs are found near large earthquakes where they are modulated by transient stress. Slip rates inferred from the new data, while generally consistent with published estimates from geodetic and surface data, offer a high-resolution image of subsurface creep along individual fault strands and resolve slip partitioning across wider zones of deformation.

Automated summary

Repeating earthquakes play a significant role in the study of seismic source processes and fault behavior, and have the potential to enhance hazard assessment and earthquake forecasting. A systematic analysis in northern California from 1984 to 2014 identified a large number of repeating earthquake sequences, showing temporal behavior ranging from random to quasi-periodic over a 30-year observation period. These sequences are found predominantly along faults within the San Andreas Fault system that exhibit surface creep.