b-value variation through the seismic cycle: Revisiting Parkfield

The Parkfield section of the San Andreas fault has a history of frequently occurring moderate (M~6) earthquakes with recurrence times ranging from 12 to 38 years. Since 1985, it has been extensively monitored as part of the experiment to predict the next moderate earthquake. Using the rich data resulting from the high-resolution monitoring, studies have revealed several interesting and consistent patterns of the frequency-magnitude distribution (FMD) of earthquakes, measured by the b-value of the Gutenberg-Richter law. The fault consists of patches of low b-values (b < 0.6) that correlate well with locked patches and with the areas that slipped in the 2004 M6 earthquake. High b-values (b > 1.3) were found to correlate with creeping section of the faults, and both observations support the hypothesis of an inverse relation between differential stress and b-values. Further, the b-value was found to increase during the aftershock periods of the 2004 earthquake, but so far, no gradual loading throughout the seismic cycle has been documented at Parkfield.

Here we revisit the b-values along the Parkfield section 19 years after the last M6 event, with the objectives to monitor and better understand the evolution of b-values in space and time as the segment approaches the next rupture. Our aim is first to benchmark and enhance approaches to map and monitor transients, to optimize uncertainty quantification, robustness, and resolving power of our statistical methods. This is best targeted by creating synthetics catalogues with known properties and then benchmarking different methods for spatial mapping and time-series analysis of b-values. We specifically investigate the recently introduced b-positive estimator and convert observed b-values and activity rates to earthquake probabilities. In a second step, we analyse the observed patterns in a context of gradual fault loading and repeated moderate events, to derive insights into the underlying physical processes. Finally, our aim to set up a ‘b-value’ observatory that will continuously monitor the space-time evolution of b-values and earthquake probabilities.