Seismic activity on neighbouring faults as a long-term precursor to large earthquakes in the San Francisco Bay area

Activity in moderate-size earthquakes accelerated in the several decades before some large California earthquakes. This type of precursor seems to require the presence of several major faults in close enough proximity to one another that moderate-size shocks are selectively triggered on surrounding faults during the latter stages of the cycle of strain buildup to large earthquakes.