Premonitory spreading of seismicity over the faults’ network in southern California: Precursor Accord

[1] We establish a connection between geometry of the faults’ network in a region and seismicity preceding the strong earthquakes in the intermediate-term timescale of years. Previous studies of observed and computer-simulated seismicity demonstrated that strong earthquakes are preceded in that timescale by the rise of seismic activity in a lower magnitude range. Here, we explore a complementary phenomenon: spreading of activity over the fault network. This phenomenon is depicted by the seismicity pattern Accord, defined as a simultaneous rise of seismicity in a sufficiently large number of fault zones (hence its name). Pattern Accord has been found recently in synthetic seismicity generated by the colliding cascades (CC) model. The present study demonstrates this pattern in observed seismicity of southern California. We applied to observations a prediction algorithm based on the pattern Accord. Scaled to the target magnitude 7.5, the pattern Accord emerges within a few years before each of the three largest earthquakes in southern California (Kern County, 1952; Landers, 1992; and Hector Mine, 1999) and at no other time. Scaled to the target magnitude interval from 6.5 to 7.4, the pattern Accord precedes six out of nine earthquakes. The duration of alarms in both cases is about 30% of the time considered. We repeated prediction with different combinations of adjustable numerical parameters of the algorithm, summing up the success and error scores on an error diagram. This numerical experiment shows that the prediction results are stable under moderate variations of adjustable parameters. The final test would be, as always, an advance prediction. The pattern Accord has a simple physical explanation connected with equilibrium of the crustal blocks’ system under the impact of tectonic driving forces. At the same time, Accord is a manifestation of a broader phenomenon: premonitory increase of earthquakes’ correlation range. The latter phenomenon was found for the first time in the CC model, although it was hypothesized by one of the authors much earlier.