Magnitude of Earthquakes Controls the Size Distribution of Their Triggered Events.

The driving concept behind one of the most successful statistical forecasting models, the Epidemic Type Aftershock Sequence (ETAS) model, has been that the seismicity is driven by spontaneously occurring background earthquakes that cascade into multitudes of triggered earthquakes. In nearly all generalizations of the ETAS model, the magnitudes of the background and the triggered earthquakes are assumed to follow Gutenberg‐Richter law with the same exponent (β value). Furthermore, the magnitudes of the triggered earthquakes are always assumed to be independent of the magnitude of the triggering earthquake. Using an expectation maximization algorithm applied to the Californian earthquake catalog, we show that the distribution of earthquake magnitudes exhibits three distinct β values: βb for background events and βa − δ and βa + δ, respectively, for triggered events below and above the magnitude of the triggering earthquake; the two last values express a correlation between the magnitudes of triggered events with that of the triggering earthquake, a feature so far absent in all proposed operational generalizations of the ETAS model. The ETAS model incorporating this kinked magnitude distribution provides by far the best description of seismic catalogs and could thus have the best forecasting potential. We speculate that the kinked magnitude distribution may result from the system tending to restore the symmetry of the regional displacement gradient tensor that has been broken by the initiating event. The general emerging concept could be that while the background events occur primarily to accommodate the symmetric stress tensor at the boundaries of the system, the triggered earthquakes are quasi‐Goldstone fluctuations of a self‐organized critical deformation state. Key Points: Magnitudes of background earthquakes are distributed according to Gutenberg‐Richter (GR) law The magnitudes of earthquakes directly triggered by a given earthquake follow a kinked GR law with two branches The location of the kink coincides with the magnitude of the triggering event [ABSTRACT FROM AUTHOR]