Can foreshocks be discriminated from seismic swarms?

The largest earthquakes control large part of the energy budget of seismogenic structures which is progressively accumulated in the adjoining volumes meanwhile the fault system is kept locked. As wide-scale destabilization is approaching, seismic activity may increase, which can produce further instability leading to the mainshock. Foreshocks are those spatially clustered seismic events preceding a large earthquake. However, increased anomalous seismic activity is far from being ubiquitous and several destructive events are known to occur without any previous foreshocks. Some sequences are rather preceded by seismic quiescence. Nevertheless, because of the rapid improvement of seismic catalogues and apparent soundness of the foreshock hypothesis, the idea is still diffuse in the scientific community that anomalies in seismic activity can be informative about the impending occurrence of large seismic events. Are foreshocks different from swarms before the occurrence of the main event? Are foreshocks fore-shocks or simply markers of local instability without prognostic value about possible future large-scale breakdown? To provide some observational evidence about this topic, we analyzed seismicity reported in the Waveform Relocated Earthquake Catalog for Southern California from 1981 to 2022. We selected seismicity above the completeness magnitude Mc = 2.5 obtained combining a goodness of fit test for the power-law scaling of frequency-size relationship and a stability condition for the b-value of the Gutenberg-Richter law. We grouped seismic events into 5486 clusters; for each one, we consider only the events occurring until the largest one only if the final group contains at least five earthquakes. We investigate whether the relationships between maximum magnitude, involved area, duration, seismic rate, number of events, cumulative nucleated seismic moment, Shannon and Tsallis entropy, inter-event times distribution, and global coefficient of variation are different for foreshocks and swarms according to our definition and procedure. We find slightly statistical difference and try to explain why they occur. Nevertheless, unlike recent research works, we conclude that foreshocks cannot be distinguished from swarms until the largest event takes place in California.