Large earthquake at immature fault system characterized by seismic moment efficiency and frequency-number distribution of small earthquakes

Seismic activity on immature fault systems is known to consist of earthquakes with various fault plane orientations. Preceding seismic activity to a large earthquake is sometimes observed in such area. The size distribution of minor earthquakes ( b-value) has been reported to be indicative about the hazard information of major earthquakes. In addition, a study suggested that proximity of large earthquake can be evaluated by fault orientation distribution. In this study, we consider the proximity evaluated from ratio of stacked seismic moment tensor to total moment release in a volume. The ratio indicates efficiency of inelastic strain due to seismic moment release by earthquakes, meaning close to the strength of the medium based on the Mohr diagram and Coulomb failure condition. We examined the change in b-value and the efficiency in the focal regions of large earthquakes in immature fault systems: 2016 Kumamoto and 2019 Ridgecrest earthquakes. The large mainshocks over magnitude of 7 were accompanied by large fore shocks and preceding small seismic activity. Both parameters show specific temporal variation for pre-foreshock, between foreshock and mainshock, and aftershock sequences. Especially, the parameters between the two shocks are different from it just after the main shocks, showing a possibility of identification whether large fore shock is followed by an even larger earthquake.