Frictional healing and induced earthquakes on conventionally stable faults.

Conventional studies suggest that faults in the shallow subsurface resist earthquake nucleation, because their frictional strength increases as slip accelerates (i.e., velocity-strengthening friction). Contrary to this widely held notion, such nominally stable faults frequently host earthquakes induced by human activities. Here, we resolve this contradiction using numerical models that simulate both geological and earthquake timescales using rate-and-state friction. Faults could develop significant interface strength, expressed as increase in static frictions by around 0.25, due to "healing" over thousands to millions of years. This strength gain can be released to nucleate earthquakes, also on velocity-strengthening faults. These earthquakes exhibit efficient frictional weakening similar to those natural earthquakes on velocity-weakening faults but follow revised nucleation stages and length scales. Seismic hazard for subsequent earthquakes is reduced and vastly different. Velocity-strengthening faults can no longer host earthquakes, because subsequent slip on human lifetimes is stable. Velocity-weakening fault segments may still nucleate earthquakes, but with sharply reduced stress drops. Neighboring ruptured velocity-strengthening segments impede rupture propagation and hence reduce anticipated future earthquake magnitudes. Both the increased hazard for the first induced earthquake and less hazardous subsequent events need to be properly assessed and communicated to maintain public confidence for using the subsurface for the energy transition.