Large nucleation before large earthquakes is sometimes skipped due to cascade‐up—Implications from a rate and state simulation of faults with hierarchical asperities

Abstract

Does a large earthquake have a large quasi‐static preparation? A hierarchical asperity model in which a large tough patch (Patch L) contains smaller fragile patches (Patch S) enables a large earthquake to start with only small preparation because of cascade‐up rupture growth. We realized such a model in a rate‐and‐state framework by heterogeneous distributions of the state evolution distance, and its consequences are investigated by earthquake sequence simulations. Focus is put on elementary processes, the interaction between two scales: one Patch L containing one Patch S is simulated, with their size ratio as a parameter. If Patch S is larger than the nucleation size of Patch L, the system falls into a limit cycle consisting of only one earthquake that starts with Patch S nucleation and grows dynamically to rupture entire Patch L. If Patch S is considerably smaller than the nucleation size of Patch L, small earthquakes never dynamically cascade‐up, and the large earthquakes are initiated by large quasi‐static nucleation. In between, large earthquakes start in various ways: by large nucleation, dynamic cascade‐up, or delayed cascade‐up. In the final stage of quasi‐static nucleation, the preseismic moment release rate increases roughly inversely proportional to the time‐to‐failure tf with its amplitude depending on the nucleation size. For a Patch S rupture to cascade up, strength in the adjacent region must have been reduced, manifested by a higher creep velocity before the Patch S nucleation starts following 1/tf acceleration. Large nucleation sometimes has a precursory small earthquake characterized by larger afterslip than nonprecursory ones.