Coseismic visibility of a small fragile patch involved in the rupture of a large patch - implications from fully dynamic rate-state earthquake sequence simulations producing variable manners of earthquake initiation

Abstract

Frictional properties on a fault cannot be uniform, and one idealization would be a hierarchical asperity concept in which a large, tough patch (Patch L, radius RL and nucleation size RL c ) includes small, fragile patches (Patches S, radius RS and nucleation size RS c ), which is consistent with the scale-independent properties of earthquakes. At the onset of large earthquakes, a minor but quick signal from an immediately preceding rupture is sometimes observed during the so-called slow nucleation phase before the moment acceleration starts increasing linearly with time. Understanding what causes such seismic characteristics is important in assessing heterogeneity on a fault. In earthquake sequence simulations with hierarchical distribution of the state-evolution distance, large earthquakes spanning Patch L may be initiated by cascade-up rupture growth from Patch S, by their own large nucleation, and by delayed cascade-up, with their occurrence ratio depending on parameters characterizing the distribution (e.g., ‘scale ratio’ α = RL/RS and ‘brittleness’ β = RL/RL c = RS/RS c ). In the present paper, we compared the coseismic moment rate and acceleration functions between different types of ruptures and between different values of α. The events that started from small nucleation showed quick onset in these functions compared with those from large nucleation. In a cascade-up large earthquake, a small wave from a small rupture spanning Patch S preceded the main wave from the main rupture if α was larger than or comparable to β. This condition is similar to that for the appearance of small events in the simulated history that are nucleated in Patch S and fail to cascade-up. If α > > β, we no longer have cascade-up large events. That is, Patch S behaves as a unit of rupture for α > > β while it merely serves as internal inhomogeneity of Patch L rupture for α < < β. The transition occurs gradually with α over the intermediate range α ~ β.