Supercycles at subduction thrusts controlled by seismogenic zone downdip width

Abstract:

Supercycles in subduction zones describe a long-term cluster of megathrust earthquakes, which recur in a similar way (Sieh et al. 2008,Goldfinger et al. 2013). It consists of two complete failures of a given subduction segment in between which, after a long period of relative quiescence, partial ruptures occur. We recognize that supercycles were proposed in those subduction zones (Sieh et al. 2008,Goldfinger et al. 2013, Metois et al. 2014, Chlieh et al. 2014) for which the seismogenic zone downdip width is estimated to be larger than average (Heuret et al. 2011, Hayes et al. 2012). We show with a two-dimensional numerical model of a subduction zone that the seismogenic zone downdip width indeed has a strong influence on the long-term seismicity pattern and rupture styles. Increasing the downdip width of the seismogenic zone leads to a transition from ordinary cycles of similar sized crack-like ruptures to supercycles consisting of a range of rupture sizes and styles. Our model demonstrates how interseismic deformation accompanied by subcritical and pulse-like ruptures effectively increases the stress throughout the seismogenic zone towards a critical state at which a crack-like superevent releases most of the accumulated stresses. We propose such stress evolution along the dip of the megathrust as the simplest explanation for supercycles. This conceptual model suggests that larger than thus far observed earthquakes could occur as part of a supercycle in subduction zones with a larger than average seismogenic zone downdip width (>120-150 km).