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Abstract

Subduction zone megathrust earthquakes may grow into giant M>9 events by unzipping the plate boundary fault along a length larger than its seismogenic thickness. Existing hypotheses on which physical properties control along strike rupture propagation have been falsified by nature: The giant 2004 Sumatra and 2011 Japan earthquakes occurred where slab age, subduction velocity and interplate roughness were regarded indicative of not generating giant events. Similarly, geometric features or creeping zones have been shown to not necessarily form a barrier to rupture propagation (e.g. during the 2010 Chile earthquake). The question of “why” therefore translates into the question of “how likely” a megathrust earthquake can grow into a giant event. Assuming that subduction zone type plate boundary faults are segmented into locked and creeping regions (potential asperities and barriers, respectively), we here argue that giant earthquakes occur more likely when neighboring fault segments are sufficiently synchronized in their seismic cycles to allow a concerted, multi-segment failure. We investigate the process of fault synchronization in the presence of stress transfer using analytical, statistical and analogue models of subduction zones. We newly define a proxy of synchronization: the degree of phase locking. The simulation results show that the latter correlates with static stress transfer between the fault segments which is a nonlinear function of asperity depth and distance. Accordingly, over few tens of simulated seismic cycles the segments can become transiently phase locked (i.e. highly synchronized failure of neighboring segments) for stress transfer as little as 1/1000 of coseimic stress drop. These observations are typical of “weakly coupled” oscillators elsewhere in physics and the values of “stress coupling” realized in the models correspond to natural asperity spacing of few tens of kilometers as is typically observed in subduction zones. We therefore argue that while giant earthquakes may theoretically occur in every subduction zone, they are more likely in those zones where asperities are narrowly spaced (< 100 km) and seismic cycles highly synchronized, both states potentially observable in nature by means of geodetic and paleoseismological methods.