Variation in slip behaviour along megathrusts controlled by multiple physical properties

Abstract

Megathrusts, faults at the plate interface in subduction zones, exhibit substantial spatiotemporal variability in their slip behaviour. Many previous attempts to discern the physical controls on their slip behaviour have focused on individual variables, often associated with the physical properties of either the subducting plate (for example, its age and roughness) or the overriding plate (for example, its thickness and rigidity). Such studies, which are often location-specific or focused on single variables, have fuelled contrasting views on the relative importance of various physical properties on megathrust slip behaviour. Here we synthesize observations of the Alaska, Hikurangi and Nankai subduction zones to ascertain the main causes of the well-documented changes in interseismic coupling and earthquake behaviour along their megathrusts. In all three cases, along-trench changes in the distribution of rigid crustal rocks in the forearc, the geometry of the subducting slab and the upper-plate stress state drive considerable variability in the downdip width of the seismogenic zone. The subducting plate is systematically rougher in creeping regions, with fault-zone heterogeneity promoting a mixture of moderate to large earthquakes, near-trench seismicity and slow-slip events. Smoother subducting plate segments (with thicker sediment cover) are more strongly correlated with deep interseismic coupling and great (>Mw 8) earthquakes. In the three regions considered, there is no one dominant variable. Rather, we conclude that several physical properties affecting the dimensions and heterogeneity of megathrusts collectively explain observed along-trench transitions in slip behaviour at these subduction zones, and potentially at many other subduction zones worldwide. Multiple factors, including slab geometry and upper-plate stress state, determine the variation in slip behaviour along most megathrusts, according to a synthesis of observations of the Alaska, Hikurangi and Nankai subduction zones.