Rheologic controls on the depth dependence of megathrust earthquakes

Abstract

The various slip behaviors of the subduction megathrust fault, including deadly megathrust earthquakes, are depth-dependent. Yet, we do not know what causes this depth dependence, in part due to variability between subduction zone thermal structures and lithological inputs. Here, we investigate controls on the nucleation depths of great earthquakes (Mw ≳ 8) and the deeper extent of seismogenesis. To do so, we create rheologic strength envelopes for six subduction zones using published constitutive relations for subducting lithologies and regional thermal models. We then compare the strength envelopes to local plate interface earthquake depths. Our synthesis shows that subducted sediments undergo a transition from frictional to predominantly viscous deformation near 300 ± 60 ° C a transition that correlates with the maximum nucleation depths of great earthquakes at 20 to 30 km. However, smaller plate interface earthquakes ( 5 ≤ Mw < 8) continue to nucleate below this depth, and the maximum earthquake size decreases until the base of the seismogenic zone at 40 to 60 km depth and ∼ 500 °C. We evaluate potential causes of seismicity below the onset of viscous deformation in these now metasediments, and conclude that the presence of lithologic heterogeneities that are loaded to frictional failure by viscously deforming metasediments is most plausible. Thus, we propose that great earthquakes can nucleate and grow into large events where all lithologies are frictional. However, the base of the seismogenic zone is deeper than the onset of viscous deformation in metasediments, which limits the growth of earthquakes into large events, and here seismicity reflects the existence and size of heterogeneity along the plate interface.