Geometrical and frictional effects on incomplete rupture and shallow slip deficit in ramp-flat structures

Recent detailed earthquake studies highlight that there is a population of moderate- to large-magnitude earthquakes (M_w > 7), termed zone-C events (Lay et al., 2012), that rupture the deeper portions of plate boundary thrust faults in regions where larger (M_w 8-9+) earthquakes are known or suspected to have occurred. For example, the 2015 Gorkha, Nepal earthquake and the 2016 Melinka, Chile earthquake both ruptured only the deep (>15 km depth) portions of megathrust faults, leaving the up-dip fault sections unbroken. Here, we investigate the effects of geometrical and frictional variations at depth on the stress accumulation and release in ramp-flat structures using 2-D finite element models. Our results show that ramp-flat structures allow for faster shear stress accumulation but lower maximum shear stress accumulation with increasing dip of the ramp (deeper fault section). These factors lead to earlier yet smaller failures of the ramp followed by larger and less frequent failures affecting both the ramp and the flat shallow fault sections when compared to structures with no change in dip. Similarly, increasing frictional strength of the faults allows more stress accumulation in a longer time before a larger earthquake. Our models thus suggest that the dynamics of strain reservoirs are related to both the frictional strength and dips of ramp-flat megathrust structures, and the failure time of the shallow fault section is affected by the stress regime at the deep fault segment.