Geometrical and Frictional Effects on Incomplete Rupture and Shallow Slip Deficit in Ramp-Flat Structures

Several recent moderate-magnitude (Mw > 7) earthquakes, such as the 2015 Gorkha, Nepal, ruptured only the deep (>15 km depth) portions of megathrust faults, leaving the updip 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 but lower shear stress accumulation with increasing dip of the deep ramp section while increasing frictional strength of the faults allows more stress accumulation. These factors lead to earlier yet smaller failures of the ramp followed by larger and less frequent failures affecting the shallow section. 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.