Undrained Gouge Response May Diminish the Effect of Fault Roughness on Earthquake Rupture

Abstract:

Crustal faults are geometrically complex at a variety of length scales, and are often characterized as fractal surfaces. Recent pioneering dynamic rupture models incorporating fault roughness showed that rupture velocity is strongly affected by geometric complexities with significant high-frequency radiation. These models assumed brittle shear failure off the fault and ignored the presence of fault gouge. Laboratory experiments show that fault gouge deform in a more ductile manner with significant phase of compaction. Using an end-cap yield criterion (similar to Cam-clay model), Hirakawa and Ma (2015) showed that large shear stress ahead of rupture front strongly compacts fault gouge (shear-enhanced compaction), which increases pore pressure and weakens the fault. Strong dilatancy during stress breakdown reduces pore pressure and strengthens the fault, promoting slip pulses. Here, we extend this undrained gouge model to rough faults. At restraining bends, large compressive stresses are expected to cause gouge compaction and thus weaken the fault (by elevated pore pressure), allowing the rupture to propagate past these bends where suppressed ruptures in previous models. On the other hand, accelerations in rupture previously seen in releasing bends may be limited due to dilatancy strengthening of gouge layer. We hypothesize that the presence of fault gouge can cause rough fault rupture to more closely resemble that of a planar fault.