Spontaneous Dynamic Rupture Propagation beyond Fault Discontinuities: Effect of Thermal Pressurization

Abstract

A range of several fault segments often sequentially ruptures during an earthquake. We investigated the effects of thermal pressurization (TP) on dynamic rupture propagation beyond fault discontinuities by simulating spontaneous rupture propagation on two vertical strike-slip fault segments. We revealed that a rupture can jump wider stepovers owing to TP, and that TP on a primary (nucleating) fault enables a rupture to jump at deep portions. In previous numerical studies on dry fault systems, it was found that a rupture sometimes fails to propagate to an unconnected fault, which is observed in the case of real earthquakes, and a rupture that successfully propagates is usually triggered near the surface of the Earth, unlike rupture evolution images obtained by seismic waveform modeling. TP can explain the inconsistencies between the previous numerical simulations and the observations, without depending on the heterogeneity of the initial stress and/or friction. Under depth-dependent stress, we showed that TP enables a rupture to jump much wider stepovers at deep portions. If TP is in effect on faults, hydraulic diffusivity along with fault geometry can strongly control the characteristics of rupture propagation at fault discontinuities.

Suppression of slip and rupture velocity increased by thermal pressurization: Effect of dilatancy

“…Numerical simulations suggested that TP can increase the breakdown stress drop [e.g., Andrews , ; Bizzarri and Cocco , ], slip‐weakening distance [ Noda and Shimamoto , ; Rice , ], peak slip rate, and rupture velocity [ Bizzarri and Cocco , ; Urata et al ., , ]. Urata et al .…”

“…[] showed that owing to TP, the slip‐weakening curves are different at different locations within a fault with uniform coefficients of friction and stress. TP and dynamic ruptures strongly depend on hydraulic diffusivity which is assumed to be constant over time and space [e.g., Bizzarri and Cocco , ; Urata et al ., , ]. Noda and Lapusta [] modeled earthquake cycles for the 1999 Chi‐Chi earthquake by two regions having different values of constant hydraulic diffusivity.…”

“…Once TP starts occurring in a fault, it significantly weakens the shear strength, and the shear stress continuously drops eventually reaching close to zero [ Rice , ]. Owing to the high‐stress drop and the weakening curves caused by TP, the rupture velocity is expected to quickly exceed the shear wave velocity in the direction of the in‐plane (mode II) rupture [ Bizzarri and Cocco , ; Urata et al ., , ], except when the initial shear stress is low. However, supershear ruptures during earthquakes are not frequently observed: the 2010 Mw 6.9 Qinghai, China, earthquake [ Wang and Mori , ], the 2012 Mw 8.6 off Northern Sumatra, Indonesia earthquake [ Wang et al ., ], and other six earthquakes listed in Bizzarri et al .…”

“…Hydraulic diffusivity can vary, owing to dilatancy processes because the hydraulic diffusivity is proportional to permeability, which can be a power of porosity [ David et al ., ], as well as the inverse of porosity. As stated above, the previous numerical studies using the constant values of hydraulic diffusivity suggested that TP and dynamic ruptures strongly depend on the values [e.g., Bizzarri and Cocco , ; Urata et al ., , ]. Therefore, changes in hydraulic diffusivity would affect dynamic ruptures significantly.…”

Steadily propagating slip pulses driven by thermal decomposition

“…Thermal pressurization is important when determining how a rupture evolves in a heterogeneous fault system. Urata et al [] showed that accounting for thermal pressurization allows a rupture to jump a wider step over; Noda and Lapusta [] showed that thermal pressurization can weaken rate‐strengthening sections of faults that would not normally slip seismically; Urata et al [] showed that the efficiency of thermal pressurization is an important consideration when determining if a rupture can branch.…”