Dynamic seismic ruptures on melting fault zones

Abstract

[1] We present a physical model that describes the behavior of spontaneous earthquake ruptures dynamically propagating on a fault zone and that accounts for the presence of frictional melt produced by the sliding surfaces. First, we analytically derive the solution for the temperature evolution inside the melt layer, which generalizes previous approximations. Then we incorporate such a solution into a numerical code for the solution of the elastodynamic problem. When a melt layer is formed, the linear slip-weakening law (initially governing the fault and relying on the Coulomb friction) is no longer valid. Therefore we introduce on the fault a linearly viscous rheology, with a temperature-dependent dynamic viscosity. We explore through numerical simulations the resulting behavior of the traction evolution in the cohesive zone before and after the transition from Coulomb friction and viscous rheology. The predictions of our model are in general agreement with the data from exhumed faults. We also find that the fault, after undergoing the breakdown stress drop controlled by the slip-weakening constitutive equation, experiences a second traction drop controlled by the exponential weakening of fault resistance due to the viscous rheology. This further drop enhances the instability of the fault, increasing the rupture speeds, the peaks in fault slip velocity, and the fracture energy density.

The mechanics of lubricated faults: Insights from 3‐D numerical models

“…We emphasize here that in the present model the final level of stress is not prescribed, as in the case of the linear slipweakening friction law, but it is itself a part of the solution. The same occurs for other governing models, such as the flash heating of asperity contacts and melting of rock and gouge, which also exhibit a dramatic fault weakening [Andrews, 2002;Bizzarri and Cocco, 2006;Bizzarri, 2009;Rice, 2006;Bizzarri, 2011a]. Indeed, Bizzarri and Spudich [2008] have shown that the inclusion of the thermal pressurization of the pore fluids can cause a supershear regime in a configuration which would produce a subshear propagation when the fluid migration is ignored (so that a linear slipweakening governs the fault), regardless the value of the initial shear stress.…”

“…The comparison between the present model and the viscous behavior of a continuous layer of an high viscosity molten material [Bizzarri, 2011a] is discussed in Appendix B.…”

“…[80] By considering a perfectly drained configuration (i.e., by assuming that mechanisms of fluid circulations are unimportant), Bizzarri [2011a] proposes a physical model accounting for the spontaneous transition between a Coulomb and a viscous rheology when the melting of rocks and gouge occurs. This model for melting fault zone has been proved to show a sufficiently good agreement with field data collected on an exhumed seismic thrust fault zone in Outer Hebrides, Scotland, and with measurements from Sibson [1975], performed on the same fault zone.…”

“…[3] The introduction of a governing law guarantees a finite energy flux at the rupture tip and makes it possible to simulate the traction evolution on the fault surface as it results from the inclusion of the various chemical and physical mechanisms which can strongly affect the traction evolution and ultimately the earthquake dynamics; the thermal weakening [Rice, 2006;Bizzarri, 2009], the pore fluid pressurization [Lachenbruch and Sass, 1980;Bizzarri and Cocco, 2006;Brantut et al, 2010], the powder lubrication due to nanograins [Han et al, 2010Reches and Lockner, 2010], the silica gel formation [Goldsby and Tullis, 2002], the thermal decomposition [Han et al, 2007;Brantut et al, 2008], the frictional melting [e.g., Jeffreys, 1942;Sibson, 1975;Bizzarri, 2011a], the acoustic fluidization [Melosh, 1996], the normal interface vibrations [Brune et al, 1993], etc. The relative importance of these different dissipative mechanisms can depend on the presence of fluids in the seismic structure, its hydraulic properties and its maturity, the level of development of the fault fabric, the rock type, etc.…”

Slip of a Two-degree-of-freedom Spring-slider Model in the Presence of Slip-dependent Friction and Viscosity

“…However, both thermal pressurization of pore fluids and flash heating predict not only a very dramatic stress drop, but also a very high peak in fault slip velocity, so that the final result is that melting temperature is very often exceeded, unless the slipping zone is extremely large [Bizzarri and Cocco, 2006b;Bizzarri, 2009]. Bizzarri [2011a] stressed that when melting occurs, the rheological behavior of the fault zone no longer obeys the Coulomb-AmontonMohr formulation, in that a viscous rheology is needed to describe the traction evolution during the ruptures.…”