Cohesive strengthening of fault zones during the interseismic period: An experimental study

Abstract

[1] There is widespread evidence indicating that faults regain a portion of their strength during the interseismic period. Here, we present experiments designed to understand and quantify the interseismic cohesive strengthening resulting from fluid-rock reactions in fault zones. The triaxial experiments consisted of fracturing cores of Fontainebleau sandstone under dry conditions, forming a localized shear failure zone (stage 1). The specimens were then reacted hydrothermally under isostatic conditions, allowing the fault damage zone to compact, consolidate and strengthen (stage 2). Following reaction, the specimens were then reloaded to failure under nominally dry conditions, so that the increase in cohesive strength of the fault could be measured (stage 3). Experiments show that cohesion increase is positively correlated to temperature and pore pressure during reaction. After 6 hours of reaction at the highest temperatures (927°C) and pore pressures (200 MPa), cohesion increases by as much as 35 MPa. Microstructural examination of the specimens showed that the gouge particles within the fault compacted and cemented together, exhibiting textures typical of pressure solution and that fractures in the surrounding damage zone had healed. A theoretical treatment of the data was conducted using these experiments in combination with results on time-dependent changes in fault cohesion presented by Tenthorey et al. (2003). We find that the rate-controlling process in our experiments has an activation energy (Q) of approximately 70 kJ mol À1. We use this information to develop a model for time-dependent cohesive strengthening in fault zones within the continental seismogenic regime. We conclude that significant cohesive strengthening of fault zones can occur during the interseismic period of medium to large earthquakes given the presence of reactive pore fluids.

Rupture nucleation and fault slip: Fracture versus friction

“…Interseismic recovery of cohesive strength is caused by compaction of gouges by dissolution-precipitation creep, and by sealing of fractures and pore spaces by mineral deposition from pore fluids. These observations are supported by experimental studies conducted at hydrothermal conditions (Cox and Paterson, 1991;Chester and Higgs, 1992;Karner et al, 1997;Kanagawa et al, 2000; www.gsapubs.org | Volume 45 | Number 9 | GEOLOGY Tenthorey and Cox, 2006;Giger et al, 2008). If frictional melting occurs during slip, melt welding provides an additional mechanism for recovery of cohesive strength after seismogenic slip events (Mitchell et al, 2016;Proctor and Lockner, 2016).…”

The Role of Pressure Solution Creep in the Ductility of the Earth’s Upper Crust

“…Although the original formulation of the rate-and-state friction law (Scholz, 1998) considers plastic deformation of contact asperities, there has been an increased awareness that fluid-assisted processes at hydrothermal conditions contribute strongly to the changes in strength. There has been a recent wave of experimental studies on gouge strengthening in the presence of aqueous fluids (Bos et al, 2000a;Bos et al, 2000b;Bos and Spiers, 2002;Kanagawa, 2002;Kay et al, 2006;Nakatani and Scholz, 2004;Niemeijer et al, 2008a;Niemeijer et al, 2010;Niemeijer et al, 2008b;Tenthorey and Cox, 2006;Yasuhara et al, 2005). They all discuss if dissolution-precipitation processes are compatible with rateand-state friction laws and to which extend they are the dominant mechanisms for strengthening.…”

The role of fault zone fabric and lithification state on frictional strength, constitutive behavior, and deformation microstructure

“…Faults that are strong, due to advanced lithification and/or intrinsically strong mineralogy (calcite, quartz, feldspar) can be substantially weakened by a variety of mechanisms. In seismogenic faults, brecciation during a seismic event would destroy fault rock cohesive strength [Sibson, 1986b], which is then regenerated during the interseismic period through fault healing processes [Sleep and Blanpied, 1992;Karner et al, 1997;Marone, 1998b;Muhuri et al, 2003;Tenthorey and Cox, 2006;Niemeijer et al, 2008]. A mechanism for weakening at a longer timescale is the authigenic formation of weak, phyllosilicate phases by fluid-rock interactions, facilitated by grain size reduction [Wintsch et al, 1995;Vrolijk and van der Pluijm, 1999;Warr and Cox, 2001;Jefferies et al, 2006] which has been observed in fault systems such as the San Andreas [Evans and Chester, 1995;Schleicher et al, 2009Schleicher et al, , 2010Holdsworth et al, 2011].…”