Low frictional strength of upper crustal faults: A model

Abstract

Abstract. Low frictional strength of faults is estimated based on existing models for earthquake recurrence, which account for compaction and hydrothermal lithification of fault rocks, as well as for the buildup of high fluid pressures in faults during interseismic periods. In areas of elevated heat flow and for a geothermal gradient of 30øC/km, fault compaction, lithification, and changes in fluid pressure gradient are inferred to occur at shallow crustal depth (3 to 7 km). Calculated shearing resistance of faults at seismogenic depths is compatible with shear stress drops ascribed to moderate and large earthquakes. The high frequency of microearthquakes, as well as the occurrence of some larger mainshock hypocenters at shallow crustal depth along several strands of the San Andreas Fault system are consistent with a predicted minimum of crustal strength at the top of a high fluid pressure regime. On the basis of field evidence from many exhumed faults for lithification of fault gouge and for faulting at near-lithostatic fluid pressures, a modified crustal strength profile is suggested to represent the strength of many active faults which involve hydrothermal fluid flow.

Symmetric and asymmetric lithospheric extension: Relative effects of frictional‐plastic and viscous strain softening

“…Strain softening is introduced in the models as frictional‐plastic strain softening (FPSS), and viscous strain softening (VSS) by respective linear decreases of the internal angle of friction and the scaling viscosity of the creep law with the second invariant of the deviatoric strain (Figures 1 and 2). Frictional‐plastic faults and brittle shear zones may be weakened by high transient, or static, fluid pressures, or mechanical weakened by gouge, or mineral transformations [ Bos and Spiers , 2002; Sibson , 1990; Streit , 1997]. For the viscous domain ductile shear zones may be weakened by a change from dislocation to diffusion creep caused by grain size reduction [ Braun et al , 1999; Karato et al , 1986].…”

Global crustal stress pattern based on the World Stress Map database release 2008

“…Despite this, Raleigh et al (1972) state that the typical strength of homogeneous rock is such that new fractures are more likely to be generated than the reactivation of non-optimal oriented pre-existing faults. However, there is also evidence that non-optimally oriented faults may be preferentially reactivated if they have a very low coefficient of friction (Hickman, 1991;Bird and Kong, 1994;Streit, 1997;Zoback, 2000). Hence, the possible discrepancy between the -, -, and -axis that are taken as proxies for the principal stress orientation, and the true orientation of the principal stress largely depends on the coefficient of friction of existing faults (McKenzie, 1969;Raleigh et al, 1972;Zoback and Healy, 1984).…”

Fluid overpressure and stress drop in fault zones

“…It is well known that seismogenic faulting is commonly associated with zones of fluid overpressure [Sibson, 1990;Hickman et al, 1995' Streit, 1997Yamashita, 1997]. The origin and magnitude of the fluid overpressure are, however, uncertain.…”