How Faulting Keeps Crust Strong? J. Townend & M.D. Zoback, 2000 Geology

Constrains of Crustal Strength Evidence that intraplate continental crust is in a state of failure equilibrium: seismicity induced by reservoir impoundment or fluid injection earthquakes triggered by other earthquakes in situ stress measured in deep wells and boreholes is consistent to Coulomb frictional- failure theory with μ of

In-situ Measurements vs. Mohr Circle

Dependence of differential stress on effective mean stress at six locations where deep stress measurements have been made. Dashed lines illustrate relationships predicted using Coulomb frictional-failure theory for various coefficients of friction → The crust contains critically stressed faults that limit its strength

Constrains of Crustal Strength Pore pressure determines frictional strength of a faulted rock: higher pore pressure causes lower frictional strength Coulomb theory with lab-derived coefficients leads to high crustal brittle strength conclusion, under conditions of hydrostatic pore pressure → How does crust maintain hydrostatic pore pressure (and thus high crustal strength)?

Scale-dependent Permeability Core measurements determine the intrinsic permeability of the rock mass, but they are not indicative of the effective permeability controlling large-scale upper crust hydraulics km scale measurements give uniform permeability of > m mD → High permeability leads to hydrostatic pore pressure, but how is high permeability maintained?

Role of critically stressed faults Barton et al. [1995] observed from televiewer images that critically stressed faults are hydraulically conductive, whereas those that are not critically stressed are not hydraulically conductive. These faults maintain the crust's high permeability. Cajon Pass Long Valley Nevada Test Site

So, here is how the crustal strengh is maintained: fractures optimally oriented for frictional sliding (critically stressed) are hydraulically conductive brecciation on critically stressed faults are countering the fault-sealing mechanism to maintain high permeability. with crustal permeability of ~ m 2, the characteristic time scale for fluid transportation over 1-10 km is yrs.

fluid pressures in the crust are expected to equilibrate over this relatively short time period, which enables quasi hydrostatic pore pressure regime to be maintained at depth of 10 km or more. the crust then maintains high brittle strength and is at the critical stress level for frictional sliding of faults with μ  the high effective stresses in the upper crust can sustain most of the plate driving tectonic forces.

Discussion: observations from stress field in California The almost normal far field horizontal stress and oblique near field horizontal stress along the SAF indicates the possible influence of fluid pressure. Based on core samples, the permeability is low and pore pressure is generally high during interseismic crack sealing, and high pore pressure is released during coseismic fracturing. The deviatoric stress along the fault is ~10 MPa, which is much smaller than the crustal deviatoric stress. However, high pore pressure does not seem to be a valid explanation based on the result of Barton et al. [1995].