Frictional slip behavior of highly overconsolidated fault gouge

On major plate-boundary fault zones, there is an expectation that large-magnitude earthquakes do not nucleate at shallow depths, but rather starting at depths of several km in the crust.  This depth dependence is generally understood  to be controlled by the frictional behavior of the sediments making up fault gouges, where velocity-strengthening friction and low effective stresses at shallow depths tends to produce stable fault slip.  The transition to velocity-weakening rocks at seismogenic depth has been suggested to be caused by a variety of diagenetic and low-grade metamorphic processes that lithify the sediments into more competent fault rocks.  Recent laboratory results on both natural fault rocks and desiccated clay-salt mixtures show that this is a viable mechanism, where velocity-weakening friction is seen in the lithified rocks having high cohesion and low porosity.  A remaining open question is whether the key ingredient for velocity-weakening friction is the porosity reduction, or the mechanical cementation.

Here, we test whether porosity reduction alone can induce velocity-weakening friction in powdered Rochester shale, an otherwise velocity-strengthening sediment.  We control the porosity by consolidating deionized water-saturated shale powders to a vertical stress of 86 MPa and shearing the samples under lower effective normal stresses of 0.1-10 MPa, for overconsolidation ratios (OCRs) of up to ~860.  We then measure the frictional properties of our overconsolidated samples with velocity-step tests from 10^-6–10^-5 m/s, repeated over long displacements to account for fading of the initial consolidation state with slip. 

We observe that overconsolidation induces an additional porosity reduction of 35-63%, relative to the porosity under normal consolidation. The velocity steps show predominantly velocity-strengthening friction; however, some scattered instances of velocity-weakening are observed for the highest tested OCR.  Analysis of the rate- and state-dependent friction parameters shows that the velocity steps in samples with the highest OCR have both larger values of “a” and large positive values of “b”, whereas the rest of the samples show predominantly negative values of “b”.  The observed pattern in “b” suggests that asperity contacts under shear, and therefore surface roughness is important.  This is supported qualitatively by photos of the shear surfaces, showing that for larger OCRs, a smaller proportion of the nominal surface area is in real contact during shear.  The results show that velocity-weakening friction begins to appear when the additional porosity reduction is ~55%, consistent with previous work.  However, the appearance of velocity-weakening friction due solely to porosity loss may require unrealistically large OCRs, and truly unstable sliding at depth likely requires cementation by mineralization.