Velocity-weakening friction induced by laboratory-controlled lithification

Abstract

Regarding the occurrence of seismicity on major plate-boundary fault zones, one leading hypothesis is that the processes of lithification is responsible transforming loose, unconsolidated sediment that does not host earthquake nucleation into the frictionally unstable rocks that inhabit the seismogenic zone. Previous laboratory studies comparing the frictional properties of intact rocks and powdered versions of the same rocks generally support this hypothesis. However, systematically quantifying frictional behavior as a function of lithification remains a challenge. Here, we simulate the lithification process in the laboratory by consolidating mixtures of halite and shale powders with halite-saturated brine, which we then desiccate. The desiccation allows precipitation of halite as cement, creating synthetic rocks. We quantify lithification by: (1) direct measurement of cohesion, and (2) measuring the porosity reduction of lithified samples compared to powders. We observe that powdered samples of each halite-shale proportion exhibit predominantly velocity-strengthening friction, whereas lithified samples exhibit a combination of velocity strengthening and significant velocity weakening when halite constitutes at least 30 wt% of the sample. Analysis of the individual rate-dependent friction parameters shows that the occurrence of velocity weakening is due to relatively low values of a for lithified samples. Larger velocity weakening is associated with cohesion of >∼1 MPa, and porosity reduction of >∼50 vol%. Microstructural images reveal that the shear surfaces for powders tend to exhibit small cracks not seen on the lithified sample shear surfaces. Our results suggest that lithification via cementation and porosity loss can facilitate slip instability, supporting the lithification hypothesis for seismogenic slip.