Brittle Asperities and Stick-Slip Motion: Insight from Friction Experiments along A Gabbro/Marble Interface

Abstract:

We conduct a series of meter-scale direct shear experiments along a gabbro/marble fault interface at NIED in Japan. Unlike the transitional behavior from stick-slip to stable sliding along a marble/marble interface under 1.3 MPa normal stress and 0.01 mm/s loading rate, the gabbro/marble case shows persistent stick-slip behavior under the same loading conditions as well as under 2.6 MPa normal stress in subsequent tests. Visual observations of the damage pattern reveal quite different features between the marble/marble case and the gabbro/marble case. For the former, the generated damage typically shows a low aspect ratio between loading-parallel and loading-perpendicular directions, suggesting that some diffusional deformation is effective during slip. For the latter, intruded gabbro pieces with preferred growing direction parallel to loading are distributed on top of the marble side, showing that hard rocks like gabbro can be partially fractured off when sheared against soft rocks like marble. Strain array data show that the apparent friction before failure is high or even above 1 near locations where fractured-off gabbro pieces are later observed, confirming that intact rock strength of gabbro has to be overcome upon the onset of fracture. Although at this moment we do not fully understand the behind mechanism, we believe that the brittleness of gabbro dominates in making the difference. If true, this result will highlight the role of brittle asperities in generating stick-slip fault behavior in a surrounding ductile-like environment. An analogous natural example may be found by the role of seamount in generating earthquakes through or underneath sediments in subduction zones (Cloos, 1992). However, instead of shearing off long-wavelength feature as illustrated by Cloos (1992), our study suggests that the collective behavior of tiny pieces along a nominally flat surface may also generate unstable ruptures macroscopically.