Understanding the Micromechanics of Rate-and-State Friction in fault gouge

The structural changes inside a fault during shearing control whether the fault can nucleate earthquakes. Rate-and-State Friction (RSF) is a well-established concept in geosciences to identify whether sliding leads to a dynamic shear instability. RSF is based on phenomenological observations and therefore lacks a mechanical basis. Here we propose to use operando 4D microtomography (4DµCT) to directly document the microstructural developments that underly different RSF-behaviours. We will do so using a direct shear assembly in our x-ray transparent deformation rig Heitt Mjölnir, which was recently successfully tested at ID19 (ES-1548) and has since been upgraded. In this setup we will shear fault gouge from the San-Andreas Fault Observatory at Depth at alternating shear velocities to monitor its frictional response to the velocity changes while collecting discrete 4DμCT data. Our experiments will allow a new and unprecedented perspective on the mechanics of RSF.