We present results from a decametre-scale, controlled field stimulation experiment (the FS experiment dedicated to the hydro-mechanical characterisation of in-situ clay fault slip) conducted in a natural fault zone at an overburden depth of 300 m. This experiment was realised in the Mont Terri underground rock laboratory located in the northwestern part of the Jura thrust-and-fold belt of the Alpine foreland (Bossart et al. 2017). The stimulated fault, called the "Main Fault", consists of a 1–5 m-wide N140°-dipping, 50-to-60°SE thrust zone intersecting the Opalinus Clay, an overconsolidated shale formation deposited around 174 Ma (Aalenian/Toarcian). The Main Fault corresponds to a third-order structure interpreted as a blind shear fault bend fold that is detached from the top of the underlying Staffelegg Formation in the hanging wall of the Mont Terri anticline (Nussbaum et al. 2017). We measured fault slip and seismicity induced by fluid-injection in four test intervals distributed in the damage zone and within the core of the Main Fault (Fig. 1A, 1B). Passive seismic monitoring was coupled to a new downhole probe (Guglielmi et al. 2013) that allowed us to conduct high flowrate injections while synchronously recording pressures, flowrate, and the full 3D displacement tensor of induced deformation of the borehole wall and of reactivated slip across the naturally fractured straddle zone (Fig. 1C).

The FS experiment aims at understanding

  • conditions for slip activation and stability of clay faults, and

  • evolution of the coupling between fault slip, pore pressure, and fluid migration.

Results obtained by the experiment are crucial to define mechanisms of natural and induced earthquakes, their precursors and earthquake risk assessment, in addition to providing information on the loss of integrity of natural low-permeability barriers, all important for safety assessment of deep geological repositories of radioactive wastes or CO2 sequestration.

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