Key questions in fault reactivation in shales relate to the potential for enhanced fluid transport through previously low-permeability aseismic formations. Here we explore the behavior of a 20 m long N0-to-170°, 75-to-80°W fault in shales that is critically stressed under a strike-slip regime (σ1 = 4 ± 2 MPa, horizontal and N162° ± 15°E, σ2 = 3.8 ± 0.4 MPa and σ3 = 2.1 ± 1 MPa, respectively 7–8° inclined from vertical and horizontal and N72°). The fault was reactivated by fluid pressurization in a borehole using a straddle packer system isolating a 2.4 m long injection chamber oriented-subnormal to the fault surface at a depth of 250 m. A three-dimensional displacement sensor attached across the fault allowed monitoring fault movements, injection pressure and flow rate. Pressurization induced a hydraulic diffusivity increase from ~2 × 10−9 to ~103 m2 s−1 associated with a complex three-dimensional fault movement. The shear (x-, z-) and fault-normal (y-) components (Ux, Uy, and Uz) = (44.0 × 10−6 m, 10.5 × 10−6 m, and 20.0 × 10−6 m) are characterized by much larger shear displacements than the normal opening. Numerical analyses of the experiment show that the fault permeability evolution is controlled by the fault reactivation in shear related to Coulomb failure. The large additional fault hydraulic aperture for fluid flow is not reflected in the total normal displacement that showed a small partly contractile component. This suggests that complex dilatant effects estimated to occur in a plurimeter radius around the injection source affect the flow and slipping patch geometries during fault rupture, controlling the initial slow slip and the strong back slip of the fault following depressurization.

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Rationale The precision obtained in routine isotope analysis of water (δ17O, δ18O, δ2H, 17O‐excess and d‐excess values) by Cavity Ring‐Down Spectroscopy is usually below the instrument specifications provided by the manufacturer. This study aimed at reducing this discrepancy, with particular attention paid to mitigating the memory effect (ME). Methods We used a Picarro L2140i analyzer coupled with a high‐precision A0211 vaporizer and an A0325 autosampler. The magnitude and duration of the ME were estimated using 24 series of 50 successive injections of samples with contrasting compositions. Four memory correction methods were compared, and the instrument performance was evaluated over a 17‐month period of routine analysis, using two different run architectures. Results The ME remains detectable after the 30th injection, implying that common corrections procedures only based on the last preceding sample need to be revised. We developed a new ME correction based on the composition of several successive samples, and designed a run architecture to minimize the magnitude of the ME. The standard deviation obtained from routine measurement of a QA water over a 7 months‐period was 0.015‰ for δ17O, 0.023‰ for δ18O, 0.078‰ for δ2H, 0.006‰ for 17O‐excess and 0.173‰ for d‐excess. In addition, we provided the first δ17O and 17O‐excess values for the GRESP certified reference material. Conclusions This study demonstrates the long‐term persistence of the ME, which is often overlooked in routine analysis of natural samples. As already evidenced when measuring labelled water, it calls for consideration of the compositions of several previous samples to obtain an appropriate correction, a prerequisite to achieve high precision data.

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