3.1 Injection protocol

A standardized injection protocol was used for the six HS experiments, to compare the influence of the targeted geological structures on the seismo-hydromechanical response. Roughly 1 m³ of fluid volume was injected per experiment (actual volumes are given in Table 1). The injection protocol consisted of four injection cycles (referred to as C1–C4, Fig. 4a), in which either the injection pressure or injection flow rate was increased in a stepwise manner after steady state was reached. All the cycles were followed by a shut-in phase, where pumping was stopped, and a venting phase, in which the pressure in the injection- and all monitoring-intervals were bled off. The first two pressure-controlled cycles C1 and C2 were conducted to determine pre-stimulation jacking pressure (i.e., the injection pressure at which the ratio between the injection pressure and flow rate deviates from linearity) and initial injectivity of the target structure. C3 was the actual flow-controlled stimulation cycle, in which the bulk part of the fluid was injected. C4 was initially pressure controlled, changing to flow-controlled injection, aimed at determining the post-stimulation jacking pressure and injectivity of the targeted structure. During all HS experiments, the flow rate did not exceed 38 L min⁻¹.

HF experiments also followed a standardized injection protocol involving a target injected volume of ∼1 m³ (actual injected volumes in Table 1). The injection protocol for the five HF experiments started with a flow-controlled formation breakdown cycle (indicated by the letter F) to initiate the hydraulic fracture. This initial cycle and all the subsequent cycles included a shut-in phase were pumping was stopped. During some of the formation breakdown cycles the shut-in phase was complemented by a bleed-off phase of the injection interval and all pressure monitoring intervals. The two subsequent cycles were aimed at propagating the previously initiated hydraulic fracture (RF1, RF2). For these two propagation cycles, water was used during HF1, HF2, HF3, and for HF5, HF6, and HF8, shear-thinning fluid (xanthan–salt–water mixture, XSW) was used. We note that the XSW mixture exhibited a viscosity of ∼35 mPa s (viscosity of water =1 mPa s). Propagation cycles RF1 were performed with maximum flow rates of 35 L min⁻¹. During experiments performed with water, the flow rate was controlled in a sinusoidal fashion (period: 2.5–20 s; amplitude: ±15 L min⁻¹) for roughly 10 minutes. For experiments in which XSW was injected, an additional cycle, RF3, was added to inject water so that XSW is flushed from the fracture network. All the HF experiments were finalized by a pressure-controlled step-rate injection test (SR) for evaluating post-stimulation jacking pressures and injectivities of the created hydraulic fracture.

3.2 Seismic monitoring and data processing

3.2.1 Seismic monitoring

A total of 26 in situ acoustic emission sensors (AE sensors) formed the passive seismic network around and inside the test volume (Fig. 3a, green cones). The sensors were manufactured by the Gesellschaft für Materialprüfung und Geophysik GmbH (GMuG) and have a bandwidth of 1 to 100 kHz, with their highest sensitivity at 70 kHz. 14 AE sensors were installed on a tunnel level (R2–R15; type: Ma-Bls-7-70), in 55 mm diameter boreholes drilled approximately 250 mm deep into the tunnel wall. The bottom view AE sensors were pressed against the polished surface of the base of the borehole. The core of the network (i.e., sensors within 5–25 m distance to the injection intervals) was composed of eight borehole AE sensors (R16–R23; type: MA-BLw-7-70-68) distributed in four water-filled monitoring boreholes (GEO1–GEO4, Fig. 3a). The borehole AE sensors have a curved front surface and were deployed in sensor shuttles in which two pneumatic cylinders (line pressure 10 bar) ensured contact pressure between the sensors and the borehole wall. Four additional sensors (R24–R27; type: MA-BLw-7-70-86) with curved front surfaces were installed in borehole SBH4 (Fig. 2b). For calibration purposes, five one-component (1C) accelerometers (R28–R32; type: Wilcoxon 736T) were installed next to five of the tunnel level AE sensors (Fig. 3a, red cones, R4, R6, R7, R9, R11). The accelerometers were factory calibrated and feature a flat frequency response from 50 to 25 000 Hz, with a sensitivity of 100 mV g⁻¹. They were mounted to brass disks (∅ 28 mm×1 mm), which were glued to the front surface of the 55 mm diameter and 100 mm deep boreholes drilled into the tunnel wall adjacent to the AE sensors (Fig. 3c). The seismic signals were recorded continuously on a 32-channel acquisition system at a 200 kHz sampling rate (GMuG, digitizer cards: Spectrum M2i.47xx). AE sensor channels had 1 kHz and accelerometer channels had 50 Hz high-pass analogue filters installed.

In addition to the passive seismic network, active seismic sources were installed; eight falling hammer sources were distributed in the AU and the VE tunnels. Two borehole piezoelectric sources were installed in borehole GEO2 and GEO4 (Fig. 3a, black arrows). The trigger signal of the seismic sources, used to determine the initiation time of each active seismic survey, was recorded on one channel of the acquisition system. The active sources were used for time-lapse 3D seismic tomography surveys during the experiments (see Schopper et al., 2020; Doetsch et al., 2018b, for details). For more information on the seismic monitoring system, see Doetsch et al. (2018b).

Figure 3(a) The seismic network consisting of 26 uncalibrated AE sensors (green cones) installed in four boreholes, the AU and VE tunnels, and the AU gallery, along with five one-component calibrated accelerometers (red cones) collocated with five AE sensors in the AU and VE tunnels. Seismic sources in the tunnels and boreholes are shown by black arrows. (b) AE tunnel sensor, insulated against acoustic noise. (c) Installed AE sensor next to a calibrated accelerometer along with their pre-amplifiers. (d) AE sensors in a sensor shuttle for deployment into water-filled boreholes. Waveforms from a small- and large-magnitude event induced during experiment HS4, including the Euclidean distance hypocenter−sensor, P-wave pick (red-stripe), and a window of a hypothetical S-wave arrival for an S-wave velocity of 2500–3000 m s⁻¹ (applied bandpass filter for small event: 1–12 kHz; large event: 1–50 kHz).

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4.1 Temporal seismic event evolution

For the HS injection experiments, most events (12 211 from a total 13 826 detections) were detected during pumping phases (Fig. 4a, b, for a selection of HS experiments). The percentage of events recorded during shut-in were in the range of 10 %. Less than 2 % of events were detected during venting. Comparing the HS experiments, significantly fewer events were detected during experiments at the S1 shear zones compared to the stimulation experiments performed at the S3 shear zones (Table 1). An exception was HS8 at the S1 shear zone south of S3, which produced a number of events comparable to the S3 injections (i.e., total detections: 3703). This may be explained by the fact that the injected fluid entered the S3 shear zone, which was evident from the seismicity cloud migrating towards the S3 shear zone (see Sect. 4.2). The number of seismic events (normalized to the total number of events per experiment) is plotted against injected volume in Fig. 5a and b. Again, a distinct behavioral difference between S1 and S3 injections is observed. During experiments in S1, the largest seismic detection rate was observed during stimulation cycle 1; more than 50 % of all events were induced with less than 100 L of fluid (< 10 % of the total volume). On the contrary, for S3 stimulations, most events were detected during cycle 3, when the largest volume of fluid was injected. Again, experiment HS8 is an exception in that the highest detection rate was observed during cycle 1 (similar to S1 stimulations), after which the event rates behave similarly to the S3 injections (HS4, HS5). Generally, a larger fraction of seismic events occurred after shut-in during injection into the S1 shear zones compared to injections into the S3 shear zones.

Over all HF injections, about half of the detections were made compared to the HS injections (see Fig. 4c, d, for a selection of HF experiments). Most of the events were detected during the pumping phases (4483 of 6731 detections). Interestingly, a comparably high percentage of detections (33 %) were made during shut-in, and no events were detected in the venting phases. We argue that the high percentage of post-shut-in detections were related to a hydraulic connection created between the injection interval and the open seismic monitoring boreholes (termed GEO) during the last two experiments HF5 and HF8. This hydraulic connection allowed observable flow from the GEO boreholes into the tunnel. We assume that this flow triggered stick-slip movements of the AE sensors. Thus, ongoing flow through GEO boreholes after shut-in would explain why many post-shut-in events were detected. Also, most of these events were only detected at the two sensors in the GEO borehole which was hydraulically connected. Note that HF6 – by mistake placed across the S1.3 shear zone close to the injection interval of HS1 – can be seen as a continuation to the HS1 experiment.

In summary, for the HS experiments, 31 % (i.e., 4342) of detected events could be located. The fraction of located shut-in events during the HS experiments is around 3 %, the fraction of events induced during the venting phase is less than 1 %. For the HF experiments, because of the large number of events without seismic origin (possibly sensor stick-slip), only 12 % (i.e., 781) of all detected events could be located. Six percent of the events were located after shut-in, and no events were located during the venting phases. The located seismic events fulfill a location uncertainty below ±1.5 m (for more information on location uncertainty see Sect. 3.2.3).

The maximum induced magnitudes M_max during both HS and HF experiments (see inset of Fig. 1 and yellow stars in Figs. 4 and 5) occurred during pumping with no evidence of a temporal trend. Events during a time interval between shut-in and the start to a new injection cycle were usually of lower magnitude. One exception was the injection experiment HF6; here the highest-magnitude event was induced during a shut-in phase (see Sect. S3).

Figure 4(a) Temporal seismicity evolution of experiment HS1 performed in shear zone S1.3 and (b) of experiment HS4 in shear zone S3.1, along with (c) the temporal seismicity evolution of experiment HF2, which was performed north of the S3 shear zones, and (d) the temporal seismicity evolution of experiment HF8, performed south of the S3 shear zones. In addition to the injection rate and pressure, the cumulative number of events and magnitudes M_A are shown. The largest-magnitude event is indicated with a yellow star. The shaded area on the plots indicate the pumping periods during an experiment (the temporal seismicity evolution of the remaining experiments is shown in Sect. S3, of this paper). Panel (a) also shows an example for a HS injection protocol with injection pressure and injection flow rate, divided into the four cycles, including shut-in and venting phases in each cycle. Panel (c) shows an example of a HF injection protocol with injection pressure and injection flow rate, including formation break down cycle (F), refrac cycles (RFs), and the final step pressure (SP) injection experiment. All of the cycles include a shut-in phase, but only after some cycles a venting phase is included. The yellow stars indicate the largest events induced in a respective experiment.

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Figure 5Cumulative fraction of detected events as a function of cumulative injected volume of (a) HS and (b) HF injection experiments. Panels (c) and (d) show absolute values of located events above a magnitude level of M_A −4.02 (maximum M_C in the experimental volume; see Sect. 4.3) for HS and HF experiments, respectively. Note that for experiment HS3 only one event was observed above the maximum M_C in the experimental volume and is thus not shown in Fig. 5c.

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4.2 Spatial properties of seismicity clouds

4.2.1 Spatial distribution

The seismicity clouds produced by the HS experiments (Fig. 6a, c) form planes with a tendency to align in the E–W direction (main direction of S3 shear zones) or in a NE–SW direction (main direction of S1 shear zones). Often these planes exhibit substructures with events grouped into clusters, which is most pronounced for experiment HS4 (see also Figs. 7 and 8). Note that we use the term “cluster” here for a distinct subgroup of seismic events within the seismicity cloud of individual experiments. These are not clusters derived from waveform similarity and relative relocation, which is the scope of future studies. The seismicity induced by the injection experiments in injection borehole INJ1 predominately propagated in an easterly direction, whereas the seismicity cloud of HS1, the only HS injection in INJ2, was oriented in a NE–SW direction (Fig. 6a). For this experiment the seismicity occurred exclusively a few meters above the injection interval (Fig. 6c). For HS8, the injection experiment closest to the top of injection borehole INJ1, there was a tendency for downward propagation. Generally, seismicity is well contained within narrow clouds surrounding the injection interval. However, interactions (i.e., hydraulic or mechanical) were evident in experiments HS4 and HS8, where part of the HS8 seismicity cloud aligns with the HS4 seismicity cloud.

The seismicity clouds of the HF injection experiments also had a tendency to propagate in the E–W direction, similar to the HS experiments. Experiments conducted in INJ1 (i.e., HF2, 3, 5) induced seismicity clouds that propagated towards the east from the injection interval, whereas injection into INJ2 (i.e., experiment HF8) induced a seismicity cloud propagating towards the west (Fig. 6b). HF6, the HF experiment misplaced at the S1.3 shear zone, induced only a few seismic events superimposed on the seismicity cloud of experiment HS1 that targeted at the same structure. Seismicity clouds that occurred during the HF experiments propagated preferentially downwards. Injection experiment HF3 stands out in that it induced a dispersed seismicity cloud, with seismic events located at sites where previous experiments (i.e., experiments HS8 and HS4) had already induced seismicity, possibly indicating interaction with the HS8 and HS4 stimulated zones. Thus, no main cloud with a distinct orientation could be identified for experiment HF3.

Figure 6(a) Overview of HS event locations in top view including interpolated shear zones and (c) side view towards east and (b) overview of HF event locations in top view including interpolated shear zones and (d) side view towards east. Injection intervals and seismic events of respective experiments are color coded. The maximum magnitude of each stimulation experiment is indicated with a yellow star. The gray events in (b) and (d) show the seismic events induced during the HS experiments, which were performed prior to the HF experiments. Note also that in order to improve visibility, the diameter of the injection intervals is exaggerated.

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Planes fitted through the seismic event clouds by orthogonal distance regression are shown in Fig. 7 as half-circles and their poles in lower-hemisphere stereographic projections. The standard deviation of orthogonal distances of the seismic event locations to the fitted planes is below ±1 m, except for experiment HS1 (standard deviation ±1.4 m). The poor plane-fit quality for HS1 events may be associated with increased location uncertainty at the bottom of injection borehole INJ2 (see Sect. 3.2.3).

For injections HS1, HS2, HS3, HS5, HF5, and HF8, single seismic clusters were observed and fitting a single plane proved to be sufficient. Three seismic clusters were observed in injection HS4, and in injection experiment HS8 and HF2 two seismic clusters were observed. Planes were fitted to each of these clusters (Fig. 7). No plane was fitted to experiment HF3 due to the dispersed character of its seismicity cloud. For experiment HF6, there were too few located seismic events (details of the fitted planes can be found in the Sect. S4).

Figure 7Orientation of fitted planes and corresponding pole points through seismic clouds in lower-hemisphere stereographic plots, including main orientations of shear zones S1 and S3 observed in the tunnels. Plots are as follows: (a) for HS1, 2, 3, and HS5 for which a single-planar orientation of stimulation was identified; (b) for the three visually identified seismic clusters of injection HS4; (c) for the two clusters of injection experiment HS8; (d) for HF5 and HF8, for which a single and planar orientation of stimulation was identified; and (e) for the two visually identified seismic clusters of injection HF2.

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Also included in Fig. 7 are the main orientations of the S1 and S3 shear zones observed in the surrounding tunnels (Krietsch et al., 2018a). Interestingly, the seismicity clouds of experiments HS2 and HS3, both targeting S1 structures, have an orientation similar to HS5 and to the main orientation of the S3 shear zones. Only the seismicity cloud of the S1 stimulation HS1 is oriented similarly to the main orientation of S1 shear zones, although its dip is slightly steeper. The HS4 seismicity produced three distinct cluster orientations: Cluster 1 formed from the injection interval and propagates sub-vertically in the ENE direction; Cluster 2 formed higher up in the injection interval and was oriented E–W, parallel to the shear zone S3.1; and Cluster 3 is a new fracture that formed during the main stimulation cycle (C3). The fracture formed at a location that was deemed to be fracture-free during geological characterization prior to the stimulation experiments. In addition, the formation of the new fracture was observed as a strong and abrupt opening by a 1 m long strain-monitoring sensor installed in a borehole (i.e., FBS2; see also Fig. 2) parallel to the S3.1 shear zone (Fig. 8d). For more information about the strain monitoring system see Doetsch et al. (2018a) and Krietsch et al. (2020). The strong tensile signal from the strain-monitoring interval at the 24 m borehole depth and the contraction of the adjacent strain-monitoring intervals began when there was a step rate increase in fluid flow. The opening lasted for about 10 min and was accompanied by the HS4 seismicity Cluster 3. Peak extensional strain occurred at shut-in. Contraction of the fracture during the shut-in phase is also associated with seismicity, after both cycles 3 and 4.

Figure 8Observation of newly formed fracture during injection experiment HS4. (a) The spatial distribution of seismicity clusters observed during period I, color coded according to cluster affiliation, along with injection borehole INJ1 and the strain-monitoring intervals at 22, 24, and 26 m in the strain-monitoring borehole FBS1. (b) The temporal evolution of seismicity including injection parameters. (c) Spatial distribution of all seismicity of the three main clusters. (d) The strain evolution of strain-monitoring intervals at the specified depths.

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The two clusters of experiment HS8 indicate an initial stimulation of shear zone S1.0 in the ENE direction, hydraulically connecting the injection interval with injection borehole INJ2. The second seismicity cluster indicates stimulation along lower regions of shear zone S3.1 in the E–W direction, possibly because the zone stimulated during experiment HS4 was reactivated during HS8.

The seismicity cloud from experiment HF8 is oriented E–W, again comparable to the orientation of S3, while the seismicity cloud of HF5 deviates from this orientation. Experiment HF2 contains two main seismicity clusters: Cluster 1 includes the events propagating from the injection interval and is oriented comparable to the orientation of HF5. With ongoing stimulation, Cluster 2 is formed and orients itself in the E-W direction.

4.3 Frequency–magnitude distributions

The Gutenberg–Richter a and b values are estimated for partial catalogs of respective injection experiments defined by the magnitude of completeness, M_C. The latter was determined per experiment using the goodness of fit method introduced by Wiemer and Wyss (2000). a and b values and their uncertainty are calculated using the modified maximum likelihood technique published by Marzocchi and Sandri (2009). a values are normalized by the injected volume to derive the so-called seismogenic index, Σ (Dinske and Shapiro, 2013). Figure 10 shows frequency–magnitude distributions (FMDs) of all injection experiments. M_C and a and b values were estimated for injection experiments exhibiting more than 20 seismic events and a goodness of fit quality of more than 90 %. Exceptions were made for injection experiment HS3 and Cluster 3 of experiment HS4, where the goodness of fit quality lies above 85 %. M_C is lowest for injections in the focal point of the seismic network (HS4: −4.90; HS5: −4.80; HF2: −4.78). For injection experiment HS4, a bimodal frequency–magnitude distribution was observed. For a and b value calculations, the higher M_C of −4.32 was used. M_C increases for injections performed outside the network focus (HS3: −4.66; HS2: −4.39; HS8: −4.38) and is highest for the injection experiments performed towards the bottom of the second injection borehole (INJ2, HS1: −4.05) and towards the tops of the two injection boreholes (HF3: −4.14; HF8: −4.02;see Fig. 9). Thus, for these experiments the range between the maximum induced magnitude and M_C is small. Moreover, when investigating spatial and statistical properties of seismicity clouds one has to be aware of the spatially varying network sensitivity. Our eight borehole AE sensors close to the injection intervals are decisive for an increased network sensitivity in the experimental volume close to the injection boreholes. In addition to the source–receiver distance, the sensitivity of the network is significantly influenced by the directivity of the AE sensor; i.e., events with incident angles > 50^∘ in the Grimsel experiment are less likely to be detected.

Figure 9Top view comparison of (a) all located seismic events with (b) seismic events exhibiting magnitudes above the maximum encountered M_C in the experimental volume (i.e., M_C−4.02) along with M_C estimates of experiments, injection boreholes, injection intervals, and borehole AE sensors.

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The HS injection experiments (Fig. 10a) targeting S1 shear zones exhibited larger b values (HS1: 1.93±0.39; HS2: 1.69±0.26; HS3: 1.93±0.37) and lower seismogenic indices (HS1: −6.6; HS2: −5.8; HS3: −7.6) compared to the b values of injections into S3 shear zones (HS4: 1.36±0.04; HS5: 1.03±0.05) with higher seismogenic indices (HS4: −3.0; HS5: −2.4). Again, HS8 – an injection into the S1 shear zone south of S3 with migration of seismicity into the S3 shear zone – forms an intermediate case between injections into S1 and S3 with a b value of 1.61±0.12 and a seismogenic index of −4.9. The b value for the bimodal FMD of injection HS4 in a magnitude range of −4.9 to −4.35 lies below 1 as compared to 1.36 above magnitude −4.35.

The b value for the HF2 experiment (Fig. 10b) north of the S3 shear zones is comparatively low at 1.35±0.08, with a seismogenic index of −4.0. Experiments HF3 and HF8 south of the S3 shear zones at a similar depth of injection borehole INJ1 and INJ2, respectively, exhibited b values of 1.55±0.26 and 2.66±0.36. Seismogenic indices for the two injection experiments were −4.8 for HF3 and −9.0 for injection HF8.

A more detailed analysis of the bimodal FMD of HS4 reveals that the bimodal character does not disappear if the FMD is split up into all four injection cycles (Fig. 10c). Also for FMDs of individual seismicity clusters (see Sect. 4.2), the seismicity cluster closest to the metabasic dikes (Cluster 1) confirms the bimodal characteristic (Fig. 10d). The cloud subparallel to the metabasic dike (Cluster 2) shows a bimodal character but with a break in scaling at higher magnitudes compared to the FMD of Cluster 1. The new fracture induced and propagated during injection cycle 3 (Cluster 3) does not show the bimodal characteristic but reveals five events with higher magnitudes than would be expected.

Figure 10Frequency–magnitude distributions for the (a) HS and the (b) HF injection experiments along with estimated b values and seismogenic indices. Injection experiments in legends are ordered in a chronological manner, whereby HS injection experiments were performed in February 2017, and HF injection experiments were executed in May 2017. Frequency–magnitude distributions for injection experiment HS4, resolved in (c) injection cycles (Cycle 1–Cycle 4) and (d) clusters, introduced in Sect. 4.2. Uncertainties in b values are estimated after Marzocchi and Sandri (2009).

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4.4 Maximum observed magnitude vs. stimulated area and a and b values

The maximum observed magnitudes per injection experiments ranged over 1.5 magnitudes. The observed maximum magnitudes showed only a slight tendency to increase as the injected fluid volumes increased (Fig. 1), possibly owing to the fact that the injected volumes were only marginally different (900–1500 L). However, a stronger relationship was seen between maximum observed seismic magnitudes and the seismically activated area (Fig. 12a; for more information on the seismically activated area we refer to Sect. S5). Injection experiment HS5 represents the highest-magnitude event as well as the largest seismically activated area (285 m²). Also during injection experiment HS4 in which several planes were seismically activated resulting in a large seismically activated area, a rather large-magnitude seismic event was induced. There were no obvious differences in the maximum induced magnitude in relation to injected volume or seismically activated area between the HS and HF injection experiments.

Gutenberg–Richter b values and seismogenic indices show a high variability but no correlation with the seismically activated area (Fig. 11). Nonetheless, injection experiment HS5, during which the largest area was activated and the largest-magnitude event was induced, also shows the lowest b value and the highest seismic productivity. A comparatively small area was activated during injection experiment HF2 with similar low b values and high seismogenic indices.

Figure 11(a) b values along with uncertainties plotted against seismically activated area and (b) seismogenic indices plotted against seismically activated area from experiments for which the b values, seismogenic indices, and areas could be estimated.

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4.5 Seismic injection efficiency and ratio of seismic/aseismic deformation

In the following, we estimate the seismic moment release (referred to as M₀ seismic) and compare it with a quantity termed hydraulic moment release (M₀ hydraulic) as well as with the total moment release (M₀ total) by stimulation experiment (Fig. 12).

The lower-bound estimate of M₀ seismic during each injection experiment was determined by adding up the seismic moment of each located seismic event during the respective injection experiment. In order to estimate the experimental specific upper bound of the seismic moment release, the Gutenberg–Richter a and b values, determined in Sect. 4.3, were used to extrapolate the seismicity rates down to a magnitude of −9. Such small magnitudes were observed on the laboratory scale by Selvadurai (2019). Also McLaskey and Lockner (2014) and Yoshimitsu et al. (2014) observed very small magnitudes (i.e., M−7) and self-similarity down to these magnitudes. In situ, Goodfellow and Young (2014) observed magnitudes down to −7.5. For an average estimate of the seismic moment release, magnitudes down to a minimum magnitude of −6 were included (symbols in Fig. 12b). A high range of possible seismic moment release was observed for injection experiments with high b values (i.e., HS1, HS3, HF8), because the small-magnitude seismic events strongly contribute to the cumulative seismic moment release. Assuming the average estimate scenario and cumulating the moment release of all possible seismic events per injection experiment into a single earthquake would have induced a moment magnitude M_w in the range of −3 to −1. Assuming a stress drop of 1 or 0.1 MPa, respectively, and a source model by Brune (1970), this would correspond to a source radius of 0.3–2.2 m/0.6–4.8 m and a ruptured area of 0.26–15.5 m²/0.28–18 m²).

The equivalent hydraulic moment (M₀ hydraulic) was calculated from the determined hydraulic injection energy. The hydraulic injection energy was estimated using $E_{\mathrm{hyd}}=\int pQ\mathrm{d}t$, where p is the injection pressure, and Q is the injection flow rate, which are both integrated over the entire injection time. The pumped hydraulic energy is then converted to an equivalent seismic moment using $M_{\mathrm{0}}=\frac{\mathit{\mu }}{\mathrm{\Delta }\mathit{\sigma }}{E}_{\mathrm{hyd}}$ (Aki and Richards, 2002; De Barros et al., 2019), where μ is the shear modulus, chosen to be 30 GPa, and Δσ represents the static stress drop assumed to be between 1 and 0.1 MPa. The average estimate represents the equivalent seismic moment averaging the aforementioned stress drop range (Fig. 12c).

The total moment (M₀ total) released by stimulation can be estimated from borehole dislocations in the injection interval that was determined from acoustic televiewer (ATV) measurements before and after each injection experiment (i.e., for injection experiment HS2: 0.95 mm; for HS4: 0.95 mm; for HS3: 1.25 mm; for HS8: 0.45 mm; and for HS1: 0.75 mm; see Krietsch et al., 2020). Note that this is only possible for HS experiments, since in the HF experiments no fault dislocations were observed (Dutler et al., 2019). For the estimate of the seismic moment from the measured displacements at the injection interval, we used M₀=μAD, where μ is the shear modulus, again chosen to be 30 GPa, A is the seismically activated area determined in Sect. 4.2, and D is the average slip on the area of rupture. For a lower-bound estimate, we assume that an average slip over the entire lower-bound seismically activated area (i.e., the concave hull area; see Sect. 4.2) is 10 % of the observed slip at the injection interval. For the upper-bound estimate, we assume that the average slip across the entire upper bound seismically activated area (i.e., the convex hull area estimate) corresponds to 50 % of the observed slip at injection intervals. Twenty-five percent of the observed slip and 50 % of the estimated seismically activated area were used for the average estimate of total moment release (symbols in Fig. 12d).

To estimate seismic injection efficiencies (i.e., the ratio between seismic moment released to equivalent hydraulic moment, Fig. 12e) and the ratio between seismic and total deformation (Fig. 12f), the average estimates of the equivalent hydraulic and total moment were used. The cumulative seismic moment release was varied according to the minimum magnitude at which seismicity rates were extrapolated. When integrating to a minimum magnitude of −6, seismic injection efficiencies lie in the range of $\mathrm{1.9}\times {\mathrm{10}}^{-\mathrm{6}}$ (HS3) and $\mathrm{5}\times {\mathrm{10}}^{-\mathrm{4}}$ (HF8); injection experiment HS4 showed a high value of $\mathrm{1}\times {\mathrm{10}}^{-}\mathrm{4}$ with minor changes as the integration magnitude decreased, due to the low b value (i.e., due to the small contribution of small-magnitude events to the cumulative seismic moment). Seismic injection efficiencies (excluding experiment HF8) tended to converge to a value in the range of $\mathrm{1.6}\times {\mathrm{10}}^{-\mathrm{5}}$ (HF2) and $\mathrm{3.2}\times {\mathrm{10}}^{-\mathrm{4}}$ (HS1) when integrating to a minimum magnitude of −9.

The ratio between seismic and total moment release (Fig. 12f), considering events with magnitudes down to −6, ranged from $\mathrm{5.8}\times {\mathrm{10}}^{-\mathrm{5}}$ (HS3) to $\mathrm{3.6}\times {\mathrm{10}}^{-\mathrm{3}}$ (HS4). Integrating the seismic moment to a minimum magnitude of −9 leads to a convergence of the ratio between seismic and total deformation to values of $\mathrm{1.3}\times {\mathrm{10}}^{-\mathrm{3}}$ (HS3) to $\mathrm{1.8}\times {\mathrm{10}}^{-\mathrm{2}}$ (HS1).

We emphasize that the cumulative seismic moment, the equivalent hydraulic moment, and the equivalent total moment from dislocation observations are prone to a high level of uncertainty. Thus, uncertainties in the seismic injection efficiencies and the ratio between seismic and total moment give only crude estimates with uncertainties that possibly exceed 1 order of magnitude.

Figure 12(a) Maximum observed magnitudes (error bars represent the standard deviation of all magnitude estimates of the respective event) with respect to seismically activated area (the estimated seismically activated area represents the mean between the upper and lower bound of the area estimate of Sect. 4.2). (b) Estimated radiated seismic moment from extrapolated Gutenberg–Richter parameter (upper-bound and average estimate) and located seismic events (lower bound) along with the equivalent moment magnitude. (c) Equivalent hydraulic moment estimated from injection parameter (i.e., flow rate, injection pressure). (d) Equivalent moment estimate from acoustic televiewer displacement measurements at the injection interval. (e) Seismic injection efficiency against the magnitude level used for seismic moment extrapolation. (f) Ratio between seismic moment and equivalent seismic moment estimated from displacement measurements against the magnitude level used for seismic moment extrapolation.

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5.1 A highly variable seismic response and the role of geology

Remarkable is the large variability in the seismic responses between experiments conducted within less than a 25 m borehole length, which is expressed in the wide range of seismogenic indices (−9 to −2) and b values (1 to 2.7) (Fig. 11). The number of detected and located events during a stimulation depends on the detection ability of the sensor network, which is primarily a function of the distance (Mignan et al., 2011). However, even at a homogeneous completeness level of −4.02, the seismic response varies widely (Fig. 5c, d). Such variability is comparable to the variability between cases worldwide, involving both projects with predominant HF stimulation in the shale gas context and HS for geothermal exploitation (Fig. 13c, Dinske and Shapiro, 2013; Mignan et al., 2017). While Dinske and Shapiro (2013) suggest that there is a large difference in the seismic response during HF-dominated stimulations in shale gas projects and HS-dominated stimulations in geothermal applications, a systematic difference between the HS and HF experiments performed in crystalline rock was not discernible here. Also, the use of the shear-thinning xanthan–salt–water mixture during the HF experiments did not have an observable effect on the seismic response. The fact that the HF experiments were conducted in a rock mass where previous HS experiments could potentially have initiated some stress relaxation may explain the tendency for fewer events during the HF experiments. Experiment HF6, which can be interpreted as continuation of the HS1 experiment, induced only a few seismic events because the zone was stimulated twice. In contrast, the dispersed character of seismic events in the HF3 experiment may be explained by the interaction of new fractures with the surrounding faults S1.0, S3.1, and S3.2. We conclude that in our experiments HS and HF are similarly seismogenic, because HF strongly interacts with the pre-existing fracture network leading to similar seismic responses as the injections directly into pre-existing fractures.

While differences in the seismic response between HF and HS were not evident, the geological setting seems important for the substantial differences seen in the seismic response in terms of magnitude distributions as well as in terms of orientation and propagation of seismicity. We observed that experiments performed directly on or in the vicinity of the highly fractured brittle–ductile S3 shear zones (Fig. 13a, i.e., experiments HS5, HS4, and HS8 and HF3, respectively) are characterized by an enhanced seismic response. This observation is in agreement with the hypothesis gained from larger-scale stimulations, which states that well-developed brittle fault zones (i.e., connecting fractures that form larger features) lead to a comparatively high seismic moment release in response to high-pressure fluid injection (McClure and Horne, 2014b; De Barros et al., 2016). An exception is experiment HF2, which shows an increased seismic response with possibly no influence from S3 structures. Injection experiment HF2 was performed between the ductile shear zones S1.1 and S1.2, north of shear zone S3. At this location the reactivated structure (i.e., Cluster 1 and Cluster 2 of HF2; see also Fig. 7) may support an increased amount of shear stress, which led to an increased seismic response.

Not only do the seismic responses (i.e., b value and the seismogenic indices) indicate a strong geological influence but also the seismicity detection rate in relation to injected volume (Fig. 5) shows a different seismic footprint for the two shear zone types. For the injection experiments on the ductile shear zones (S1) more than 50 % of all detections are made during the injection of the initial 100 L of fluid. In contrast, the S3 shear zones experienced a gradual increase in detections with injected fluid volume (Fig. 5a).

The spatial distribution and propagation also appear to be affected by the geology. A concentric growth of seismicity clouds was rarely observed, indicating that the spatial fracture zone heterogeneity had a substantial impact. Seismicity clouds of experiments on ductile shear zones S1 show changing propagation directions and a planar character. Comparing the two S3 stimulations (HS4 and HS5), distinct differences in seismicity patterns were observed, even for stimulations within 3 m from each other in similar geological structures. During HS5, propagation directions changed along an extended seismicity cloud (of 16 m diameter) with a clustered character and regions of increased seismic event density. During the HS4 experiment the seismicity was mostly limited to patches/clusters within a 9 m radius from the injection interval but with a complex 3D and nonplanar architecture (Figs. 6, 8).

Beside their tendency of being very seismogenic, the highly fractured S3 shear zones stand out as being the most hydraulically conductive structures in the experimental volume compared to the less conductive S1 shear zones (see injectivities of HS4 and HS5 intervals in Fig. 13b). Injectivities at these intervals only increased marginally during stimulation. On the contrary, injectivities for the S1 stimulation experiments on the ductile shear zones and in the intact intervals increased by 2–3 orders of magnitude. Again, these observations agree with cases in the literature, for which the most permeable fractures were also found to be the most critically stressed and thus the most seismogenic zones (e.g., Barton et al., 1988; Barton et al., 1995; Barton and Zoback, 1998; Evans et al., 2005a; Davatzes and Hickman, 2010; Baisch et al., 2015; Hirschberg et al., 2015). It is also noteworthy that the injectivities for all experiments performed at the brittle–ductile shear zones, the ductile shear zones, and in the intact intervals end up having the same order of magnitude (Fig. 13b). While initial injectivities are highly dependent on the local geology, final injectivities are very similar (as are transmissivities; Brixel et al., 2020).

Figure 13(a) Seismic responses (seismogenic indices, b values) along with (b) pre- and post-injectivity values of experiments along the depth of the injection boreholes. Location of S3 shear zones and experiment HS4 and HS5 therein are highlighted. Injectivity values of experiment HF5 and HF6 for which the number of located seismic events renders a determination of b value and seismogenic index impossible, are also included. (b) b values and seismogenic indices of various high-pressure fluid injections at different sites (source seismogenic indices and b values from other locations: Dinske and Shapiro, 2013; Shapiro et al., 2013; and Mignan et al., 2017).

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With the aforementioned observations in mind, it is possible to imagine what would have happened if a large open-hole stimulation would have been conducted in INJ1 and INJ2, as it was done in most of the previous EGS projects (e.g., Basel, Häring et al., 2008; Soultz, Evans et al., 2005b), instead of several stimulations at selected short intervals. Because of their high transmissivity, flow would have preferentially entered the shear zones S3.1 and S3.2 leading to induced seismicity, mostly dominated by the seismogenic properties of these structures. The result would have been a very limited transmissivity increase together with a strong seismic response. Thus, for larger-scale EGS stimulations, it appears quite promising to selectively stimulate multiple short borehole intervals with comparatively small fluid volumes (i.e., zonal isolation, Meier et al., 2015), during which the transmissivity of low-transmissive structures would be strongly enhanced, while stimulations in intervals at seismogenic fault zones should be avoided if possible. Of course, hydraulic stimulation of short intervals could also be combined with alternative injection schemes (such as described by Zang et al., 2017). However, the pronounced influence of geology on the aforementioned stimulation parameters in our experiments may imply that the impact of alternative injection strategies on induced seismicity (such as those discussed and proposed in the literature by McClure and Horne, 2011; Zimmermann et al., 2014; McClure et al., 2016; and Zang et al., 2018) is limited, since their effects are unlikely to emerge above the strong variability of orders of magnitudes imposed by the geological conditions.

5.2 Impact of the stress field

Compared to the observed main orientation of the S1 (NE–SW) and the S3 (E–W) shear zones in the tunnels surrounding the experimental volume, the orientation of individual fractures within the S1 and S3 fault zones do show a similar WSW–ENE orientation. Also, the orientation of fractures found in the host rock are predominantly WSW–ENE in orientation with some random joint orientations. We combine the orientation of these pre-existing fractures with the slip tendencies inferred from the stress conditions measured 30 m south of shear zone S3.1 (i.e., the unperturbed stress state) and the stress conditions measured in borehole SBH4 (Fig. 2) in the vicinity of shear zone S3.1 (i.e., the perturbed stress state). It can be seen that there is an increased susceptibility for the S1 and S3 structures to slip (see also Krietsch et al., 2018a) when considering the perturbed stress state (Fig. 14a, b).

By including both the inferred orientation of the seismicity clouds or their clusters resulting from the injection experiments performed on the shear zones (i.e., the HS experiments) and the stress field, the combined influence of geology and stress field becomes evident. The predominant orientation of seismicity clouds is E–W, in agreement with the orientation of pre-existing fractures. Surprisingly, the predominant orientation also holds for the S1 stimulation experiments, even though the main orientation of the S1 shear-zones is NE–SW. Only the seismicity cloud of injection experiment HS1 is oriented in the main S1 direction. However, the orientation of seismicity in the E–W direction, also for S1 experiments, is not surprising when considering the fracture inventory of the experimental volume and the overlapping pole points of S1 and S3 structures, as well as the increased fracture density with the same orientation (Fig. 14a–d).

Hydraulic fractures in a strict sense, meaning fractures which form in intact rock, perpendicular to the minimum principal stress, at injection pressures higher than the minimum principal stress, are conceivable for the initiated fractures in experiment HF5 and the initial fracture (Cluster 1) of experiment HF2, oriented perpendicular to the minimum principal stress of the perturbed stress state where directional geological features are sparse. Cluster 2 of experiment HF2 formed at a later time compared to Cluster 1; it possibly formed because of the leak-off of fluids through Cluster 1 to the formation. The associated reduction in pore pressure through Cluster 1 may suggest a geology-dominated E–W orientation of the seismicity cloud of Cluster 2.

The new fracture created during experiment HS4 (Cluster 3) orients in a direction perpendicular to the minimum principal stress of the perturbed stress field (Villiger et al., 2019). We suggest that this fracture opens in connection to shear dislocation along shear zone S3.1 (Jung, 2013) induced during the HS4 injection.

In conclusion, the perturbed stress field – measured closer to the target rock volume than the unperturbed stress field – explains most of the observed seismicity cloud orientations well. HFs growing through intact rock tend to form normal to the minimum principal stress, while the other seismicity clouds are mostly guided by the predominant set of geological features that have comparably high slip tendency. However, it is likely that local stress variations may locally lead to a combination of opening mode deformation (i.e., mode I opening) and shear dislocation (mode II, mode III). Also, HFs show a strong tendency to connect with the pre-existing fracture network, which might explain why the seismic response during HF experiments is similar to the one during HS experiments.

Figure 14Principal stress directions of the unperturbed (σ₁=13.1 MPa, 104^∘ dip direction/39^∘ dip; σ₂=9.2 MPa, $\mathrm{259}{}^{\circ }/\mathrm{48}{}^{\circ }$; and σ₃=8.7 MPa, $\mathrm{4}{}^{\circ }/\mathrm{13}{}^{\circ }$) and perturbed stress state (σ₁=13.1 MPa, 134^∘ dip direction/14^∘ dip; σ₂=8.2 MPa, $\mathrm{026}{}^{\circ }/\mathrm{50}{}^{\circ }$; and σ₃=6.5 MPa, $\mathrm{235}{}^{\circ }/\mathrm{36}{}^{\circ }$) along with slip tendencies determined from the respective stress state in lower-hemisphere stereographic plots, along with (a, b) the fracture inventory from borehole observations, (c, d) pole points of seismicity cloud orientations of HS experiments, their targeted structures and (e, f) orientation of seismicity clouds of HF stimulation experiments.

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5.3 Aseismic deformation

Our experiments indicate that deformation in HS experiments (for which a displacement was measured at the injection interval) is to a large extent aseismic (i.e., < 2 % seismic). We also observed the tendency for the amount of aseismic deformation to be larger for experiments targeting the S1 structures (Fig. 12f). These overall values agree with values determined from hydraulic reactivation of a fault zone in limestone on a decameter scale, where 0.1 % to 3.9 % of shear deformation was estimated to be seismic (Duboeuf et al., 2017). Similar studies in shale materials report that less than 0.1 % of deformation is seismic (De Barros et al., 2016). An increased value of 4 % to 8 % for released seismic energy was reported for hydraulic fracturing experiments on granite samples at the laboratory scale (Goodfellow et al., 2015). Also, at the field scale, a large amount of aseismic deformation is suspected due to the observed slip dislocation of up to 4 cm on an acoustic televiewer log of an injection interval in granite at Soultz-sous-Forêts, which is much larger than the slip motion associated with the recorded seismic events (Cornet et al., 1997).