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Induced seismicity with unexpectedly large magnitude often occurs after shut‐in or end of stimulation, generating concerns at the end of stimulation. We investigated the physical mechanism of large‐magnitude induced seismicity during shut‐in following the hydraulic stimulation at Basel, Switzerland. Larger postinjection events occurred at the periphery of the seismic cloud. We estimated the pore pressure required to cause shear slip using Coulomb failure criteria from stress information, geometry of the fault planes of microseismic events, and a constant coefficient of friction. Time series analysis of pore pressure distribution indicated that pore pressure migrated to the far field even after shut‐in. Redistribution of pore pressure at shut‐in brought sufficient pore pressure increase to induce seismicity in the peripheral region. After shut‐in, the pore pressure gradient away from the well lessened and eventually pressure became uniform. These observations suggest that the higher pore pressure, which remained in the vicinity of the injection point, shifted to the farthest field. Shut‐in pressure migration caused uniform pore pressure distribution at the edge of the seismic zone. Shut‐in pressure destabilized a large part of the fault located at the edge of the seismic cloud, resulting in the shear slip of a large section of the fault. Meanwhile, during stimulation, only some parts of the fault entered the critical state because of the pressure gradient. The resulting shear slip on that specific part causes moderate magnitude events at most.

A B S T R A C TThe stimulation of a geothermal well in Basel, Switzerland produced a distribution of microseismic event locations with an overall alignment in the direction of the maximum horizontal stress. Fault plane solutions of individual larger events indicated movements on fracture planes at an angle to the maximum horizontal stress that could not be reliably interpreted from the event locations. To obtain higher resolution images of the microseismic event locations, events with similar waveforms have been identified by multiplet analysis. A number of receivers were used in the multiplet processing to ensure each multiplet is represented by a unique group of waveforms. The location accuracy within each multiplet has been significantly improved using cross-correlation to refine the shear-wave traveltime picks. The distribution of events within each multiplet can be interpreted as being due to movements on a single fracture or a number of near parallel fractures. It is shown that whilst the overall distribution of events is around the direction of the maximum horizontal stress, the individual multiplets representing fracture planes have a variety of azimuths and dips.