[1] During an Enhanced Geothermal System (EGS) experiment, fluid is injected at high pressure into crystalline rock, to enhance its permeability and thus create a reservoir from which geothermal heat can be extracted. The fracturing of the basement caused by these high pore-pressures is associated with microseismicity. However, the relationship between the magnitudes of these induced seismic events and the applied fluid injection rates, and thus pore-pressure, is unknown. Here we show how pore-pressure can be linked to the seismic frequency-magnitude distribution, described by its slope, the b-value. We evaluate the dataset of an EGS in Basel, Switzerland and compare the observed event-size distribution with the outcome of a minimalistic model of porepressure evolution that relates event-sizes to the differential stress s D . We observe that the decrease of b-values with increasing distance of the injection point is likely caused by a decrease in pore-pressure. This leads to an increase of the probability of a large magnitude event with distance and time.

SUMMARY Geothermal energy is becoming an important clean energy source, however, the stimulation of a reservoir for an Enhanced Geothermal System (EGS) is associated with seismic risk due to induced seismicity. Seismicity occurring due to the water injection at depth have to be well recorded and monitored. To mitigate the seismic risk of a damaging event, an appropriate alarm system needs to be in place for each individual experiment. In recent experiments, the so‐called traffic‐light alarm system, based on public response, local magnitude and peak ground velocity, was used. We aim to improve the pre‐defined alarm system by introducing a probability‐based approach; we retrospectively model the ongoing seismicity in real time with multiple statistical forecast models and then translate the forecast to seismic hazard in terms of probabilities of exceeding a ground motion intensity level. One class of models accounts for the water injection rate, the main parameter that can be controlled by the operators during an experiment. By translating the models into time‐varying probabilities of exceeding various intensity levels, we provide tools which are well understood by the decision makers and can be used to determine thresholds non‐exceedance during a reservoir stimulation; this, however, remains an entrepreneurial or political decision of the responsible project coordinators. We introduce forecast models based on the data set of an EGS experiment in the city of Basel. Between 2006 December 2 and 8, approximately 11 500 m3 of water was injected into a 5‐km‐deep well at high pressures. A six‐sensor borehole array, was installed by the company Geothermal Explorers Limited (GEL) at depths between 300 and 2700 m around the well to monitor the induced seismicity. The network recorded approximately 11 200 events during the injection phase, more than 3500 of which were located. With the traffic‐light system, actions where implemented after an ML 2.7 event, the water injection was reduced and then stopped after another ML 2.5 event. A few hours later, an earthquake with ML 3.4, felt within the city, occurred, which led to bleed‐off of the well. A risk study was later issued with the outcome that the experiment could not be resumed. We analyse the statistical features of the sequence and show that the sequence is well modelled with the Omori–Utsu law following the termination of water injection. Based on this model, the sequence will last 31+29/−14 years to reach the background level. We introduce statistical models based on Reasenberg and Jones and Epidemic Type Aftershock Sequence (ETAS) models, commonly used to model aftershock sequences. We compare and test different model setups to simulate the sequences, varying the number of fixed and free parameters. For one class of the ETAS models, we account for the flow rate at the injection borehole. We test the models against the observed data with standard likelihood tests and find the ETAS model accounting for the on flow rate to perform best. Such a model may in future serve as...

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an unusual series of 47 earthquakes occurred over a 12-hr period in central Switzerland. The earthquakes occurred at the end of 3-d period of intensive rainfall, with over 300 mm of precipitation. The highest seismicity occurred as two distinct clusters in the region of Muotatal and Riemenstalden, Switzerland, a well-known Karst area that received a particularly large amount of rainfall. The large increase in seismicity, compared to the background, and the short time delay between the onset of the intense rainfall and the seismicity strongly suggest that earthquakes were triggered by rainfall. In our preferred model, an increase in fluid pressure at the surface due to a large amount of rain leads to a local increase in pore fluid pressure at depth. The increase in pore fluid pressure will reduce the shear strength of a porous medium by counteracting normal stress and, at the end, provoke failure. The series of triggered earthquakes in central Switzerland occurred in regions that have been seismically active in the past, showing similar hypocentre locations and magnitudes. This suggests that these earthquakes occurred on existing faults that were critically stressed. We modelled the intense rainfall as a step increase in fluid pressure at the surface that migrates to greater depths following the solution of the one-dimensional diffusion equation in a homogeneous half space. This allowed us to estimate the hydraulic diffusivity by plotting triggered seismicity in a time-depth plot. We found values of hydraulic diffusivity in the range from 0.01 to 0.5 m 2 s −1 for our study area. These values are in good agreement with previous studies on earthquakes that were triggered by fluids, supporting the idea that the observed earthquake series was triggered by the large amount of rainfall.

With the global challenge to satisfy an increasing demand for energy while at the same time stabilizing or reducing carbon dioxide (CO2) concentrations in the atmosphere, geothermal energy from enhanced geothermal systems (EGSs) increasingly is being recognized as an attractive alternative energy source throughout the world. However, the risks associated with the seismicity necessarily induced during the development of an EGS constitute a significant challenge for the widespread implementation of this technology. This article provides a preliminary overview of lessons learned from an attempt to develop an EGS beneath the city of Basel, Switzerland.