Geothermal energy is a green source of power that could play an important role in climate-conscious energy portfolios; enhanced geothermal systems (EGS) have the potential to scale up exploitation of thermal resources. During hydraulic fracturing, fluids injected under high-pressure cause the rock mass to fail, stimulating fractures that improve fluid connectivity. However, this increase of pore fluid pressure can also reactivate pre-existing fault systems, potentially inducing earthquakes of significant size. Induced earthquakes are a significant concern for EGS operations. In some cases, ground shaking nuisance, building damages, or injuries have spurred the early termination of projects (e.g., Basel, Pohang). On the other hand, EGS operations at Soultz-sous-Forêts (France), Helsinki (Finland), Blue Mountain (Nevada, USA), and Utah FORGE (USA) have adequately managed induced earthquake risks. The success of an EGS operation depends on economical reservoir enhancements, while maintaining acceptable seismic risk levels. This requires state-of-the-art seismic risk management. This article reviews domains of seismology, earthquake engineering, risk management, and communication. We then synthesize “good practice” recommendations for evaluating, mitigating, and communicating the risk of induced seismicity. We advocate for a modular approach. Recommendations are provided for key technical aspects including (a) a seismic risk management framework, (b) seismic risk pre-screening, (c) comprehensive seismic hazard and risk evaluation, (d) traffic light protocol designs, (e) seismic monitoring implementation, and (f) step-by-step communication plans. Our recommendations adhere to regulatory best practices, to ensure their general applicability. Our guidelines provide a template for effective earthquake risk management and future research directions.

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Geothermal energy is a source of clean, renewable, and sustainable power that could play an important role in future energy portfolios. Enhanced Geothermal Systems (EGS) in particular have the potential to scale up accessibility to thermal resources. Seismic events of significant size associated with the development of EGSs display a negative socio-economic impact, posing a risk to the local infrastructure and the public, undermining the public acceptance of pending and future projects. To achieve a sustainable exploitation of geothermal resources using EGS, reservoir productivity needs to be enhanced while keeping seismic risk at an acceptable level during all stages of the project. In this document, we recommend good practices to evaluate, mitigate and communicate the risk of induced seismicity for EGS projects. This document is produced by the DEEP consortium, with input from more than 17 experts, with expertise in geophysics, seismology, earthquake engineering, risk management and communication. We advocate for a modular approach, providing recommendations on (1) seismic risk pre-screening, (2) data acquisition and research, (3), communication and outreach, (4) comprehensive seismic risk analysis, (5) seismic monitoring, (6) traffic light protocols, and (7) operational mitigation strategies. Our framework is designed to facilitate various stakeholders, such as regulators, operators, independent experts and affected communities.
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Deep geothermal energy (~3-6 km depth) is a candidate for sustainable and carbon-free energy supply. One of the main concerns of deep geothermal systems is induced seismicity that may produce earthquakes of economic concerns, challenging the development of this form of alternative energy. So far, cold water injection has been overlooked but may contribute to induced seismicity due to fault reactivation through thermal stresses also beyond the cooling region. This can be of importance, in particular, in fractured and faulted geothermal reservoirs. In this study, we first compare different approaches to estimate induced seismic risk from slip-tendency analysis, rate-and-state friction theory and modified Gutenberg-Richter statistics based on frictional Coulomb-stress perturbations. Then, we systematically investigate effects of both, intrinsic geological parameters (e.g., fault-, host rock properties and in-situ stress), and operational parameters (e.g., well geometry and placement, injection schemes, induced pressure perturbation) on induced seismicity. @en

The induced seismicity in the Groningen gas field, The Netherlands, presents contrasted spatio-temporal patterns between the central area and the south west area. Understanding the origin of this contrast requires a thorough assessment of two factors: (1) the stress development on the Groningen faults and (2) the frictional response of the faults to induced stresses. Both factors have large uncertainties that must be honoured and then reduced with the observational constraints. Ensembles of induced stress realizations are built by varying the Poisson's ratio in a poro-elastic model incorporating the 3-D complexities of the geometries of the Groningen gas reservoir and its faults, and the historical pore pressure distribution. The a priori uncertainties in the frictional response are mapped by varying the parameters of a seismicity model based on rate-and-state friction. The uncertainties of each component of this complex physics-based model are honoured through an efficient data assimilation algorithm. By assimilating the seismicity data with an Ensemble-Smoother, the prior uncertainties of each model parameter are effectively reduced, and the posterior seismicity rate predictions are consistent with the observations. Our integrated workflow allows us to disentangle the contributions of the main two factors controlling the induced seismicity at Groningen, induced stress development and fault frictional response. Posterior distributions of the model parameters of each modelling component are contrasted between the central and south west area at Groningen. We find that, even after honouring the spatial heterogeneity in stress development across the Groningen gas field, the spatial variability of the observed induced seismicity rate still requires spatial heterogeneity in the fault frictional response. This work is enabled by the unprecedented deployment of an Ensemble-Smoother combined with physics-based modelling over a complex case of reservoir induced seismicity.

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For induced seismicity, the non-stationary, heterogeneous character of subsurface stress perturbations can be a source of spatiotemporal variations in the scaling of event sizes; one of the critical parameters controlling seismic hazard and risk. We demonstrate and test a systematic, statistical, penalized-likelihood approach to analysing both spatial and temporal variations in event size distributions. The methodology used is transferable to the risk analysis of any subsurface operation, especially for small earthquake catalogues. We explore the whole solution space and circumvent conventional, arbitrary choices that require a priori knowledge of these variations. We assess the effect of possible bias in the derivation, e.g., due to tapering of the earthquake-size distribution, correlation between the b-value and the magnitude of completeness and correlation between the b-value and the largest magnitude observed. We analyse the spatiotemporal variations in the earthquake-size distribution of the Groningen induced seismicity catalogue (December 1991–November 16, 2021). We find statistically significant spatial variations without any compelling, statistical evidence of a temporal variation. Furthermore, we find that the largest magnitudes observed are inconsistent with the sampling statistics of an unconstrained earthquake-size distribution. Current risk assessment models likely overestimate the probability of larger magnitude events (M ≥ 3.0) and thus the risk posed.

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Prospects for geothermal energy in the Netherlands have renewed concerns around induced earthquakes. Risks from induced earthquakes are managed by traffic light protocols (TLPs), where the red-light is chosen as the stop-point before exceeding a tolerance to risk. Here, we simulate post-shut-in earthquake scenarios based on realistic information for the Netherlands. We focus on three risk metrics: aggregates like nuisance and damage impacts and also local personal risk (LPR) – a likelihood of building collapse fatality for an individual. Our results show that the severity of these risks varies spatially by orders of magnitude. Prior induced seismicity (e.g., the 2012 Huizinge event) provides a reference baseline to calibrate the Dutch earthquake risk tolerances. We find that these calibrated risk tolerances are similar to those observed in North America, suggesting an underlying sociological ‘license to operate.’ Furthermore, the use of calibrated risk tolerances results in nuisance concerns completely eclipsing the other two metrics. We compare our results to a hypothetical Groningen geothermal operation and find that our approach sets red-light thresholds approximately one magnitude unit below the M_L 3.6 Huizinge event. Overall, our results provide a first-order recommendation for red-light thresholds and proactive management of Dutch enhanced geothermal induced seismicity.

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We present an overview of induced seismicity due to subsurface engineering in the Netherlands. Our overview includes events induced by gas extraction, underground gas storage, geothermal heat extraction, salt solution mining and post-mining water ingress. Compared to natural seismicity, induced events are usually small (magnitudes ≤ 4.0). However, due to the soft topsoils in combination with shallow hypocentres, in the Netherlands events exceeding magnitude 1.5–2.0 may be felt by the public. These events can potentially damage houses and infrastructure, and undermine public acceptance. Felt events were induced by gas production in the north of the Netherlands and by post-mining water ingress in the south-east. Notorious examples are the earthquakes induced by gas production from the large Groningen gas field with magnitudes up to 3.6. Here, extensive non-structural damage incurred and public support was revoked. As a consequence, production will be terminated in 2022 leaving approximately 800 billion cubic metres of gas unexploited. The magnitudes of the events observed at underground gas storage, geothermal heat production and salt solution mining projects have so far been very limited (magnitudes ≤ 1.7). However, in the future larger events cannot be excluded. Project- or industry-specific risk governance protocols, extensive gathering of subsurface data and adequate seismic monitoring are therefore essential to allow sustainable use of the Dutch subsurface now and over the decades to come.

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Over 190 gas fields have been exploited in The Netherlands and only 15-20% have been associated with induced seismicity. We assess the geomechanical characteristics of stress changes on faults due to gas depletion for 180 producing gas fields in the Netherlands. We confirm findings from earlier generic studies that inter-reservoir offset faults require less reservoir depletion to reach failure compared to bounding and small offset faults. However, the stress changes on the offset faults alone are not sufficient to explain the observed seismicity. We find that the presence of the visco-elastic Zechstein formation probably has a crucial influence on the in-situ stress field in the Dutch subsurface and significantly impacts the fault stability of the gas reservoirs in the Netherlands. By accounting for this influence, our results show remarkable consistency with the observed (non)occurrence of induced seismicity in the Dutch gas fields. A more detailed study taking into account the detailed geological information of reservoir and fault geometry available at the operators and the slip weakening behavior of the frictional strength is required to further refine the predictive power of our analysis.

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The main objective of this paper is to give an overview of the risks seen in the exploration and production of geothermal energy from the viewpoint of the regulator. The risks are categorised as conventional risks, ultra-deep risks and enhancing factors. These risks are similar to those seen in the oil and gas industry, but the maturity of the geothermal sector in terms of managing such risks is much lower.

Another objective of this paper is to discuss how these risks are managed and mitigated by the sector and the supervisor, State Supervision of Mines (SodM). Portfolio operators developing multiple projects, using skilled employees and embracing continuous improvement are seen as the way forward for the sector to grow safely and sustainably.

This paper concludes that positive developments have started, but a lot of work still needs to be done to ensure safe growth of the geothermal energy sector.@en

The Groningen gas field in the north of the Netherlands is one of the largest gas fields in the world. Since the early 1990s induced seismicity has been recorded. The largest magnitude event observed so far was a Mw = 3.6 event at the town of Huizinge in 2012. The risk posed by the induced events urged the necessity to build comprehensive seismological models capable of explaining the spatial-temporal distribution of the recorded seismicity and evaluating the regional seismic hazard and risk. The link between the occurrence of the seismicity and pressure depletion due to the production of the gas has been firmly established. However, the construction of comprehensive seismological models as well as hazard assessment is complicated by the fact that it is difficult to distinguish between induced and clustered events (events triggered by stress transfer of preceding, neighbouring events). This paper explores the contribution of clustered populations (i.e. aftershocks) to the Groningen induced seismic catalogue based on a statistical methodology in the time-space-magnitude domain. Specifically, the distributions of space-time distances between pairs of nearest-neighbour earthquakes, referred to as cluster style, is analysed. The cluster style of the Groningen induced seismicity is found to be very diffuse and characterized by a very low proportion of fore-/aftershock sequences and swarms (∼5 per cent) and a large proportion of repeater events (∼10 per cent). In contrast to human-induced seismicity in other regions, the background seismicity rate of Groningen is very low. Temporal variations in background seismicity rates can be related to changes in fault loading rates induced by gas production. Furthermore, a significant amount of independent, coincidental events (events occurring very close in time, but long distances apart) are observed. As the large gas field is fully connected, loading of the faults occurs roughly simultaneously throughout the field. Hence, the statistical probability of events occurring very close in time, but spatially far apart is significantly larger than in areas of fluid-injection induced seismicity The significant amount of repeaters and coincidental events cause an overabundance of events at intermediate time- and space-distances. This is further enhanced by the larger location errors in the catalogue increasing the estimated space-distance for non-relocated events. The diffusivity due to this overabundance of events at intermediate time- and space-distances, and the low-proportion of true fore-/aftershocks renders the statistical method used incapable of deriving a proper mode-separation value. However, this is not unique to this method. Any statistical method aimed at resolving two populations will break down if one of the populations analysed is too small. Hence, it is advisable to use caution when distinguishing fore-/aftershocks sequences or swarms for induced seismicity where the relative proportion of clustered events may be significantly lower than for tectonic events. In addition, given the small proportion of clustering and the general uncertainty in earthquake statistics, the results of this paper indicate that a distinction for earthquake risk modelling in Groningen is unnecessary.

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We implement a Coulomb rate-and-state approach to explore the nonlinear relation between stressing rate and seismicity rate in the Groningen gas field. Coulomb stress rates are calculated, taking into account the 3-D structural complexity of the field and including the poroelastic effect of the differential compaction due to fault offsets. The spatiotemporal evolution of the Groningen seismicity must be attributed to a combination of both (i) spatial variability in the induced stressing rate history and (ii) spatial heterogeneities in the rate-and-state model parameters. Focusing on two subareas of the Groningen field where the observed event rates are very contrasted even though the modeled seismicity rates are of similar magnitudes, we show that the rate-and-state model parameters are spatially heterogeneous. For these two subareas, the very low background seismicity rate of the Groningen gas field can explain the long delay in the seismicity response relative to the onset of reservoir depletion. The characteristic periods of stress perturbations, due to gas production fluctuations, are much shorter than the inferred intrinsic time delay of the earthquake nucleation process. In this regime the modeled seismicity rate is in phase with the stress changes. However, since the start of production and for two subareas of our analysis, the Groningen fault system is unsteady and it is gradually becoming more sensitive to the stressing rate.@en

In different subsurface energy technologies, traffic light systems (TLSs) have been implemented for limiting the strength of induced seismicity. Despite their widespread application, fundamental assumptions regarding the controllability of induced seismicity were usually not reviewed. This is the focus of the current article, in which we discuss limitations of a TLS in the context of seismicity induced by fluid injection and gas production.

Most existing TLSs are based on a critical earthquake magnitude or vibration level that should be prevented to occur. Operational measures are defined to be taken after an induced earthquake exceeds predefined threshold values. This concept rests on the tacit assumptions that induced earthquakes of a critical strength announce themselves by precursory events of smaller strength and that future earthquakes of a critical strength can be prevented by modifying or stopping subsurface operations. We investigate to what extent these assumptions can be justified by studying observation data from a dozen fluid‐injection operations in geothermal reservoirs as well as from gas production in 26 gas fields in The Netherlands.

In our case studies, whereas fluid injection–induced seismicity generally starts at a low‐magnitude level and exhibits a gradual temporal increase of the maximum earthquake magnitude with the duration of the injection, the largest magnitude event frequently occurs postinjection. The temporal evolution of the seismicity induced by gas production in The Netherlands is less systematic. In some gas fields, seismicity started at a comparatively large‐magnitude level (⁠ML ≥ 2.7) without detectable precursors. A correlation between seismic activity and the gas production rate is only observed in the largest gas field.

Our findings indicate that the precision to what an earthquake of a given strength can be prevented by a TLS has more limitations than typically assumed.@en

In recent years public concern about earthquakes induced by gas production has increased in the Netherlands. This has mainly been caused by numerous seismic events related to gas depletion in the Groningen gas field, the largest gas field in Western Europe. Induced seismicity has also been observed in 31 smaller gas fields located on land (onshore) or in the area close to the Dutch coast. Earthquakes with magnitudes as high as ML = 3.5 have occurred in Roswinkel and Bergermeer causing damage to buildings.

In 2016 State Supervision of Mines (SSM), with input from the geological survey of the Netherlands (TNO) and the onshore operators, proposed a guideline for a qualitative seismic risk analysis for depletion induced seismicity arising from gas production in the small fields in the Netherlands. The guideline follows international practices for risk assessment using a risk matrix approach. This paper elaborates the seismic risk guideline and reports on the application of the guideline to the gas fields in the Netherlands.

Risk is a combination of hazard and consequences. The result of the seismic risk analysis is qualitative and gives a relative scoring of the producing gas fields in the Netherlands in terms of risk. In order to obtain more information on the quantitative assessment of the risk, more detailed studies are needed. The Groningen gas field clearly poses a much larger seismic risk than that obtained for the other, smaller gas fields, most of which fall into the lowest risk category. Because of the large difference in risk between the Groningen field and the other smaller gas fields, the guideline of SodM deems it sufficient to carry out a qualitative risk analysis for the other gas fields in the Netherlands, as performed in this paper. Based on the combination of the hazards and consequences, the risk can be further interpreted and, if necessary, appropriate measures can be implemented.@en

Prediction of gas-production-induced subsidence and seismicity is much more difficult and uncertain than generally recognised in the past. It is now widely accepted that uncertainties in predicted subsidence and seismicity are large prior to and during the initial stages of production. At later stages, predictions remain highly uncertain for periods more than three to five years into the future. This requires a different regulatory framework to ensure that associated risks remain within accepted boundaries. Previously, single-scenario operator predictions were checked against field measurements. When subsidence or seismicity started to deviate beyond claimed uncertainties, the operator was asked to provide prediction updates. The practice was long considered acceptable, as structural damage to buildings and infrastructure or personal risk to people was not expected. This all changed following the 2012 Huizinge seismic event, necessitating better identification, assessment and ranking of risks, the use of scenarios, probabilistic forecasting and a much intensified field monitoring and control loop. It requires that the regulator becomes actively involved in assuring the integrated control loop of risk identification, predictions, monitoring, updating, mitigation measures and the closing of knowledge gaps, to ensure that subsidence (rate) and induced seismicity remain within acceptable limits. And it requires that this increased involvement of the regulator is supported in the mining law and by appropriate conditions in the Production Plan assent.@en

Shaking and damage in the province of Groningen, the Netherlands, resulting from production-induced seismicity has caused increased public anxiety. Since 2014, production offtake has been reduced stepwise by over 50% in an attempt to minimise production-induced seismicity. The earthquake catalogue, combined with comprehensive data of the changes in production offtake, shows a clear response of seismic activity following the production measures taken. Associated temporal variations in the proportionality between smaller- and larger-magnitude events (the b-value of the Gutenberg–Richter relation) are observed. Since production measures were imposed, the b-value has tended to increase, thus lowering the probability of a larger-magnitude event. The analysis also shows increases in activity rate and b-value prior to larger-magnitude events. Subsequently, the probability of a larger-magnitude event seems to be decreasing prior to the events occurring. This implies that for short-term earthquake prediction of hydrocarbon-production-induced seismicity, these types of analysis could be misleading. However, regional analysis is necessary to explain the observations in terms of rupture initiation. At present, each event felt still draws the interest of both public and press. As some clustering of events in both time and space is still observed, managing both the seismicity and the public perception provides a continuing challenge.@en

Halite solution mining and natural gas production in the west of the province of Fryslân in the Netherlands cause subsidence significantly exceeding original predictions. Subsidence at the centre of the salt subsidence bowl reached 33 cm in 2015 while only 7 cm had been predicted in the 1995 forecast. Maximum subsidence above the nearby Harlingen gas field was around 33 cm in 2015 compared to 10 cm in the 2003 forecast for the end of field life. In addition, subsidence accelerated over time. From the regulator's point of view, the development of the discrepancies was unacceptable and led to an early reassessment of the salt mining project and to suspension of the gas production by the operator in 2008. Extensive research into the causes of the exceedances was able to provide a proper explanation for both occurrences. For the salt solution mining, cavern convergence rates at the in-situ stresses and temperatures at the depth of the project are much higher than initially estimated. This leads to a different production mechanism (squeeze mining), which gives much more subsidence. After convergence and subsidence modelling were corrected for this in 1998, predicted and observed subsidence are in much better agreement. The additional and accelerating subsidence above the Harlingen gas field can only be explained by pore-collapse of the gas-bearing reservoir chalk. During the laboratory compaction tests used for the original subsidence predictions, pore collapse only occurred at stress levels beyond those possible in-situ. The explanation for the inconsistency was found to be the strong impact of loading rate on the pore-collapse stress level. The applicability of an existing rate type compaction model for the Harlingen chalk was confirmed by new variable loading rate laboratory experiments. Using the model to translate the laboratory measured pore-collapse stresses into values applicable under field conditions, a much better agreement with the observed field behaviour was obtained. The Harlingen case thus provides further evidence that the large influence of loading rate on the chalk pore collapse stress seen during laboratory experiments applies down to field loading rates and that corrections for this need to be applied in compaction and subsidence forecasts.@en

The Wadden Sea is a shallow tidal sea in the north of the Netherlands where gas production is ongoing since 1986. Due to the sensitive nature of this area, gas extraction induced subsidence must remain within the "effective subsidence capacity" for the two tidal basins (Pinkegat and Zoutkamperlaag) affected. We present a probabilistic method to monitor the "effective subsidence capacity" and ensure that subsidence is below the long term (18.6 years) volumetric rate for relative sea level rise that can be accommodated by the tidal basins without environmental harm. The role of sedimentation volume rate, relative sea level rise and subsidence volume rate due to gas depletion are taken into account including their uncertainties. The probability of exceeding the acceptable subsidence limit for the period 2012 to 2050 is 2.8% for the tidal basin called Zoutkamperlaag and 1% for the tidal basin of Pinkegat for climate scenarios that fit the current relative sea level rise observations on the Dutch coast. The values are shown to be dominated by the effect of relative sea level rise, and not due to subsidence induced by gas depletion in the Wadden Sea. To current knowledge no harm is done to nature.@en

Reliable prediction of the induced subsidence resulting from gas production is important for a near sea level country like the Netherlands. Without the protection of dunes, dikes and pumping, large parts of the country would be flooded. The predicted sea-level rise from global warming increases the challenge to design proper mitigation measures. Water management problems from gas production induced subsidence can be prevented if measures to counter its adverse effects are taken timely. This requires reliable subsidence predictions, which is a major challenge. Since the 1960's a number of large, multi-decade gas production projects were started in the Netherlands. Extensive, well-documented subsidence prediction and monitoring technologies were applied. Nevertheless predicted subsidence at the end of the Groningen field production period (for the centre of the bowl) went from 100 cm in 1971 to 77 cm in 1973 and then to 30 cm in 1977. In 1984 the prediction went up again to 65 cm, down to 36 cm in 1990 and then via 38 cm (1995) and 42 cm (2005) to 47 cm in 2010 and 49 cm in 2013. Such changes can have large implications for the planning of water management measures. Until 1991, when the first event was registered, production induced seismicity was not observed nor expected for the Groningen field. Thereafter the number of observed events rose from 5 to 10 per year during the 1990's to well over a hundred in 2013. The anticipated maximum likely magnitude rose from an initial value of less than 3.0 to a value of 3.3 in 1993 and then to 3.9 in 2006. The strongest tremor to date occurred near the village of Huizinge in August 2012. It had a magnitude of 3.6, caused significant damage and triggered the regulator into an independent investigation. Late 2012 it became clear that significantly larger magnitudes cannot be excluded and that values up to magnitude 5.0 cannot be ruled out. As a consequence the regulator advised early 2013 to lower Groningen gas production by as much and as fast as realistically possible. Before taking such a decision, the Minister of Economic Affairs requested further studies. The results became available early 2014 and led to the government decision to lower gas production in the earthquake prone central area of the field by 80 % for the next three~years. In addition further investigations and a program to strengthen houses and infrastructure were started. Important lessons have been learned from the studies carried out to date. It is now realised that uncertainties in predicted subsidence and seismicity are much larger than previously recognised. Compaction, subsidence and seismicity are strongly interlinked and relate in a non-linear way to production and pressure drop. The latest studies by the operator suggest that seismic hazard in Groningen is largely determined by tremors with magnitudes between 4.5 and 5.0 even at an annual probability of occurrence of less than 1 %. And that subsidence in 2080 in the centre of the bowl could be anywhere between 50 and 70 cm. Initial evaluations by the regulator indicate similar numbers and suggest that the present seismic risk is comparable to Dutch flooding risks. Different models and parameters can be used to describe the subsidence and seismicity observed so far. The choice of compaction and seismicity models and their parameters has a large impact on the calculated future subsidence (rates), seismic activity and on the predicted response to changes in gas production. In addition there are considerable uncertainties in the ground motions resulting from an earthquake of a given magnitude and in the expected response of buildings and infrastructure. As a result uncertainties in subsidence and seismicity become very large for periods more than three to five years into the future. To counter this a control loop based on interactive modelling, measurements and repeated calibration will be used. Over the coming years, the effect of the production reduction in the centre of the field on subsidence and seismicity will be studied in detail in an effort to improve understanding and thereby reduce prediction uncertainties. First indications are that the reduction has led to a drop in subsidence rate and seismicity within a period of a few months. This suggests that the system can be controlled and regulated. If this is the case, the integrated loop of predicting, monitoring and updating in combination with mitigation measures can be applied to keep subsidence (rate) and induced seismicity within limits. To be able to do so, the operator has extended the field-monitoring network. It now includes PS-InSAR and GPS stations for semi-permanent subsidence monitoring in addition to a traditional network of levelling benchmarks. For the seismic monitoring 60 shallow (200 m) borehole seismometers, 60 + accelerometers and two permanent downhole seismic arrays at reservoir level will be added. Scenario's spanning the range of parameter and model uncertainties will be generated to calculate possible subsidence and seismicity outcomes. The probability of each scenario will be updated over time through confrontation with the measurements as they become available. At regular intervals the subsidence prediction and the seismic risk will be re-evaluated. Further mitigation measures, possibly including further production measures will need to be taken if probabilities indicate unacceptable risks.@en

In the Netherlands, seismicity is induced by the reactivation of faults because of the extraction of gas. The Dutch mining law requires a seismic-risk assessment as part of the license application process. For this purpose, a risk-assessment guideline has been developed over the past decade. The guideline contains three assessment levels. At the first level, a screening occurs to assess the potential of inducing seismicity. On the basis of three key parameters and an analysis of the maximum potential magnitude, each field can be classified for induced-seismicity risk prior to the onset of production. For fields with a low seismicity potential, the existing national monitoring network suffices. At the second level, for fields with medium and high seismicity potential, a qualitative assessment of hazard and risk is required based on a risk-matrix approach. The large Groningen gas field is considered a field with high seismic risk requiring a level 3 assessment. For such fields, a probabilistic seismic-risk assessment and risk-management plan are required. The risk assessment requires special attention because most of the seismic risk is associated with low-probability events that can induce large ground accelerations.@en