Abstract. 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 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 impac...

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.

Subsidence caused by extraction of hydrocarbons and solution salt mining is a sensitive issue in the Netherlands. An extensive legal, technical and organisational framework is in place to ensure a high probability that such subsidence will stay within predefined limits. The key question is: how much subsidence is acceptable and at which rate? And: how can it be reliably assured that (future) subsidence will stay within these limits?To address the issue for the Wadden Sea area, the concept of ‘effective subsidence capacity’ is used. To determine the ‘effective subsidence capacity’, the maximum volumetric rate of relative sea-level rise, that can be accommodated in the long term, without environmental harm, is established first. The volume of sediment that can be transported and deposited by nature into the tidal basin where the subsidence is expected, ultimately determines this ‘limit of acceptable average subsidence rate’. The capability of the tidal basins to ‘capture’ sediment over the lunar cycle period of 18.6 years is the overall rate-determining step. Effective subsidence capacity is then the maximum average subsidence rate available for planning of human activities. It is obtained by subtracting the subsidence volume rate ‘consumed’ by natural relative subsidence in the area (sealevel rise plus natural shallow compaction) from the total long-term acceptable subsidence volume rate limit.In the operational procedure for mining companies, six-years-average expectation values of subsidence rates are used to calculate the maximum allowable production rates. This is done under the provision that production will be reduced or halted if the expected or actual subsidence rate (natural + man induced) is likely to exceed the limit of acceptable subsidence. Monitoring and management schemes ensure that predicted (6-year average) and actual (18.6-year average) subsidence rates stay within the limit of acceptable subsidence rate and that no damage is caused to the protected nature. A GPS based early warning system is used for early detection of unexpected behaviour. In support of SSM (State Supervision of Mines, the government regulator), TNO-AGE (an independent government advisory group) applies an independent Bayesian statistical analysis of all data, as they become available, to calculate the probability of scenario's under which future subsidence will exceed the defined limits. It is external to the operator's annual measurement and control loop and ensures that preventive actions can be taken in time in case such scenarios emerge.Regular communication keeps the authorities and the general public informed on the use of the effective subsidence capacity to demonstrate that the actual average subsidence rate stays strictly within the defined bounds and that, from a scientific point of view, there is no reasonable doubt that damage to the tidal system will not occur now or in the future.

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.

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.