Estimating the maximum possible earthquake magnitude using extreme value methodology: the Groningen case

The area-characteristic, maximum possible earthquake magnitude $T_M$ is required by the earthquake engineering community, disaster management agencies and the insurance industry. The Gutenberg-Richter law predicts that earthquake magnitudes $M$ follow a truncated exponential distribution. In the geophysical literature several estimation procedures were proposed, see for instance Kijko and Singh (Acta Geophys., 2011) and the references therein. Estimation of $T_M$ is of course an extreme value problem to which the classical methods for endpoint estimation could be applied. We argue that recent methods on truncated tails at high levels (Beirlant et al., Extremes, 2016; Electron. J. Stat., 2017) constitute a more appropriate setting for this estimation problem. We present upper confidence bounds to quantify uncertainty of the point estimates. We also compare methods from the extreme value and geophysical literature through simulations. Finally, the different methods are applied to the magnitude data for the earthquakes induced by gas extraction in the Groningen province of the Netherlands

Rise and reduction of induced earthquakes in the Groningen gas field, 1991-2018:statistical trends, social impacts, and policy change

Over 320 induced earthquakes with magnitude M1.5, including 38 with M2.5 in the Groningen gas field, the Netherlands, are statistically analysed, and their societal impacts and recent policy changes reviewed. Increased seismicity indicates that the 900km(2) large, 3km deep and 100m thick sandstone reservoir has become increasingly vulnerable to further extraction, especially after 2001 and 60% depletion of the total 2800billion cubic meters (bcm). Regardless of stepwise reductions in annual extraction: from 54 bcm in 2013 to 20 bcm in 2018, well-fitting trends over 1991-2018 reveal a steady growth of seismic activity per unit of gas extraction. This would imply that, before full resource depletion, some 500 more earthquakes with M1.5 might occur, including 50 with M2.5, 6 with M3.5, and 1 with M4.5. Meanwhile, thousands of residents have been suffering from advanced building damage, diminishing property values, disturbing home reinforcement, and various stress-related health complaints. This has spurred a cascade of judgements, decisions and actions by responsible authorities during 2013-2018, topped by the Dutch cabinet's March 2018 decision to reduce Groningen gas extraction to below 12 bcm in 2022 and to end all field operations by 2030. This would reduce the remaining number of risky earthquakes with M2.5 to some seven or eight, with one expected M-max approximate to 4.0. Until 2022, however, seismic hazard and risk would only decrease under average' winter conditions. By December 2018, there has been considerable uncertainty about the actual course of decreasing extraction. Meanwhile, a controversial building-reinforcement programme is being greatly reduced

Earthquake hazard and risk analysis for natural and induced seismicity: towards objective assessments in the face of uncertainty.

The fundamental objective of earthquake engineering is to protect lives and livelihoods through the reduction of seismic risk. Directly or indirectly, this generally requires quantification of the risk, for which quantification of the seismic hazard is required as a basic input. Over the last several decades, the practice of seismic hazard analysis has evolved enormously, firstly with the introduction of a rational framework for handling the apparent randomness in earthquake processes, which also enabled risk assessments to consider both the severity and likelihood of earthquake effects. The next major evolutionary step was the identification of epistemic uncertainties related to incomplete knowledge, and the formulation of frameworks for both their quantification and their incorporation into hazard assessments. Despite these advances in the practice of seismic hazard analysis, it is not uncommon for the acceptance of seismic hazard estimates to be hindered by invalid comparisons, resistance to new information that challenges prevailing views, and attachment to previous estimates of the hazard. The challenge of achieving impartial acceptance of seismic hazard and risk estimates becomes even more acute in the case of earthquakes attributed to human activities. A more rational evaluation of seismic hazard and risk due to induced earthquakes may be facilitated by adopting, with appropriate adaptations, the advances in risk quantification and risk mitigation developed for natural seismicity. While such practices may provide an impartial starting point for decision making regarding risk mitigation measures, the most promising avenue to achieve broad societal acceptance of the risks associated with induced earthquakes is through effective regulation, which needs to be transparent, independent, and informed by risk considerations based on both sound seismological science and reliable earthquake engineering

Geomechanical modelling of subsidence and induced seismicity in a gas reservoir

Reservoir compaction and associated surface subsidence, fault reactivation and induced earthquakes are observed in many petroleum fields worldwide. A better understanding of the geomechanical behaviour of reservoir rocks and neighbouring rock bodies is therefore becoming increasingly important within the petroleum industry. Several monitoring techniques for these phenomena exist, but methods of modelling reservoir geomechanical behaviour are hindered by clear limitations. This study discusses different suspected mechanisms of induced seismicity related to oil and gas production and their significance in varying reservoir environments. In support of this discussion, relevant background theory is presented together with a case study of induced seismicity in the Groningen Gas Field in the northern Netherlands. The aim of this thesis is to use a Modified Discrete Element Method proposed by (H. T. Alassi, 2008) to model the geomechanical behaviour in a depleting gas reservoir. Multiple scenarios have been modelled to investigate the significance of the suspected underlying mechanisms of seismicity and subsidence observed in the Groningen Field. It was found that depletion of a reservoir has the potential to induce rock failure on faults inside and in contact with the depleted zone as well as causing significant surface subsidence. It is also emphasized that improvements of the method and further research is needed to fully understand the significance of the underlying mechanisms

Improving the assessment of seismic hazard in the North Sea

The following PhD thesis provides a comprehensive reassessment of probabilistic seismic hazard assessment (PSHA) in the North Sea. PSHA provides probabilistic representations of the expected ground-shaking at sites of interest, which can be used to assess the seismic risk for structures located at (or proximal to) said sites. In the North Sea, the seismic risk for offshore infrastructure including (1) oil and gas platforms and (2) wind turbine facilities must be considered. The seismic risk of this offshore infrastructure is important to consider because certain levels of seismic damage can result in negative impacts upon (1) the environmental health of the North Sea, (2) the personal health of employees on or near the considered infrastructure and (3) the economic health of governments and corporations which are reliant upon this infrastructure. The most recent publicly available North Sea PSHA was undertaken by Bungum et al. (2000). Two decades have passed since this study, since which substantial developments in PSHA have been made, and additional North Sea ground-motion data has been collected. Furthermore, the 2001 Ekofisk earthquake was the first hydrocarbon production induced earthquake in the North Sea to have been deemed of engineering significance for platforms in the region, but was not considered within the Bungum et al. (2000) study. In this investigation, North Sea PSHA is reassessed in several ways. Firstly, a pre-existing ground-motion prediction equation (GMPE) which performs well in the North Sea is identified as a base model for a North Sea GMPE using an additional 20 years of ground motion records available since the Bungum et al. (2000) study. This base model GMPE is then improved incrementally through the constrainment of North Sea path and site effects using novel techniques. Following the development of this North Sea GMPE, the seismogenic source model of Bungum et al. (2000) is updated using an additional two decades of North Sea earthquake observations. The impact of the North Sea GMPE and the updated source model are evaluated using (1) macroseismic earthquake observations and (2) assessment of the seismic risk of offshore infrastructure in the region. The updated PSHA formulation developed within this investigation results in moderate but significant differences in the seismic risk for offshore infrastructure in the North Sea. These seismic risk estimates are potentially more appropriate than those computed using the Bungum et al. (2000) PSHA formulation due to the additional ground-motion data and the PSHA advancements available since the Bungum et al. (2000) PSHA study. Ultimately, the improved seismic hazard estimates potentially help to better assess the structural health of offshore North Sea infrastructure, and subsequently minimise the likelihood of levels of seismic damage which could be detrimental to the North Sea environment or the personnel and/or economies operating within the region.The following PhD thesis provides a comprehensive reassessment of probabilistic seismic hazard assessment (PSHA) in the North Sea. PSHA provides probabilistic representations of the expected ground-shaking at sites of interest, which can be used to assess the seismic risk for structures located at (or proximal to) said sites. In the North Sea, the seismic risk for offshore infrastructure including (1) oil and gas platforms and (2) wind turbine facilities must be considered. The seismic risk of this offshore infrastructure is important to consider because certain levels of seismic damage can result in negative impacts upon (1) the environmental health of the North Sea, (2) the personal health of employees on or near the considered infrastructure and (3) the economic health of governments and corporations which are reliant upon this infrastructure. The most recent publicly available North Sea PSHA was undertaken by Bungum et al. (2000). Two decades have passed since this study, since which substantial developments in PSHA have been made, and additional North Sea ground-motion data has been collected. Furthermore, the 2001 Ekofisk earthquake was the first hydrocarbon production induced earthquake in the North Sea to have been deemed of engineering significance for platforms in the region, but was not considered within the Bungum et al. (2000) study. In this investigation, North Sea PSHA is reassessed in several ways. Firstly, a pre-existing ground-motion prediction equation (GMPE) which performs well in the North Sea is identified as a base model for a North Sea GMPE using an additional 20 years of ground motion records available since the Bungum et al. (2000) study. This base model GMPE is then improved incrementally through the constrainment of North Sea path and site effects using novel techniques. Following the development of this North Sea GMPE, the seismogenic source model of Bungum et al. (2000) is updated using an additional two decades of North Sea earthquake observations. The impact of the North Sea GMPE and the updated source model are evaluated using (1) macroseismic earthquake observations and (2) assessment of the seismic risk of offshore infrastructure in the region. The updated PSHA formulation developed within this investigation results in moderate but significant differences in the seismic risk for offshore infrastructure in the North Sea. These seismic risk estimates are potentially more appropriate than those computed using the Bungum et al. (2000) PSHA formulation due to the additional ground-motion data and the PSHA advancements available since the Bungum et al. (2000) PSHA study. Ultimately, the improved seismic hazard estimates potentially help to better assess the structural health of offshore North Sea infrastructure, and subsequently minimise the likelihood of levels of seismic damage which could be detrimental to the North Sea environment or the personnel and/or economies operating within the region

Gas exploration and production at the Dutch continental shelf: an assessment of the 'Depreciation at Will'.

This report analyses the effects of Depreciation at Will (DAW) on offshore gas production, government budget and employment in the gas industry. The DAW enables firms to accelerate deprecation of investments in platforms and other offshore equipment. The interest advantage due to the postponed payments of taxes raises the profitability of investment projects and, hence, could raise the level of investments. The key question in the debate on the DAW is whether the higher tax base compensates for the interest losses due to postponed tax receipts. The econometric analysis has shown that the DAW increased only the number of development drillings during the period this measure was implemented (1996-2002). A moving long-run average of the oil price has appeared to be a significantly explaining variable behind the level of exploration drillings as well as development drillings. Using the current value of that oil price, 25 dollar per barrel, we find a large number of profitable exploration projects. In the current circumstances, introduction of the DAW will not raise the level of investments in the near future, as several non-financial factors appear to be bottlenecks, such as the duration of licensing procedures. The econometric analysis is also published in A. ten Cate en M. Mulder, 2007, "Impact of the oil price and fiscal facilities on offshore mining at the Dutch Continental Shelf", Energy Policy , vol 35, pp 5601-5613.