Physics-Based Forecasting of Induced Seismicity at Groningen Gas Field, The Netherlands: Post Hoc Evaluation and Forecast Update

Abstract

Abstract Induced seismicity due to natural gas extraction from the Groningen reservoir, The Netherlands, has been occurring since the early 1990s. Prospective forecasts of the possible maximum earthquake magnitude for different gas extraction scenarios have previously been published. However, their accuracy and continued relevance are unknown. Here, we evaluate our prior Groningen seismicity forecast and propose a revised model. We compare the forecast to earthquakes that occurred after its publication, between 1 February 2017 and 1 July 2022. We used volume scaling to account for actual gas extraction being 35% less than considered in the forecast. After this correction, we found that the forecast slightly underestimated the number of M ≥2.5 events. However, it was consistent with the largest event in that period, an M 3.4. Then, we updated the forecast model for a new depressurization scenario, testing its pseudoprospective (out-of-sample) performance using L-test and N-test. This revision considered two possible magnitude–frequency distributions (MFD): (1) tapered Gutenberg–Richter (GR), in which induced ruptures are confined within the reservoir, and (2) unbounded GR, in which ruptures can propagate into the underburden. A tapered GR is the more likely scenario due to clear rollover in the MFD and a lack of events beneath the reservoir. However, we cannot exclude the possibility of an unbounded GR, because direct stress measurements in the underburden are not currently available. For the period 2021–2030, this new forecast suggests a 1% likelihood that the largest event will exceed M 3.8 for the tapered model and M 5.4 for the unbounded model.