Reflections and Some Questions about Assessing the Maximum Possible Earthquake in the Long-Exploited Groningen Gas Field

Abstract

Abstract This article is inspired by an official expert reassessment (September 2022), for the next several decades, of the maximum possible earthquake magnitude, Mmax, induced by sixty years of extraction from the rich Groningen gas field in the Netherlands. Basic considerations, advisory inputs, and the assessment of Mmax are briefly reviewed. Comments and questions are given on the range and weights of possible Mmax values, and on probabilities, “weights”, as expert degrees of belief. It is argued that plausible shifts in conditional beliefs (e.g., 100% rather than 90% induced) might have led to a lower future maximum-magnitude range (3.5≤Mmax≤5.5, with average 4.2) than the reported 4.0≤Mmax≤6.5 with weighted-average 4.6. As a possible event, Mmax should have a nonzero probability that (seismo) logically should go down as Mmax goes up. Given that Mmax weights assigned are based on expert beliefs, are assessors willing to bet—substantially—on the outcome of an Mmax the probability of which is being estimated? For the Groningen field, the assumption of a stationary seismic source and Mmax may be disputed. Instead, it is proposed that an eventually observable Mmax must be related to total-cumulative extraction since 1963. Hence, after Groningen gas extraction has ended in October 2023, Mmax will rapidly decrease. Unfortunately missing is a sensitivity analysis to seize the practical meaning of an (decreasing) Mmax distribution, for example, for ground motions, seismic risk (including residents’ anxiety), and possible building reinforcement. Such implications bolster the requirement that seismic hazard assessment be thoroughly designed, well understood, and clearly communicated.