Statistics of seismic events at the Groningen field

Depletion of gas fields, even in a tectonically inactive area can induce earthquakes. This is the case for the Groningen gas field, located in the north of the Netherlands. Increased seismic activity raised public concern which led to the government trying to understand the cause of the earthquakes and optimize production such as to minimize the risk of induced seismicity. The main question is how production is correlated with induced seismicity. In this paper we deal with the statistics of seismic events using Bayesian model comparison and a Bayesian change point model. We have developed a method to assess seismic event rate, its changes and tendencies. These statistical analyses are in agreement with each other and find a constant event rate up to 2003, an increasing event rate from 2003 to 2014 and a preference for a decreasing event rate from early 2014 to now. Seasonality in the production and the number of events is present. The seasonality indicates a delay ranging between 2 and 8 months between seismicity and production changes. The question of interest is whether the production reduction since January 2014 has had an effect on the seismicity occurring in the Groningen field. The number of events in the Groningen field in the area affected by the production change has been reduced significantly. We present evidence that changes in seismicity are closely related to changes in production.