Natural time and nowcasting induced seismicity at the Groningen gas field in the Netherlands

The Groningen gas field in the Netherlands has a significant level of induced seismicity; this seismicity is attributed to fluid withdrawal not fluid injection. This is one of the most productive gas fields in Europe. Production began in 1963 and the first induced earthquake occurred in 1991. Increasing levels of seismicity led to the installation of a sensitive seismic network in 1995 with a minimum magnitude completeness of M >= 1.5. The largest induced earthquake was a magnitude 3.6 in 2012, which caused thousands of damage claims and led to a reduction in gas production in 2014 January. In this paper, we show that the induced earthquakes satisfy Gutenberg-Richter (GR) frequency-magnitude statistics over the range 1.5 <= M <= 3.0. We utilize truncated GR scaling to introduce a possible maximum earthquake with M = 3.85. We also demonstrate an almost complete absence of aftershocks following the largest induced earthquakes. To quantify the temporal evolution of the induced seismicity, we perform our nowcasting method. Nowcasting uses the catalogue of seismicity in the region. Two earthquake magnitudes are selected, one large say M-lambda >= 2.5, and one small say M-sigma >= 1.5. The method utilizes the number of small earthquakes that occur between pairs of large earthquakes. A major advantage of nowcasting is that it relies on 'natural time', earthquake counts, between events rather than clock time. Thus, the results are applicable to induced seismicity that varies in time. We count the rate of occurrence of small induced earthquakes to nowcast the probability of occurrence of larger earthquakes.