Earthquake Nucleation on Reactivated Faults with Velocity-Strengthening Friction

Gas production in the Groningen field in the Netherlands reactivates faults and induces earthquakes, triggering severe societal unrest. Laboratory experiments indicate the relevant lithologies are velocity-strengthening [1], which favors stable creeping rather than instabilities and thus in theory prohibits earthquake nucleation. This makes the observed seismicity hard to explain. However, slide-hold-slide experiments on the same rocks reveal healing: stress drop and energy release of the subsequent slip after reactivation increase as healing time increases [2]. Given that the Groningen region has been tectonically inactive for millions of years, we will show that the healing history is a key factor for the nucleation of induced earthquakes.

We simulate earthquake sequences on a normal fault governed by rate-and-state friction, crosscutting a depleting gas reservoir. We reproduced the healing behavior observed in the laboratory and extended healing time up to millions of years. Our simulations suggest induced seismicity occur after fault reactivation, no matter whether the frictional behavior is velocity-weakening (VW) or -strengthening (VS). Unlike under VW friction where larger earthquakes with longer recurrence intervals occur as production continues, no subsequent earthquakes are expected under VS friction. No theoretically predictable nucleation size is measured: nucleation zone keeps expanding simultaneously when shear stress is dropping. Rupture propagation is mostly restricted to within the reservoir. Its reduced possibility of propagating into the under- and overburden reduces energy release and thus moment magnitude. The more VS the fault is, the lower the maximum slip rate will be, if under the same healing time. In other words, for a larger (a-b), a longer healing time is required before a large earthquake can happen. Therefore, if the healing history is well constrained, we can have an estimate of the seismic hazard of the coming event before it occurs. However, in order to have an accurate hazard estimate, dynamic weakening mechanisms activated at slip rate of cm/s may not be ignored. Adding flash heating shortens the required healing time by at least eight orders of magnitude in one of our representative models. Parameter space is explored systematically to build analytical expressions of earthquake magnitude as a function of healing time and frictional parameters. This study will help to understand in which tectonic histories, induced seismicity can still occur under VS friction. This is important for forecasting induced seismicity and minimizing the hazard in similar lithologies across northwestern Europe and in subsurface production and storage sites worldwide.