Earthquake Nucleation on Reactivated Faults with Velocity-Strengthening Friction

Abstract

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.