Abstract
Geological faults may produce earthquakes under the increased stresses
associated with hydrocarbon recovery, geothermal extraction, CO2
storage. The associated risks depend on the frequency and magnitude of
these earthquakes. Within seismic risk analysis, the exceedance
probability of seismic moments, Μ, is treated as a pure power-law
distribution, Μ^{-β}, where the power-law exponent, β, may vary in
time or space or with stress. Insights from statistical mechanics
theories of brittle failure, statistical seismology, and acoustic
emissions experiments all indicate this pure power-law may contain an
exponential taper, Μ^{-β}e^{-ζ Μ}, where the taper strength,
ζ, decreases with increasing stress. The role of this taper is to
significantly reduce the probability of earthquakes larger than
ζ^{-1} relative to the pure power-law. We review the existing
theoretical and observational evidence for a stress-dependent
exponential taper to motivate a range of magnitude models suitable for
induced seismicity risk analysis. These include stress-invariant models
with and without a taper, stress-dependent β models without a taper, and
stress-dependent ζ models. For each of these models, we evaluated their
forecast performance within the Groningen gas field in the Netherlands
using a combination of Bayesian inference, and simulations. Our results
show that the stress-dependent ζ-model with constant β likely offer
(75–85%) higher performance forecasts than the stress-dependent
β-models with ζ = 0. This model also lowers the magnitudes with a 10%
and 1% chance of exceedance over the next 5 years of gas production
from 4.3 to 3.7 and from 5.5 to 4.3 respectively.