On the b-value Dependency of Injection-Induced Seismicity on
Geomechanical Parameters
Abstract
Variability in the b-value, which describes the frequency
distribution of earthquake magnitudes, is usually attributed to
variations in differential stress in the setting of natural and
laboratory earthquakes. However, differential stress is unlikely to
explain b-value variations on the reservoir scale of
injection-induced seismicity cases where significant differential stress
variability can hardly be expected. We investigate the responsible
geomechanical parameters for the b-value reduction observed in
injection-induced seismicity at the Basel EGS field in Switzerland, for
which a measured in-situ stress model and fault orientations of
numerous microseismic events are available. We estimate the shear and
normal stresses along faults, differential stress, pore pressure
increase at failure, and the Coulomb failure stress, for each event.
Event magnitude and shear stress display the most systematic and clear
correlation between each other, while other parameters do not show a
clear correlation with event magnitude. We further examine the
relationship between the b-value and these geomechanical
parameters. We discover that the b-value systematically decreases
with increasing shear stress. Again, other geomechanical parameters do
not show a clear correlation with the b-value. We conclude that
b-value variability is explained by variations in shear stress in
the injection-induced seismicity setting, where near constant
differential stress conditions are expected. Furthermore, we observe
that b-value reduction with time also correlates with an
increasing number of events along faults having high shear stress, which
strongly supports our conclusions. Thus, we discovered a profound
physical mechanism behind b-value variation in injection-induced
seismicity beyond general understandings of b-value variation.