The Role of Background Stress State in Fluid-Induced Aseismic Slip and
Dynamic Rupture on a 3-meter Laboratory Fault
Abstract
Fluid injection stimulates seismicity far from active tectonic regions,
however the details of how fluids modify on-fault stresses and initiate
seismic events remains poorly understood. We conducted laboratory
experiments using a biaxial loading apparatus with a 3 m saw-cut granite
fault and compared events induced at different background shear stress
levels. Water was injected at 10 mL/min and normal stress was constant
at 4 MPa. In all experiments, aseismic slip initiated on the fault near
the location of fluid injection and dynamic rupture eventually initiated
from within the aseismic slipping patch. When the fault was near
critically stressed, seismic slip initiated only seconds after MPa-level
injection pressures were reached and the dynamic rupture propagated
beyond the fluid pressure perturbed region. At lower stress levels,
dynamic rupture initiated hundreds of seconds later and was limited to
regions where aseismic slip had significantly redistributed stress from
within the pressurized region to neighboring locked patches. We find
that slow slip initiated when local stresses exceeded Coulomb failure
criteria, but initiation of dynamic rupture required additional criteria
to be met. Even high background stress levels required aseismic slip to
modify on-fault stress to meet initiation criteria. We also observed
slow slip events prior to dynamic rupture. Overall, our experiments
suggest that initial fault stress, relative to fault strength, is a
critical factor in determining whether a fluid-induced rupture will
“runaway” or whether a fluid-induced rupture will remain localized to
the fluid pressurized region.