PhD dissertation of Pritom Sarma

 

Coupled Micromechanical and Pore Pressure Processes in Seismic and Aseismic Deformation
Fluids play a large role in subsurface processes associated with seismic and aseismic deformation. Large
subsurface fluid volume injection is increasingly common across a range of economic (enhanced oil
recovery, geothermal energy extraction) and remediation (carbon sequestration, wastewater injection)
activities. This fluid injection is often associated with increased seismicity, usually ascribed to reduction in
fault strength on pre-stressed faults once diffusing pore-pressure reaches the fault. In other situations,
fluid-injection hasinstead induced aseismic fault slip. A research gap exists in the understanding of coupled
fluid-solid behaviour leading to seismic and aseismic deformation processes. This PhD investigates seismic
and aseismic deformation processes that arise from the coupling between pore-fluid flow and
micromechanical grain-scale mechanics, from laboratory to field scales. In the first three chapters we use

a coupled fluid-DEM (Discrete Element Method) numerical model to simulate a water-saturated gouge-
filled fault zone under shear. DEM is a discrete numerical technique that predicts the behavior of granular

aggregates by calculating interactions between a large number of grains. The first chapter examines fluid-
induced failure in granular fault gouge, developing analytical characterizations of steady and transient

behavior, revealing hysteretic response of the fault under cycles of loading and unloading; a key distinction
from dry gouge is a delay to failure in the wet fault gouge, governed by dilatant hardening. The second
chapter analyzes the role of fluid injection rate on failure threshold for a pre-stressed fault, showing that
faster injection increases the fluid pressure required for failure, consistent with prior experiments, and

derives an analytical framework for failure pressures across rates. The third chapter studies cyclic pore-
pressure variations, demonstrating that under cyclic injection failure can occur below the classical Mohr–

Coulomb criterion, and that varying oscillation frequency produces distinct failure regimes; this transition
between sliding modes is governed by a characteristic timescale. The final (4th) chapter applies these
coupled fluid–solid processes to large-scale geophysical settings, modeling seasonal and decadal vertical
ground motions in Coastal Louisiana. Theoretical poroelastic analysis predicts that ground water infiltration
from the Mississippi River into the Pleistocene Baton Rouge Fault Zone follows seasonal fluctuations that
modulate the resulting ground deformation.