Electronic Theses and Dissertations Archive

Date

2026

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Earth Sciences

Committee Chair

Thomas Goebel

Committee Member

Charles Camp

Committee Member

Chris Cramer

Committee Member

Christodolous Kyriakopoulos

Committee Member

Eunseo Choi

Abstract

The spatial, temporal, and magnitude distribution of earthquakes is governed by fault zone properties including stress state, pore pressure, gouge composition, and structural heterogeneity. Variations in the b-value of the Gutenberg–Richter frequency–magnitude distribution and in precursory fault slip behavior are thought to reflect these underlying controls, yet isolating the contribution of individual factors from natural observations remains a central challenge in earthquake physics. Here, we investigate the effects of pore pressure and compositional heterogeneity on seismicity statistics and preparatory processes through two complementary sets of laboratory experiments. In the first study, we conducted triaxial compression experiments on faulted Westerly granite under nominally dry, fluid-saturated, and transient injection conditions to examine how pore pressure modulates the b-value (slope of the frequency-magnitude distribution) across multiple seismic cycles. Increasing differential stress exhibits an inverse linear relationship with b-value, and elevated pore pressure amplifies this stress dependence close to failure. Higher pore pressures, on average, lead to lower b-values. Spatial analysis during fluid injection reveals that elevated b-values near the injection well result from distributed rock damage rather than pore-pressure gradients alone, a pattern consistent with observations at the Basel Enhanced Geothermal System. In the second study, we performed direct shear friction experiments on binary mixtures of velocity-weakening soda lime beads and velocity-strengthening quartz powder, systematically varying composition from 100\% to 0\% weakening material to isolate the effect of compositional variations on fault slip behavior and earthquake recurrence. Faults with variable gouge composition slide at intermittent critical stresses close to failure for most of the interseismic period, exhibiting transient slip acceleration events that significantly exceed the loading rate. Prefailure dilation amplitude and duration decrease systematically with increasing heterogeneity. Compositional variation thus weakens the dilational signal while extending the preparatory slip phase before failure. The transition from stick-slip to stable sliding occurs due to small compositional changes (~5\% less weakening gouge), with peak slip velocities differing by approximately 3.8 orders of magnitude. Homogeneous faults follow time-predictable recurrence, whereas heterogeneous faults deviate progressively from this behavior, compositional changes alone are sufficient to produce complex recurrence intervals independent of loading conditions. The results from the two studies emphasize that b-value variations stem from at least three distinct factors, stress, pore pressure, and fault zone heterogeneity, and that compositional variations, similar to geometric heterogeneity, lead to complex pre-slip processes and less-predictable earthquake recurrence. These findings underscore that neither b-value anomalies nor precursory signals can be attributed to a single cause. Laboratory experiments isolate the role of individual factors under controlled conditions, providing testable hypotheses for seismicity patterns observed in nature. The applicability of these hypotheses to natural fault requires direct testing through comparison with field observation on well-characterized fault system.

Comments

Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to ProQuest/Clarivate.

Notes

Open Access

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