Fractures, Stress, and Seismic Refraction

A New Perspective on Critical Zone Dynamics

Dr. Brady A. Flinchum

The critical zone (CZ) is the region extending from treetops down to fresh, unaltered bedrock. In the CZ, water drives weathering. In crystalline rock, fractures serve as the primary conduits for water flow, and this flow is governed by in situ stresses resulting from the interaction between topography and regional stress fields. Consequently, it is hypothesized that these stress regimes influence CZ evolution.

To investigate these stress-related processes, geophysical methods—particularly seismic refraction—have become essential tools for characterizing the CZ at large spatial scales. Over the past decade, seismic refraction has proven instrumental in imaging and interpreting processes that shape the CZ, especially in eroding landscapes underlain by crystalline bedrock. However, the CZ remains a challenging environment for seismic investigations. A single seismic profile may encounter a wide range of porosities (0–60%), fractured and porous media, varying degrees of saturation (0–100%), and effective pressures near zero. Moreover, obtaining high-quality samples for laboratory analysis under low effective pressures (≤ 1 MPa) and small elastic strains (10⁻⁵ to 10⁻⁶) remains difficult.

This presentation explores a series of puzzling seismic observations that reveal anomalously low Poisson’s ratios, with Vp/Vs values ≤ 1.4. These observations are primarily confined to saprolite and fractured bedrock weathered in eroding crystalline terrains. I propose that weathering-induced microcracks cause inelastic deformation that reduces compressibility more than shear strength, leading to a greater reduction in P-wave velocity than S-wave velocity. These low Vp/Vs values are likely observable only near the surface due to the low effective pressures and minimal stresses and strains associated with seismic wave propagation in the CZ.

About the speaker

Dr. Brady A. Flinchum Lecturer in Earth Sciences, University of Newcastle (NSW)

Dr Brady A. Flinchum is a Lecturer in Earth Sciences at the University of Newcastle, with over eight years of post-Ph.D. experience in near-surface geophysics and Critical Zone science. He earned his Ph.D. in Geology and Geophysics from the University of Wyoming in 2017 and has held research and teaching positions at CSIRO and Clemson University. His work integrates seismic refraction and reflection, ground penetrating radar (GPR), nuclear magnetic resonance (NMR), and DC electrical resistivity methods to investigate subsurface processes, regolith formation, and ecosystem resilience. Dr Flinchum has led major National Science Foundation (NSF) funded projects, published 34 peer-reviewed papers in top-tier journals, and contributed to policy-relevant research cited internationally. He is an active mentor and reviewer, committed to advancing interdisciplinary geoscience.

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