About

W. Steven Holbrook is a Professor of Geophysics in the Department of Geosciences at Virginia Tech. His research utilizes geophysical techniques, primarily seismology, to examine Earth processes, with a focus on critical zone geophysics. He explores the Earth's 'breathing skin,' where water, life, and rock converge, aiming to understand the structure of Earth's regolith at landscape scales and how this structure influences water storage and movement in terrestrial environments. Holbrook's work has contributed to understanding continental margins, volcanic arcs, and methane hydrate systems, and he was a key developer of seismic oceanography, which images thermohaline fine-structure within the oceans. His career includes positions at the Woods Hole Oceanographic Institution and the University of Wyoming before joining Virginia Tech. He has mentored numerous students and postdoctoral researchers, many of whom have gone on to successful careers in industry or academia. His educational background includes a Ph.D. and M.S. in Geophysics from Stanford University and a B.S. in Geoscience from Penn State.

Research topics

• Geomorphology
• Geology
• Mineralogy
• Environmental science
• Ecology
• Earth science
• Geochemistry
• Geophysics

Selected publications

• Deep Subsurface Water Stores Sustain Giant Sequoias

  Research Square · 2025-12-16

  preprintOpen access
  PublisherOA PDFDOI

• Using Seismic Refraction Data to Estimate a Relationship Between Landscape Curvature and Deep Critical Zone Structure in the South Carolina Piedmont, USA

  Journal of Geophysical Research Earth Surface · 2025-11-01 · 1 citations

  articleOpen access

  Abstract P‐wave velocity profiles from seismic refraction reveal deep critical zone (CZ) architecture along profiles hundreds of meters long. However, extrapolating local velocity measurements to infer CZ architecture at regional scales (1–20 km 2 ) remains challenging. Here, we present a strategy that transforms seismic observations from individual profiles into maps of CZ architecture spanning tens of square kilometers. Data from 15 seismic refraction profiles (approximately 6.6 km total length) collected in weathered crystalline rocks of the South Carolina Piedmont, USA, revealed approximately 400,000 m 2 of deep CZ architecture. Using casing depths from four boreholes, we show that the boundary dividing saprolite and fractured rock corresponds to a velocity of 1,870 m/s. Using velocity measurements from an outcrop within the survey area, we identify the bedrock velocity as 4,550 m/s. These velocities define a three‐layer CZ structure comprising soil and saprolite, fractured bedrock, and unweathered bedrock. We developed an empirical relationship between CZ structure and minimum and maximum principal curvatures, enabling prediction of CZ architecture over approximately 17 km 2 . The correlation between seismically inferred CZ structure and principal curvatures at our study site suggests that curvature metrics can be used to predict CZ structure at larger scales in crystalline terrains under subtropical climates. However, the empirical relationship struggled to predict CZ structure where landscape curvatures were near zero, suggesting that other variables likely contribute to local heterogeneity. Given that curvature is an important variable for erosion and groundwater flow, our results suggest it could be a promising metric for predicting CZ structure.

  PublisherOA PDFDOI

• Creating a Critical Zone: Feedbacks Between Bedrock Geology, Hydrology, and Vegetation on an Exposed Bedrock Surface, Panola Mountain, Georgia, USA

  2025-04-07

  preprintOpen access

  Most of Earth's present-day terrestrial surface is covered by regolith --- the layers of soil, saprolite, and weathered bedrock that together comprise the critical zone. Recent research has focused on understanding fluxes of minerals, water, and energy through the critical zone under steady state assumptions. However, in eroding landscapes, regolith and soil are produced from the bedrock as it is exhumed. Therefore, at some point in time, every location on the Earth's surface currently mantled by regolith experienced an onset of weathering processes. This initial creation of a critical zone from rock is poorly understood. Here we study initial critical zone formation from exposed bedrock by combining surface and subsurface geophysical observations at a site where regolith appears to be forming from bedrock on a granodiorite outcrop in Panola Mountain State Park, Georgia, USA. Vegetation gains an initial foothold on the outcrop by colonizing microtopographic depressions created by differential weathering of contrasting bedrock compositions. We observe a range of colonization stages, from moss to grasses to small bushes and eventually to large trees. Subsurface signatures of the vegetation include enhanced radar reflectance and reduced seismic velocities, with larger vegetation associated with stronger subsurface signals. Using a space-for-time substitution approach, we propose an evolutionary sequence for critical zone development. While disentangling the chicken-and-egg questions that pervade this topic remains challenging, our results suggest that geological heterogeneity can provide the initial catalyst for colonization, but ultimately vegetation itself plays a strong role in producing subsurface structures we associate with the critical zone.

  PublisherOA PDFDOI

• Multi‐Scale Geophysical Imaging of a Hydrothermal System in Yellowstone National Park, USA

  Journal of Geophysical Research Solid Earth · 2025-03-28 · 2 citations

  articleOpen access

  Abstract Little is known about the local plumbing systems that fuel Yellowstone's famous hot springs, geysers and mud pots. A multi‐method, multi‐scale geophysical investigation was carried out in the Obsidian Pool Thermal Area (OPTA) to: (a) delineate the lateral extent of the hydrothermal area and associated surface features; (b) estimate the dimensions of the upflow zone and identify its main controlling structures; (c) assess fluids circulation pathways from depth to surface. Ground and airborne geophysical data were acquired to connect local and regional scales, from shallow to large depths. Maps of surface electrical resistivity show a strong correlation with hydrothermal features. At intermediate depths, electrical resistivity permits delineating the upper limit of the upflow zone, while Poisson's ratio highlights differences in subsurface fluid content. Combining these results with surface observations and topographic information, we speculate that differential mixing of hydrothermal and fresh water could explain the wide diversity of features observed at OPTA. Low electrical resistivity observed at large depths also suggest that a vast upflow zone, controlled by rhyolite flows and conjugate faults, underlies the OPTA. We speculate that hydrothermal fluids rise along fractures and reach the surface in topographic lows to form hydrothermal features. Our results show that synoptic, multi‐scale geophysical measurements provide a roadmap for understanding where and how geologic heterogeneity, topography, fluid‐gas separation, and the mixing of thermal and meteoric waters conspire to produce the wide variety of Yellowstone's renowned hydrothermal features.

  PublisherOA PDFDOI

• Uncovering the Secrets Beneath Giant Sequoias using Seismic Refraction Tomography Coupled with Advanced Seismic First Arrival Picker

  2025-01-24

  preprint

  Giant sequoia trees (sequoiadendron giganteum) are the world's largest single-stemmed tree and an iconic species in the Sierra Nevada mountains. Giant sequoias require large volumes of groundwater for transpiration, but little is known about subsurface critical zone structure beneath giant sequoias, hindering our understanding of the resilience of these giants to environmental change. We determined the quasi-3D seismic P-wave velocity structure beneath a grove of four giant sequoia trees in the Mariposa Grove, Yosemite National Park, CA, using seismic refraction tomography. A key feature of this study is the implementation of an interactive, semi-automatic picker based on the dynamic time warping algorithm to precisely identify seismic first arrivals. This advanced picking method enhances the precision of seismic first arrivals and significantly reduces data processing time. The resulting detailed seismic velocity structure provides valuable insights into the subsurface characteristics beneath these ancient trees. High seismic velocities are observed directly beneath standing sequoia stems, but not under a recently fallen sequoia, suggesting that enhanced pressure from the mass of the tree changes soil structure beneath the trees. The velocity structure also reveals important details about root zone structure and the extent of weathering in the critical zone beneath the giant sequoias, giving us a better understanding of the subsurface environment that supports the giant sequoias.

  PublisherDOI

• On the role of inherited rock fabric in critical zone porosity development: Insights from seismic anisotropy measurements using surface waves

  Earth Surface Processes and Landforms · 2025-07-01 · 3 citations

  articleOpen access

  Abstract Within Earth's critical zone, weathering processes influence landscape evolution and hillslope hydrology by creating porosity in bedrock, transforming it into saprolite and eventually soil. In situ weathering processes drive much of this transformation while preserving the rock fabric of the parent material. Inherited rock fabric in regolith makes the critical zone anisotropic, affecting its mechanical and hydrological properties. Therefore, quantifying and studying anisotropy is an important part of characterising the critical zone, yet doing so remains challenging. Seismic methods can be used to detect rock fabric and infer mechanical and hydrologic conductivity anisotropy across landscapes. We present a novel way of measuring seismic anisotropy in the critical zone using Rayleigh and Love surface waves. This method leverages multi‐component surface seismic data to create a high‐resolution model of seismic anisotropy, which we compare with a nuclear magnetic resonance log measured in a nearby borehole. The two geophysical data sets show that seismic anisotropy and porosity develop at similar depths in weathered bedrock and both reach their maximum values in saprolite, implying that in situ weathering enhances anisotropy while concurrently generating porosity in the critical zone. We bolster our findings with in situ measurements of seismic and hydrologic conductivity anisotropy made in a 3 m deep soil excavation. Our study offers a fresh perspective on the importance of rock fabric in the development and function of the critical zone and sheds new insights into how weathering processes operate.

  PublisherOA PDFDOI

• Anatomy of a deep Piedmont critical zone: Evaluating hypotheses on regolith depth controls through comparison of ridge and valley boreholes

  Earth Surface Processes and Landforms · 2024-11-19 · 3 citations

  articleOpen access

  Abstract Controls on the physical and chemical architecture of the subsurface critical zone are somewhat controversial, with multiple hypotheses proposed to account for variations in the depth of weathering between sites, and with landscape position at a site. In the Piedmont region of the Mid‐Atlantic US weathering of crystalline bedrock has been observed to extend tens of meters below the surface and groundwater in a'bow‐tie’ shape – i.e. weathering extends to lower elevations below ridges than below channels. The chemical and physical structure of a hillslope transect in the Maryland Piedmont was explored with a 45 m borehole in the ridge, as well as shallow bedrock boreholes at the toe of the slope and valley. Chemical weathering fronts were characterized using elemental abundances and mineralogical analysis. The ridge borehole did not extend deeper than the chemically and physically weathered rock. Surface and borehole geophysics and density measurements were used to characterize the weathered rock and saprolite. Na and Ca results suggest that plagioclase feldspar weathering is similar between samples collected from 45 m under the ridge and 2.2 m under the valley bottom. A narrow Fe oxidation garnet weathering front co‐insides with the transition from weathered bedrock to saprolite, suggesting that this reaction may generate initial saprolite porosity. Muscovite weathering co‐occurs with complete depletion of plagioclase a few meters above the Fe oxidation front. These nested weathering fronts in the saprolite appear to follow a subdued version of the surface topography. The location and shape of the nested saprolite weathering fronts may be controlled by the feedback between the transport of reactants and solutes and reaction‐generated porosity, consistent with the conceptual “valve” hypothesis. Differing dominant control mechanisms on deep bedrock weathering and saprolite initiating reactions may explain the thickness and structure of the critical zone at our site.

  PublisherOA PDFDOI

• Sims et al. A Tale of Two Pools: The Dynamic Influence of Subsurface Geological Processes on the Assembly and Diversification of Thermophilic Microbial Communities in Hydrothermal Systems - Supplementary Datasets

  Zenodo (CERN European Organization for Nuclear Research) · 2024-01-01

  datasetOpen access

  These datasets comprise the supplementary datasets found in Sims et al. A Tale of Two Pools: The Dynamic Influence of Subsurface Geological Processes on the Assembly and Diversification of Thermophilic Microbial Communities in Hydrothermal Systems, <strong><em>Geochimica et Cosmiochimica Act </em></strong>

  PublisherDOI

• 2D Near‐Surface Full‐Waveform Tomography Reveals Bedrock Controls on Critical Zone Architecture

  Earth and Space Science · 2024-02-01 · 10 citations

  articleOpen access

  Abstract For decades, seismic imaging methods have been used to study the critical zone, Earth's thin, life‐supporting skin. The vast majority of critical zone seismic studies use traveltime tomography, which poorly resolves heterogeneity at many scales relevant to near‐surface processes, therefore limiting progress in critical zone science. Full‐waveform tomography can overcome this limitation by leveraging more seismic data and enhancing the resolution of geophysical imaging. In this study, we apply 2D full‐waveform tomography to match the phases of observed seismograms and elucidate previously undetected heterogeneity in the critical zone at a well‐studied catchment in the Laramie Range, Wyoming. In contrast to traveltime tomograms from the same data set, our results show variations in depth to bedrock ranging from 5 to 60 m over lateral scales of just tens of meters and image steep low‐velocity anomalies suggesting hydrologic pathways into the deep critical zone. Our results also show that areas with thick fractured bedrock layers correspond to zones of slightly lower velocities in the deep bedrock, while zones of high bedrock velocity correspond to sharp vertical transitions from bedrock to saprolite. By corroborating these findings with borehole imagery, we hypothesize that lateral changes in bedrock fracture density majorly impact critical zone architecture. Borehole data also show that our full‐waveform tomography results agree significantly better with velocity logs than previously published traveltime tomography models. Full‐waveform tomography thus appears unprecedentedly capable of imaging the spatially complex porosity structure crucial to critical zone hydrology and processes.

  PublisherOA PDFDOI

• Low V_p/V_s Values as an Indicator for Fractures in the Critical Zone

  Geophysical Research Letters · 2024-01-19 · 13 citations

  articleOpen accessSenior author

  Abstract Poisson's ratio for earth materials is usually assumed to be positive (V p /V s &gt; 1.4). However, this assumption may not be valid in the critical zone because near Earth's surface effective pressures are low (&lt;1 MPa), porosity has a wide range (0%–60%), there are significant texture changes (e.g., unconsolidated vs. fractured media), and saturation ranges from 0% to 100%. We present P‐wave (V p ) and S‐wave (V s ) velocities from seismic refraction profiles collected in weathered crystalline environments in South Carolina and Wyoming. Our data show that ∼20% of the subsurface has negative Poisson's ratios (V p /V s values &lt; 1.4), a conclusion supported by borehole sonic logs. The low V p /V s values are confined to the fractured bedrock and saprolite. Our data support the hypothesis that weathering‐generated microcracks can produce a negative Poisson's ratio and that V p /V s values can thus provide insight into important critical zone weathering processes.

  PublisherOA PDFDOI

Recent grants

• RAPID: Collaborative Research: A Short, Open-Access 2D MCS Acquisition Program off Washington State

  NSF · $52k · 2012–2014

• Collaborative Research: Seismic measurements of magma flux, arc composition, and lower-plate serpentinization in the Central American subduction factory

  NSF · $1.5M · 2004–2012

• Collaborative Research: Network Cluster: Bedrock controls on the deep critical zone, landscapes, and ecosystems

  NSF · $1.6M · 2020–2026

• Collaborative Research: Quantitative Estimates of Oceanic Turbulence and Temperature Structure from Seismic Reflection Data

  NSF · $404k · 2007–2011

• Methane Release in Submarine Landslides: A Seismic, Sedimentological, and Geochemical Study of the Storegga Slide, Offshore Norway

  NSF · $629k · 2003–2008

Frequent coauthors

• Bradley J. Carr

  University of Wyoming

  65 shared

• Brady Flinchum

  61 shared

• Daniel Lizarralde

  48 shared

• Sylvain Pasquet

  Centre National de la Recherche Scientifique

  43 shared

• Linda Lakemacher

  State Science and Technology Institute

  39 shared

• John B. Wachtman

  38 shared

• J. L. Hayes

  Dickinson College

  38 shared

• John R. Hopper

  Geological Survey of Denmark and Greenland

  37 shared

Labs

• Department of GeosciencesPI

Education

• Ph.D., Geophysics, Geophysics

  Stanford University

  1989

• B.S., Geosciences, Geosciences

  Pennsylvania State University

  1982

Awards & honors

• Walter Munk Award for Distinguished Research in Oceanography…
• Fellow, American Geophysical Union (2012)
• Ocean Drilling Program (JOI/USSAC) Distinguished Lecturer (2…
• Fellow, Geological Society of America (1998)