Browsing by Author "Hole, John Andrew"
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- Earthquakes in complex fault settings: Examples from the Oregon Cascades, Eastern California Shear Zone, and San Andreas faultVadman, Michael John (Virginia Tech, 2023-06-22)The surface expression of upper crustal deformation varies widely based on geologic settings. Normal faults within an intra-arc basin, strike-slip faulting within a wide shear zone, and creeping fault behavior all manifest differently and require a variety of techniques for analysis. In this dissertation I studied three different actively deforming regions across a variety of geologic settings. First, I explored the drivers of extension within the La Pine graben in the Oregon Cascades. I mapped >20 new Quaternary faults and conducted paleoseismic trenching, where I found evidence for a mid-late Holocene earthquake on the Twin Lakes maar fault. I suggest that tectonics and not volcanism is responsible for the most recent deformation in the region based on fault geometries and earthquake timings, although more research is needed to tease out finer temporal and genetic relationships between tectonics and volcanism regionally. Second, I investigated the rupture pattern and earthquake history of the Calico fault system in the Eastern California Shear Zone. We mapped ~18 km of continuous rupture, with a mean offset of 2.3 m based on 39 field measurements. We also found evidence for two earthquakes, 0.5 - 1.7 ka and 5.5 - 6.6 ka through paleoseismic trenching. We develop a number of different multifault rupture scenarios using our rupture mapping and rupture scaling relationships to conduct Coulomb stress change modeling for the most recent earthquake on the Calico fault system. We find that the most recent event places regions adjacent to the fault in a stress shadow and may have both delayed the historic Landers and Hector Mine ruptures and prevented triggering of the Calico fault system during those events. Last, I studied the spatial distribution of the southern transition zone of the creeping section of the San Andreas fault at Parkfield, CA to determine if it shifted in response to the M6 2004 Parkfield earthquake. I used an Iterative Closest Point algorithm to find the displacement between two lidar datasets acquired 13 years apart. I compared creep rates measured before the 2004 earthquake to creep rates calculated from my lidar displacement results and found that there is not a discernible change in the overall pattern or distribution of creep as a response to the 2004 earthquake. Peaks within the lidar displacement results indicate complexity in the geometry of fault locking.
- Evidence of Hydro-Seismicity in the Tennessee Seismic ZoneKing, Gregory Lester (Virginia Tech, 2023-11-17)Reservoir level and discharge data were collected for three Tennessee Valley Authority (TVA) dams (Chickamauga, Watts Bar and Douglas) on the Tennessee river in the Eastern Tennessee Seismic Zone (ETSZ). Earthquake catalog data was also collected for the ETSZ from 1980-2018. Well levels from 2 U.S. Geological Survey groundwater monitoring wells were also collected. Rainfall data were collected for the ETSZ. Reservoir, well and rainfall data were examined for correlation with earthquake occurrence rates. High Pearson correlation and anti-correlation coefficients (.7-.9) were obtained for the reservoir level and dam discharge volume rates vs monthly earthquake counts. Pearson correlation coefficients for rainfall and well level vs. monthly earthquake counts were small (.0-.2). A large difference in earthquake rates between the months of February (low rate) and April (high rate) over the 39 year study period was observed. The difference in earthquakes rates for February and April are statistically significant at the 95% confidence interval using the two sample Poisson rate test. The high correlations for the reservoir level and flow discharge vs. earthquake counts provides strong evidence of hydro-seismicity occurring in the ETSZ from seasonal fluctuations in reservoir level and discharge flow.
- Investigating volcano tectonic interactions in the Natron Rift of the East African Rift SystemJones, Joshua Robert (Virginia Tech, 2021-06-10)Continental rifting, like other plate tectonic processes, plays a large role in shaping the Earth's crust. Active rift zones evolve from repeated tectonic and magmatic events including volcanic activity. Through investigations of currently and previously active rifts, scientists have discovered considerable interactions between these tectonic and magmatic processes during a rift's evolution; however questions remain about these interactions especially in youthful stages of rifts. We investigate an early phase magma-rich section of the East African Rift System (EARS), named the Eastern Branch to assess volcano-tectonic interactions. The Eastern Branch of the EARS consists of volcanically rich rifts that are actively spreading the Nubian Plate, Somalian plates, and Victoria block at different evolutionary stages making it an ideal study area for volcano-tectonic interactions. Our initial investigation of active volcano-tectonic interactions centered on a rifting event that occurred between 2007-2008 in the Natron Rift, a rift segment in the southern Eastern Branch located in Northern Tanzania. This rifting event contained multiple occurrences of tectonic, magmatic, and volcanic activity in close proximity. We examine the stress transferred from these events to the Natron Fault, which is the major border fault in the area, with analytical modeling using the USGS program Coulomb 3.4. We processed Global Positioning System (GPS) data that recorded slip on the major border fault in the region in early January 2008 and test which events could generate large enough stress changes to trigger the observed slip using a previously defined threshold of 0.1 MPa. These initial models were created using simplified model parameters, such as an elastic homogeneous half-space, and find that 1) magmatically induced stress perturbations have the potential to trigger fault slip on rift border faults, 2) magmatic events have the potential to trigger strike‐slip motions on a rift border fault, and 3) the proximity of magmatic activity may affect occurrences of slip on adjacent border faults. We then further investigate volcano-tectonic interactions in the Natron Rift by testing using numerical modeling with the CIG finite element code PyLith. We systematically test how adding topography, heterogeneous materials, and various reservoir volumes to a deflating 3 km deep magma reservoir system at the active volcano Ol Doinyo Lengai can affect stress transfer to the adjacent Natron Fault. We compare eight models with variations in topography, material properties, and reservoir volumes to calculate the percent differences between the models; to test their effects on the stress change results. We find that topography plays the largest role with the effect increasing with reservoir size. Finally, we seek to improve the capability of investigating volcano-tectonic interactions in the Natron Rift at faster time- scales by improving Global Navigation Satellite System (GNSS) positioning data (latitude, longitude, and height) collection and distribution capabilities. In the final part of this work, we describe a new Python-based data broker application, GNSS2CHORDS, that can stream real-time centimeter precision displacement data distributed by UNAVCO real-time GNSS data services to an online EarthCube cybertool called CHORDS. GNSS2CHORDS is applied to the TZVOLCANO GNSS network that monitors Ol Doinyo Lengai in the Natron Rift and its interactions with the adjacent rift border fault, the Natron Fault. This new tool provides a mechanism for assessing volcano-tectonic interactions in real-time. In summary, this work provides a new avenue for understanding volcano-tectonic interactions at unprecedented, 1-second time-scales, demonstrates slip can be triggered by small stress changes from magmatic events during early phase rifting, and provides insights into the key role of volcanic topography during volcano-tectonic interactions.
- Novel Multitemporal Synthetic Aperture Radar Interferometry Algorithms and Models Applied on Managed Aquifer Recharge and Fault CreepLee, Jui-Chi (Virginia Tech, 2024-02-09)The launch of Sentinel-1A/B satellites in 2014 and 2016 marked a pivotal moment in Synthetic Aperture Radar (SAR) technology, ushering in a golden era for SAR. With a revisit time of 6–12 days, these satellites facilitated the acquisition of extensive stacks of high-resolution SAR images, enabling advanced time series analysis. However, processing these stacks posed challenges like interferometric phase degradation and tropospheric phase delay. This study introduces an advanced Small Baseline Subset (SBAS) algorithm that optimizes interferometric pairs, addressing systematic errors through dyadic downsampling and Delaunay Triangulation. A novel statistical framework is developed for elite pixel selection, considering distributed and permanent scatterers, and a tropospheric error correction method using smooth 2D splines effectively identifies and removes error components with fractal-like structures. Beyond geodetic technique advancements, the research explores geological phenomena, detecting five significant slow slip events (SSEs) along the Southern San Andreas Fault using multitemporal SAR interferometric time series from 2015-2021. These SSEs govern aseismic slip dynamics, manifesting as avalanche-like creep rate variations. The study further investigates Managed Aquifer Recharge (MAR) as a nature-engineering-based solution in the Santa Ana Basin. Analyzing surface deformation from 2004 to 2022 demonstrates MAR's effectiveness in curbing land subsidence within Orange County, CA. Additionally, MAR has the potential to stabilize nearby faults by inducing a negative Coulomb stress change. Projecting into the future, a suggested 2% annual increase in recharge volume through 2050 could mitigate land subsidence and reduce seismic hazards in coastal cities vulnerable to relative sea level rise. This integrated approach offers a comprehensive understanding of geological processes and proposes solutions to associated risks.
- Radar Imaging Applications for Mining and Landmine DetectionAbbasi Baghbadorani, Amin (Virginia Tech, 2022-08-02)The theme of this dissertation is to advance safety hazard mitigation by detecting and characterizing hidden targets of concern. Ground-penetrating radar (GPR) is used to detect and characterize hidden targets that pose safety hazards at Earth's surface, within shallow soil, and within rock. The resulting images detect unexploded ordnance (UXO) and detect fractures that pose collapse hazards in a mine. Detecting and characterizing fractures and voids within rock prior to excavation can enable mitigation of mine collapse hazards. GPR data were acquired on the wall of a pillar in an underground mine. Strong radar reflections in the field data correlate with fractures and a cave exposed on the pillar walls. Pillar wall roughness was included in migration, a wavefield imaging algorithm, to quantitatively locate fractures and voids and map their spatial relationships within the rock. Quantifying the radar reflection amplitudes enabled mapping the distance between fracture walls. Detecting and characterizing UXO and landmines from a safe distance can enable de-mining. Migration was used to improve GPR imaging for unmanned aerial vehicle (UAV) data acquisitions. Existing algorithms were adapted for UAV flight irregularities and surface topography, and a new algorithm was developed that does not depend on the unknown soil wavespeed. Errors associated with wavespeed and raypath assumptions were quantified. The algorithms were tested with real and synthetic datasets. The improved and new algorithms are more successful than previous algorithms. To detect linear targets at all orientations, fully polarized GPR data are needed. Polarity combinations were investigated to optimize the detection of surface and subsurface small targets and linear targets. Scattering caused by topographic roughness is the primary shallow subsurface noise. For subsurface targets, detection is optimized by migration plus a polarity combination that captures all scattered energy. Strong reflection and scattering from the air-ground boundary can hide surface targets. Detection is optimized by removing the strong isotropic surface scattering, imaging targets by their anisotropic scattering.
- Remote Sensing of 21st Century Water Stress for Hazard Monitoring in CaliforniaCarlson, Grace Anne (Virginia Tech, 2023-02-02)California has experienced an unusually dry past two decades punctuated by three intense multi-year droughts from 2007-2010, 2012-2015, and 2020-2022. A portion of the water lost during these two decades is due to intense groundwater overdraft of the Central Valley Aquifer. This groundwater overdraft has led to poroelastic compaction of the aquifer system and subsidence of the land surface. Water mass loss also causes elastic deformation of the solid Earth, an opposite and smaller amplitude response than the poroelastic deformation of aquifer systems. These mass changes can disturb the regional stress field, which may influence earthquake activity. Both the elastic and poroelastic deformation responses can be observed using satellite-based geodetic tools including Global Navigation Satellite System (GNSS) station displacements and Interferometric Synthetic Aperture Radar (InSAR). In this dissertation, I model aquifer-system compaction at depth using InSAR-based vertical land motion during the 2007-2010 drought and evaluate hazards related to Earth fissures, tensional cracks that form at the edges of subsidence zones. Next, I forward-calculate the predicted elastic deformation response to groundwater mass loss over the same period and calculate crustal stress change to evaluate what, if any, impact this has on seismicity in California. In addition to modeling deformation caused by water storage change, I also introduce a new method to jointly invert elastic vertical displacements at GNSS stations with water storage anomalies from the Gravity Recovery and Climate Experiment (GRACE) to solve for water storage changes from 2003-2016 over California. Finally, I expand on this joint inversion framework to include poroelastic deformation measured using InSAR over the Central Valley aquifer-system to solve for a change in water storage and groundwater storage over water years 2020-2021, the most recent drought period in California.
- Seismic Imaging of a Granitoid-Greenstone Boundary in the Paleoarchean Pilbara CratonPrasad, Anusha (Virginia Tech, 2023-03-13)The mode of tectonics by which early Archean proto-continents were deformed was investigated in the Pilbara Craton in Western Australia, which has not been substantially tectonically deformed since ~3.2 Ga. The craton consists of a unique dome and keel structure where vertical, low-grade metamorphism basaltic greenstone keels surround large granitic (TTG) domes. The dominant model for 3.5-3.2 Ga deformation in the Pilbara is gravity-driven vertical tectonics, or partial convective overturn in a hot crust. In this model, the granitic bodies rose upward as solid-state diapirs, and the greenstones "sagducted" downward around the granitic bodies. Australian scientists acquired deep seismic reflection data crossing a granitoid-greenstone boundary. Their processing did not image the geologically mapped steep dip of the boundary because standard methods limit the maximum dip. A 37-km section of these data were reprocessed using 2D Kirchhoff prestack depth migration to include vertical dips. The western half of the migrated section images a granitoid dome with weak to no reflectivity that extends deeper than 4 km. The eastern half images 2-3 km of layered volcanic rocks of the Fortescue Group overlying the greenstones. Seismic velocity models created using travel-time tomography suggest a thin weathering layer overlying slightly fractured crystalline rocks. These fractures close within 200-300 m depth, and velocity reaches bedrock speeds consistent with expected values of granitoids to the west and volcanic rocks of the Fortescue Group to the east. The best migrated image contains several reflections with dips (~45-55˚) cross-cutting each other from both directions at the location of the expected granitoid-greenstone boundary. This strongly suggests the presence of steep dips in the upper ~1.5 km but does not provide a definitive image. This inconclusive result is due to strong surface-wave noise, the crooked 2D seismic line, and the 3D nature of the geologic boundary at the seismic line. A very small seismic velocity gradient within the crystalline bedrock limits the maximum depth to which vertical features can be imaged.
- Seismic Imaging of the Global Asthenosphere using SS PrecursorsSun, Shuyang (Virginia Tech, 2023-09-21)The asthenosphere, a weak layer beneath the rigid lithosphere, plays a fundamental role in the operation of plate tectonics and mantle convection. While this layer is often characterized by low seismic velocity and high seismic attenuation, the global structure of the asthenosphere remains poorly understood. In this dissertation, twelve years of SS precursors reflected off the top and bottom of the asthenosphere, namely, the LAB and the 220-km discontinuity, are processed to investigate the boundaries of the asthenosphere at a global scale. Finite-frequency sensitivities are used in tomography to account for wave diffraction effects that cannot be modeled in global ray-theoretical tomography. Strong SS precursors reflected off the LAB and the 220-km discontinuity are observed across the global oceans and continents. In oceanic regions, the LAB is characterized by a large velocity drop of about 12.5%, which can be explained by 1.5%-2% partial melt in the oceanic asthenosphere. The depth of the Lithosphere Asthenosphere Boundary is about 120 km, and its average depth is independent of seafloor age. This observation supports the existence of a constant-thickness plate in the global oceans. The base of the asthenosphere is imaged at a depth of about 250 km in both oceanic and continental areas, with a velocity jump of about ∼ 7% across the interface. This finding suggests that the asthenosphere in oceanic and continental regions share the same defining mechanism. The depth perturbations of the oceanic 220-km discontinuity roughly follow the seafloor age contours. The 220-km topography is smoother beneath slower-spreading seafloors while it becomes rougher beneath faster-spreading seafloors. In addition, the roughness of the 220-km discontinuity increases rapidly with spreading rate at slow spreading seafloors, whereas the increase in roughness is much slower at fast spreading seafloors. This observation indicates that the thermal and compositional structures of seafloors formed at spreading centers may have a long-lasting impact on asthenospheric convections. In continental regions, a broad correlation is observed between the 220-km discontinuity depth structure and surface tectonics. For example, the 220-km discontinuity depth is shallower along the southern border of the Eurasian plate as well as the Pacific subduction zones. However, there is no apparent correlation between 3-D seismic wavespeed in the upper mantle and the depths of the 220-km discontinuity, indicating that secular cooling has minimum impact on the base of the asthenosphere.