VTechWorks Repository :: Browsing by Author "Zhou, Ying"

Browsing by Author "Zhou, Ying"

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3D trench-parallel flow in the subduction region and correlation with seismic anisotropy direction
Maiti, Tannistha (Virginia Tech, 2012-07-27)
The motivation of this study is to understand the seismic anisotropy observations from various subduction regions of the world. In subduction zone backarcs both trench-parallel and trench-normal seismic anisotropy, or fast wave polarization direction of shear wave, are observed. In the mantle the general assumption is that seismic anisotropy is caused by Lattice Preferred Orientation (LPO) of olivine minerals and that the direction of anisotropy is an indicator of the direction of mantle flow. The complex pattern of seismic anisotropy observations suggests that the flow geometry in the vicinity of subduction zones differs at different subduction zones with some subduction zones having trench perpendicular flow, consistent with corner flow in the mantle wedge while other subduction zones have trench parallel flow, consistent with a mode of flow where material from the mantle wedge flows around the edges of the slab. It should be noted that the direction of LPO orientation can also be modified by the presence or absence of water, pressure, and temperature in the mantle and that it is possible that the difference in anisotropy observations reflects a difference in water content or thermal structure of back arcs. The aim of this study is to test whether the flow geometry of mantle in numerical subduction calculations can influence the direction of seismic anisotropy and if we parameters that control the pattern of flow can be identified. In this study we explicitly assume that seismic anisotropy occurs only due to plastic and dynamic re-crystallization of mantle mineral forming LPO. To approach the problem two different models are formulated. In one of the models the trench evolves self-consistently, with no prescribed artificial zones of weakness. The self-consistent model has a sticky-air layer at the top of the model domain that mimics a "free-surface." The other model has the same initial conditions but a trench-migration velocity boundary condition is imposed to the model. The mantle flow pattern for the self-consistent model is consistent with the 2D corner flow with no flow around the trench and no trench migration. However when the trench-migration velocity boundary condition is imposed, 3D flow around the mantle is observed. The stress field from these simulations are used to calculated instantaneous strain axis directions which correlate with LPO directions. The LPO orientations are measured from the models showing that the seismic-anisotropy direction is primarily trench-perpendicular for both models. Because the models have different flow patterns, the trench-perpendicular anisotropy alignment that is calculated for both the models is a bit puzzling. It could be that factors such as high temperature and non-linear rheology cause the LPO direction to align trench perpendicular in both the cases. It can also be possible that the 3D vertical flow is not strong enough to cause change in orientation of the LPO direction. From the present study it can be concluded that by looking at the LPO direction nature of mantle flow might not be predicted. This suggests that in addition to flow direction other factors such as the presence of water in mantle wedge, pressure, and high temperature due to viscous coupling modify the seismic anisotropy directions.
Age-Independent Oceanic Plate Thickness and Asthenosphere Melting From SS Precursor Imaging
Sun, Shuyang; Zhou, Ying (2023-02-01)
The Earth's asthenosphere is a mechanically weak layer characterized by low seismic velocity and high attenuation. The nature of this layer has been strongly debated. In this study, we process 12 years of seismic data recorded at the global seismological network stations to investigate SS waves reflected at the upper and lower boundaries of this layer in global oceanic regions. We observe strong reflections from both the top and the bottom of the asthenosphere, dispersive across all major oceans. The average depths of the two discontinuities are 120 and 255 km, respectively. The SS waves reflected at the lithosphere and asthenosphere boundary are characterized by anomalously large amplitudes, which require a ∼12.5% reduction in seismic velocity across the interface. This large velocity drop can not be explained by a thermal cooling model but indicates 1.5%–2% localized melt in the oceanic asthenosphere. The depths of the two discontinuities show large variations, indicating that the asthenosphere is far from a homogeneous layer but likely associated with strong and heterogeneous small-scale convection in the oceanic mantle. The average depths of the two boundaries are largely constant across different age bands. In contrast to the half-space cooling model, this observation supports the existence of a constant-thickness plate in oceanic regions with a complex and heterogeneous origin.
Body and surface wave ambient noise seismic interferometry across the Salton Sea Geothermal Field, California
Sabey, Lindsay Erin (Virginia Tech, 2015-01-13)
Virtual source gathers were generated using the principles of seismic interferometry from 135 hours of ambient noise recorded during a controlled-source survey across the Salton Sea Geothermal Field in southern California. The non-uniform nature of the noise sources violated a primary assumption of the method and generated artifacts in the data. The artifacts generated by the high-energy impulsive sources (e.g. earthquakes, shots) were removable using traditional methods of amplitude normalization prior to cross-correlation. The continuous source artifacts generated by the geothermal wells and highways required an unconventional approach of utilizing only normalized impulsive sources to successfully reduce the artifacts. Virtual source gathers were produced successfully that contained strong surface waves at 0.4-2.5 Hz, an order of magnitude below the corner frequency of the geophones, and modest body waves at 22-30 Hz, which are generally more difficult to obtain due to the need for many large, well-distributed subsurface sources. The virtual source gathers compare well to nearby explosive shots and are more densely spaced, but have a much lower signal-to-noise ratio. Analysis of the surface waves was complicated by strong higher-order modes. Spectral analysis of virtual source gathers required utilization of the geothermal plant energy, which produced usable signal at offsets required for mode separation. The virtual source dispersion curve compared well to a dispersion curve from a nearby explosive shot. P-waves were observed on the virtual source gathers. Creation of a low-quality multichannel reflection stack revealed two weak reflectors in the upper 2 km.
Comparison of 4.5 Hz Geophones and a Broadband Seismometer in a Real Field Deployment
Rasmussen, Tyler Wyatt (Virginia Tech, 2019-06-18)
An analysis of waveforms, power spectral density and array responses was performed using geophones and broadband seismometers, co-deployed as part of a geologically motivated study. Broadband seismometers record excellent waveforms but, due to cost and deployment effort, wavefields are usually spatially aliased above ~0.1 Hz. Industry rapidly deploys many thousands of inexpensive, passive geophones to record full, unaliased seismic wavefields; however, waveform quality is limited below the instrument's natural frequency of ≥2 Hz. In 2012, coincident passive and controlled-source seismic surveys were deployed to investigate tectonics in Idaho and Oregon. Broadband stations were deployed at quiet sites every 15 km, taking experienced professionals >1 person-days per station. Fifty 4.5 Hz geophones and "Texan" seismographs at 200-m spacing were deployed per person-day by inexperienced students. Geophone data were continuously recorded for 3 nights and 1 day, while broadband seismometers were deployed for ~2 years. The spectral and array responses of these real deployments were compared. For a M7.7 teleseismic event, the broadband seismometer and geophone recorded nearly identical waveforms down to <0.03 Hz (32 s) and matching power spectral density down to 0.02 Hz (50 s). For quiet ambient noise, the waveforms strongly correlate down to <0.25 Hz (4 s) and the power spectral density match to the low-frequency side of the microseismic peak at ~0.15 Hz (~7 s). By deploying a much larger number of geophones, waveforms can be stacked to reduce instrument self-noise and beamforming can be used to identify wavefield azimuth and apparent velocity. Geophones can be an effective tool in ambient noise seismology down to ~7 seconds and can be used to record large seismic events effectively down to tens of seconds, well below the natural frequency of the instruments. A well-designed deployment of broadbands and geophones can enable full wavefield studies from long period to short period. Scientific and societal applications that could benefit from the improved unaliased wavefield bandwidth include local to regional seismicity, strong ground motion, magma migration, nuclear source discrimination, and crustal studies.
Continental Arc Processes in British Columbia and Earthquake Processes in Virginia: Insights from Seismic Imaging
Wang, Kai (Virginia Tech, 2014-02-07)
Travel times from a refraction and wide-angle reflection seismic survey across the Coast Plutonic Complex and Stikine terrane of British Columbia were inverted to derive two dimensional P and S-wave seismic velocity models of the crust and uppermost mantle. A felsic upper crust and a felsic to intermediate middle crust are observed in both the batholith complex and the accreted Stikine island arc terrane. The P and S wave models demonstrate a high-velocity (P 7.0 km/s, S 3.8 km/s) layer in the lower crust beneath the youngest (late Cretaceous to Eocene) portion of the continental arc complex. In contrast, the lower crust under the Stikine terrane has lower velocities consistent with amphibolite or other hydrated mafic rocks. The Moho is at ~35 km depth under the Stikine terrane, deepens to ~38 km beneath the youngest portion of the arc, then shallows towards the coast. The high velocity zone under the younger portion of the Coast Plutonic Complex has a 1.81 Vp/Vs ratio and is interpreted to have a bulk composition of mafic garnet granulite. This garnet granulite and large volumes of granodiorite-dominated melt were created by arc dehydration melting of amphibolite (or hydrated gabbro) in the pre-existing lower crust Reverse time migration method was applied to image aftershocks recorded by a dense array deployed after the 2011 Virginia earthquake. Events as tiny as magnitude -2 were successfully imaged as point sources. The propagation of energy release as a function of time and space was observed for events larger than magnitude 2.5. Spatial resolution of the images was ~200 m, which synthetic data tests show was primarily limited by the temporal sampling rate. Improved temporal and spatial sampling could produce images with sharper resolution.
Continental Tectonics from Dense Array Seismic Imaging: Intraplate Seismicity in Virginia and a Steep Cratonic Margin in Idaho
Davenport, Kathy (Virginia Tech, 2016-09-21)
Dense array seismic techniques can be applied to multiple types of seismic data to understand regional tectonic processes via analysis of crustal velocity structure, imaging reflection surfaces, and calculating high-resolution hypocenter locations. The two regions presented here include an intraplate seismogenic fault zone in Virginia and a steep cratonic margin in eastern Oregon and Idaho. The intraplate seismicity study in Virginia consisted of using 201 short-period vertical-component seismographs, which recorded events as low as magnitude -2 during a period of 12 days. Dense array analysis revealed almost no variation in the seismic velocity within the hypocentral zone, indicating that the aftershock zone is confined to a single crystalline-rock terrane. The 1-2 km wide cloud of hypocenters is characterized by a 29° strike and 53° dip consistent with the focal mechanism of the main shock. A 5° bend along strike and a shallower dip angle below 6 km points toward a more complex concave shaped fault zone. The seismic study in Idaho and Oregon was centered on the inversion of controlled-source wide-angle reflection and refraction seismic P- and S-wave traveltimes to determine a seismic velocity model of the crust beneath this part of the U.S. Cordillera. We imaged a narrow, steep velocity boundary within the crust that juxtaposes the Blue Mountains accreted terranes and the North American craton at the western Idaho shear zone. We found a 7 km offset in Moho depth, separating crust with different seismic velocities and Poisson's ratios. The crust beneath the Blue Mountains terranes is consistent with an intermediate lithology dominated by diorite. In the lower crust there is evidence of magmatic underplating which is consistent with the location of the feeder system of the Columbia River Basalts. The cratonic crust east of the WISZ is thicker and characterized by a felsic composition dominated by granite through most of the crust, with an intermediate composition layer in the lower crust. This sharp lithologic and rheologic boundary strongly influenced subsequent deformation and magmatic events in the region.
Evidence of Hydro-Seismicity in the Tennessee Seismic Zone
King, 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.
The Evolution of the Galapagos Mantle Plume: From Large Igneous Province to Ocean Island Basalt
Trela, Jarek (Virginia Tech, 2017-04-21)
Mantle plumes are anomalously hot, narrow upwellings of mantle material that originate at the core-mantle boundary. As plumes rise they may form volumetrically large "heads" (~1000 km in diameter) with narrower (~100 km) "tails." Plume head melting is thought to form Large Igneous Provinces (LIPs), vast outpourings of basaltic lava (~106 km3), while plume tail melting forms linear chains of ocean island basalts (OIBs) similar the Emperor-Hawaii Seamount chain. Mantle plume derived melts indicate that these structures sample deep Earth geochemical and lithological heterogeneities. Studying plume-derived lavas can clarify important planetary-scale questions relating to the accretion of the Earth, primordial geochemical reservoirs, the fate of subducted materials, planetary differentiation, and convective mixing.
A Geodynamic Investigation of Continental Rifting and Mantle Rheology: Madagascar and East African Rift case studies
Rajaonarison, Tahiry A. (Virginia Tech, 2021-02-18)
Continental rifting is an important geodynamic process during which the Earth's outer-most rigid shell undergoes continuous stretching resulting in continental break-up and theformation of new oceanic basins. The East African Rift System, which has two continentalsegments comprising largely of the East African Rift (EAR) to the West and the easternmostsegment Madagascar, is the largest narrow rift on Earth. However, the driving mechanismsof continental rifting remain poorly understood due to a lack of numerical infrastructure tosimulate rifting, the lack of knowledge of the underlying mantle dynamics, and poor knowl-edge of mantle rheology. Here, we use state-of-art computational modeling of the upper660 km of the Earth to: 1) provide a better understanding of mantle flow patterns and themantle rheology beneath Madagascar, 2) to elucidate the main driving forces of observedpresent-day∼E-W opening in the EAR, and 3) to investigate the role of multiple plumesor a superplume in driving surface deformation in the EAR. In chapter 1, we simulate EdgeDriven convection (EDC), constrained by a lithospheric thickness model beneath Madagas-car. The mantle flow associated with the EDC is used to calculate induced olivine aggregates'Lattice Preferred Orientation (LPO), known as seismic anisotropy. The predicted LPO isthen used to calculate synthetic seismic anisotropy, which were compared with observationsacross the island. Through a series of comparisons, we found that asthenospheric flow result-ing from undulations in lithospheric thickness variations is the dominant source of the seismicanisotropy, but fossilized structures from an ancient shear zone may play a role in southern Madagascar. Our results suggest that the rheological conditions needed for the formationof seismic anisotropy, dislocation creep, dominates the upper asthenosphere beneath Mada-gascar and likely other continental regions. In chapter 2, we use a 3D numerical model ofthe lithosphere-asthenosphere system to simulate instantaneous lithospheric deformation inthe EAR and surroundings. We test the hypothesis that the∼E-W extension of the EAR isdriven by large scale forces arising from topography and internal density gradients, known aslithospheric buoyancy forces. We calculate surface deformation solely driven by lithosphericbuoyancy forces and compare them with surface velocity observations. The lithosphericbuoyancy forces are implemented by imposing observed topography at the model surfaceand lateral density variations in the crust and mantle down to a compensation depth of 100km. Our results indicate that the large-scale∼E-W extension across East Africa is driven bylithospheric buoyancy forces, but not along-rift surface motions in deforming zones. In chap-ter 3, we test the hypothesis that the anomalous northward rift-parallel deformation observedin the deforming zones of the EAR is driven by viscous coupling between the lithosphereand deep upwelling mantle material, known as a superplume, flowing northward. We testtwo end-member plume models including a multiple plumes model simulated using high res-olution shear wave tomography-derived thermal anomaly and a superplume model (Africansuperplume) simulated by imposing a northward mantle-wind on the multiple plumes model.Our results suggest that the horizontal tractions from northward mantle flow associated withthe African Superplume is needed to explain observations of rift-parallel surface motions indeforming zones from GNSS/GPS data and northward oriented seismic anisotropy beneaththe EAR. Overall, this work yields a better understanding of the geodynamics of Africa.
A Geodynamic Investigation of Magma-Poor Rifting Processes and Melt Generation: A Case Study of the Malawi Rift and Rungwe Volcanic Province, East Africa
Njinju, Emmanuel A. (Virginia Tech, 2021-01-12)
Our understanding of how magma-poor rifts accommodate strain remains limited largely due to sparse geophysical observations from these rift systems. To better understand magma-poor rifting processes, chapter 1 of this dissertation is focused on investigating the lithosphere-asthenosphere interactions beneath the Malawi Rift, a segment of the magma-poor Western Branch of the East African Rift (EAR). Chapter 2 and 3 are focused on investigating the sources of melt beneath the Rungwe Volcanic Province (RVP), an anomalous volcanic center located at the northern tip of the Malawi Rift. In chapter 1, we use the lithospheric structure of the Malawi Rift derived from the World Gravity Model 2012 to constrain three-dimensional (3D) numerical models of lithosphere-asthenosphere interactions, which indicate ~3 cm/yr asthenospheric upwelling beneath the thin lithosphere (115-125 km) of the northern Malawi Rift and the RVP from lithospheric modulated convection (LMC) that is decoupling from surface motions. We suggest that the asthenospheric upwelling may generate decompression melts which weakens the lithosphere thereby enabling extension. The source of asthenospheric melt for the RVP is still contentious. Some studies suggest the asthenospheric melt beneath the RVP arises from thermal perturbations in the upper mantle associated with plume head materials, while others propose decompression melting from upwelling asthenosphere due to LMC where the lithosphere is thin. Chapter 2 of this dissertation is focused on testing the hypothesis that asthenospheric melt feeding the RVP can be generated from LMC using realistic constraints on the mantle potential temperature (Tp). We develop a 3D thermomechanical model of LMC beneath the RVP and the entire Malawi Rift that incorporates melt generation. We find decompression melt associated with LMC upwelling (~3 cm/yr) occurs at a maximum depth of ~150 km localized beneath the RVP. Studies of volcanic rock samples from the RVP indicate plume signatures which are enigmatic since the RVP is highly localized, unlike the large igneous provinces in the Eastern Branch of the EAR. In chapter 3, we test the hypothesis that the melt beneath the RVP is generated from plume materials. We investigate melt generation from plume-lithosphere interactions (PLI) beneath the RVP by developing a 3D seismic tomography-based convection (TBC) model beneath the RVP. The seismic constraints indicate excess temperatures of ~250 K in the sublithospheric mantle beneath the RVP suggesting the presence of a plume. We find a relatively fast upwelling (~10 cm/yr) beneath the RVP which we interpret as a rising plume. The TBC upwelling generates decompression melt (~0.25 %) at a maximum depth of ~200 km beneath the RVP where the lithosphere is thinnest (~100 km). Our results demonstrate that an excess heat source from may be plume materials is necessary for melt generation in the sublithospheric mantle beneath the RVP because passive asthenospheric upwelling of ambient mantle will require a higher than normal Tp to generate melt. Studies of volcanic rock samples from the RVP indicate plume signatures which are enigmatic since the RVP is highly localized, unlike the large igneous provinces in the Eastern Branch of the EAR. In chapter 3, we test the hypothesis that the melt beneath the RVP is generated from plume materials. We investigate melt generation from plume-lithosphere interactions (PLI) beneath the RVP by developing a 3D seismic tomography-based convection (TBC) model beneath the RVP. The seismic constraints indicate excess temperatures of ≈ 250K in the sublithospheric mantle beneath the RVP suggesting the presence of a plume. We find a relatively fast upwelling (≈10 cm/yr) beneath the RVP which we interpret as a rising plume. The TBC upwelling generates decompression melt (≈0.25 %) at a maximum depth of ≈200 km beneath the RVP where the lithosphere is thinnest (≈100 km). Our results demonstrate that an excess heat source from may be plume materials is necessary for melt generation in the sublithospheric mantle beneath the RVP because passive asthenospheric upwelling of ambient mantle will require a higher than normal Tp to generate melt.
Geophysical Imaging of Earth Processes: Electromagnetic Induction in Rough Geologic Media, and Back-Projection Imaging of Earthquake Aftershocks
Beskardes, Gungor Didem (Virginia Tech, 2017-06-04)
This dissertation focuses on two different types of responses of Earth; that is, seismic and electromagnetic, and aims to better understand Earth processes at a wider range of scales than those conventional approaches offer. Electromagnetic responses resulting from the subsurface diffusion of applied electromagnetic fields through heterogeneous geoelectrical structures are utilized to characterize the underlying geology. Geology exhibits multiscale hierarchical structure which brought about by almost all geological processes operating across multiple length scales and the relationship between multiscale electrical properties of underlying geology and the observed electromagnetic response has not yet been fully understood. To quantify this relationship, the electromagnetic responses of textured and spatially correlated, stochastic geologic media are herein presented. The modelling results demonstrate that the resulting electromagnetic responses present a power law distribution, rather than a smooth response polluted with random, incoherent noise as commonly assumed; moreover, they are examples of fractional Brownian motion. Furthermore, the results indicate that the fractal behavior of electromagnetic responses is correlated with the degree of the spatial correlation, the contrasts in ground electrical conductivity, and the preferred orientation of small-scale heterogeneity. In addition, these inferences are also supported by the observed electromagnetic responses from a fault zone comprising different lithological units and varying wavelengths of geologic heterogeneity. Seismic signals generated by aftershocks are generally recorded by local aftershock networks consisted of insufficient number of stations which result in strongly spatially-aliased aftershock data. This limits aftershock detections and locations at smaller magnitudes. Following the 23 August 2011 Mineral, Virginia earthquake, to drastically reduce spatial aliasing, a temporary dense array (AIDA) consisting of ~200 stations at 200-400 m spacing was deployed near the epicenter to record the 12 days of the aftershocks. The backprojection imaging method is applied to the entire AIDA dataset to detect and locate aftershocks. The method takes advantage of staking of many seismograms and improves the signal-to-noise ratio for detection. The catalog obtained from the co-deployed, unusually large temporal traditional network of 36 stations enabled a quantitative comparison. The aftershock catalog derived from the dense AIDA array and the backprojection indicates event detection an order of magnitude smaller including events as small as M–1.8. The catalog is complete to magnitude –1.0 while the traditional network catalog was complete to M–0.27 for the same time period. The AIDA backprojection catalog indicate the same major patterns of seismicity in the epicentral region, but additional details are revealed indicating a more complex fault zone and a new shallow cluster. The b-value or the temporal decay constant were not changed by inclusion of the small events; however, they are different for two completeness periods and are different at shallow depth than greater depth.
Global Structure of the Mantle Transition Zone Discontinuities and Site Response Effects in the Atlantic and Gulf Coastal Plain
Guo, Zhen (Virginia Tech, 2019-09-03)
This thesis focuses on two different topics in seismology: imaging the global structures of the mantle transition zone discontinuities and studying the site response effects in the Atlantic and Gulf Coastal Plain. Global structures of the mantle transition zone discontinuities provide important constraints on thermal structures and dynamic processes in the mid mantle. In this thesis, global topographic structures of the 410- and 660-km discontinuities are obtained from finite-frequency tomography of SS precursors. The finite-frequency sensitivities of SS waves and precursors are calculated based on a single-scattering (Born) approximation and can be used for data selection. The new global models show a number of smaller-scale features that were absent in back-projection models. Good correlation between the mantle transition zone thickness and wave speed variations suggests dominantly thermal origins for the lateral variations in the transition zone. The high-resolution global models of the 410- and 660-km discontinuities in this thesis show strong positive correlation beneath western North America and eastern Asia subduction zones with both discontinuities occurring at greater depths. Wavespeed and anisotropy models support vertical variations in thermal structure in the mid mantle, suggesting return flows from the lower mantle occur predominantly in the vicinity of stagnant slabs and the region overlying the stagnant slabs. In oceanic regions, the two discontinuities show a weak anti-correlation, indicating the existence of a secondary global far-field return flow. The Atlantic and Gulf Coastal Plain is covered by extensive Cretaceous and Cenozoic marine sediments. In this thesis, the site response effects of sediments in the Coastal Plain region relative to the reference condition outside that region are investigated using Lg and coda spectral ratios. The high-frequency attenuation factors (kappa) in the Coastal Plain are strongly correlated with the sediment thickness. At frequencies between 0.1-2.86 Hz, the Lg spectral ratio amplitudes are modeled as functions of frequency and thickness of the sediments in the Coastal Plain. Analysis of the residuals from the stochastic ground motion prediction method suggests that incorporating the site response effects as functions of sediment thickness may improve ground motion prediction models for the Coastal Plain region.
Hypocenter Locations and Focal Mechanism Solutions of Earthquakes in the Epicentral Area of the 1886 Charleston, SC, Earthquake
Hardy, Anna Corella (Virginia Tech, 2015-02-03)
The Charleston earthquake of 1886 was one of the largest shocks to occur on the eastern coast of North America. The geological cause has long been a controversial issue and a variety of source models have been proposed. Previous potential field modeling and reinterpretation of seismic reflection and well data collected in the early 1980s indicate that the crust between approximately 1 and 4.5 km depth is comprised primarily of Mesozoic mafic rocks, with extensive faulting that is spatially coincident with modern seismicity in the epicentral area (Chapman and Beale, 2010). This thesis proposes a new and testable hypothesis concerning the fault source of the 1886 shock that is very different from all previous interpretations. It is based on data collected during 2011-2012 from a local seismic network deployment in the immediate epicentral area. The 8-station temporary network was designed to better constrain earthquake hypocenter locations and focal mechanisms. Hypocenter locations of 134 earthquakes indicate a south-striking, west-dipping seismogenic zone in the upper 12 km of the crust. Over 40% of the 66 well-constrained focal mechanisms show reverse faulting on approximately north-south trending nodal planes, consistent with the orientation of the tabular hypocenter distribution. I offer the following hypothesis: The 1886 shock occurred by compressional reactivation of a major, south-striking, west-dipping early Mesozoic extensional fault. The modern seismicity can be regarded as a long-term aftershock sequence that is outlining the 1886 damage zone. Variability of shallow focal mechanisms is due to the complex early Mesozoic fault structure in the upper 4-5 km.
Imaging Resolution of the 410-km and 660-km Discontinuities
Deng, Kai (Virginia Tech, 2014-08-26)
The structure of seismic discontinuities at depths of about 410 km and 660 km provides important constraints on mantle convection as the associated mineral phase transformations in the transition zone are sensitive to thermal perturbations. Teleseismic P-to-S receiver functions have been widely used to map the depths of the two discontinuities. In this study, we investigate the resolution of receiver functions in imaging topographic variations of the 410-km and 660-km discontinuities based on wave propagation simulations using a Spectral Element Method (SEM). We investigate finite-frequency effects of direct P waves as well as P-to-S converted waves by varying the length scale of discontinuity topography in the transition zone. We show that wavefront healing effects are significant in broadband receiver functions. For example, at a period of 10 to 20 seconds, the arrivaltime anomaly in P-to-S converted waves is about 50% of what predicted by ray theory when the topography length scale is in the order of 400 km. The observed arrival anomaly further reduces to 10-20% when the topography length scale reduces to about 200 km. We calculate 2-D boundary sensitivity kernels for direct P waves as well as receiver functions based on surface wave mode summation and confirm that finite frequency-effects can be properly accounted for. Three-dimensional wavespeed structure beneath seismic stations can also introduce significant artifacts in transition zone discontinuity topography if time corrections are not applied, and, the effects are dependent on frequency.
New Constraints on Fault-Zone Structure from Seismic Guided Waves
Wu, Jiedi (Virginia Tech, 2008-09-01)
The structure of fault zones (FZs) plays an important role in understanding fault mechanics, earthquake rupture and seismic hazards. Fault zone seismic guided waves (GW) carry important information about internal structure of the low-velocity fault damage zone. Numerical modeling of observed FZGWs has been used to construct models of FZ structure. However, the depth extent of the waveguide and the uniqueness of deep structure in the models have been debated. Elastic finite-difference synthetic seismograms were generated for FZ models that include an increase in seismic velocity with depth both inside and outside the FZ. Strong GWs were created from sources both in and out of the waveguide, in contrast with previous homogenous-FZ studies that required an in-fault source to create GW. This is because the frequency-dependent trapping efficiency of the waveguide changes with depth. The near-surface fault structure efficiently guides waves at lower frequencies than the deeper fault. Fault structure at seismogenic depth requires the analysis of data at higher frequencies than the GWs that dominate at the surface. Adapting a two-station technique from surface wave studies, dispersive differential group arrival times between two earthquakes can be used to solve for FZ structures between the earthquakes. This method was tested with synthetic data and shallow events recorded in the SAFOD borehole in the San Andreas Fault. A pair of deep earthquakes recorded in the SAFOD borehole indicate a ~150 m wide San Andreas Fault waveguide with >20% velocity contrast at 10-12 km depth. With additional earthquakes, the full FZ structure at seismogenic depth could be imaged. Subsurface FZ structure can also be derived from a surface source and receiver array analogous to a body-wave refraction survey. Synthetic seismograms for such source-receiver geometry were generated and verified that FZGWs are refracted by the increase in velocity with depth. Synthetic data from a surface array were successfully inverted to derive FZ structure in the subsurface. The new methods presented in this dissertation extend the potential of FZGWs to image deeper FZ structure than has been uniquely constrained in the past.
Reduced Firing of Nucleus Accumbens Parvalbumin Interneurons Impairs Risk Avoidance in DISC1 Transgenic Mice
Zhou, Xinyi; Wu, Bifeng; Liu, Wenhao; Xiao, Qian; He, Wei; Zhou, Ying; Wei, Pengfei; Zhang, Xu; Liu, Yue; Wang, Jie; He, Jufang; Zhang, Zhigang; Li, Weidong; Wang, Liping; Tu, Jie (2021-06-18)
A strong animal survival instinct is to approach objects and situations that are of benefit and to avoid risk. In humans, a large proportion of mental disorders are accompanied by impairments in risk avoidance. One of the most important genes involved in mental disorders is disrupted-in-schizophrenia-1 (DISC1), and animal models in which this gene has some level of dysfunction show emotion-related impairments. However, it is not known whether DISC1 mouse models have an impairment in avoiding potential risks. In the present study, we used DISC1-N terminal truncation (DISC1-N-TM) mice to investigate risk avoidance and found that these mice were impaired in risk avoidance on the elevated plus maze (EPM) and showed reduced social preference in a three-chamber social interaction test. Following EPM tests, c-Fos expression levels indicated that the nucleus accumbens (NAc) was associated with risk-avoidance behavior in DISC1-N-TM mice. In addition, in vivo electrophysiological recordings following tamoxifen administration showed that the firing rates of fast-spiking neurons (FS) in the NAc were significantly lower in DISC1-N-TM mice than in wild-type (WT) mice. In addition, in vitro patch clamp recording revealed that the frequency of action potentials stimulated by current injection was lower in parvalbumin (PV) neurons in the NAc of DISC1-N-TM mice than in WT controls. The impairment of risk avoidance in DISC1-N-TM mice was rescued using optogenetic tools that activated NAcPV neurons. Finally, inhibition of the activity of NAcPV neurons in PV-Cre mice mimicked the risk-avoidance impairment found in DISC1-N-TM mice during tests on the elevated zero maze. Taken together, our findings confirm an impairment in risk avoidance in DISC1-N-TM mice and suggest that reduced excitability of NAcPV neurons is responsible.
Seismic Imaging of the Global Asthenosphere using SS Precursors
Sun, 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.
Seismic Source and Attenuation Studies in the Central and Eastern United States
Wu, Qimin (Virginia Tech, 2017-05-16)
To better understand the ground motion and associated seismic hazard of earthquakes in the central and eastern United States (CEUS), this dissertation focuses on the source parameters and wave propagation characteristics of both tectonic earthquakes and induced earthquakes in the CEUS. The infrequent occurrence of significant earthquakes in the CEUS limits the necessary observations needed to understand earthquake processes and to reduce uncertainty in seismic-hazard maps. The well-recored aftershock sequence of the 2011 Mineral, Virginia, earthquake offers a rare opportunity to improve our understanding of earthquake processes and earthquake hazard in this populous region of the United States. Moreover, the rapid increase of seismicity in the CEUS since 2009 that has been linked to wastewater injection has raised concern regarding the potential hazard. In this dissertation, I first present a detailed study of the aftershock sequence of the 2011 Mw 5.7 Mineral, Virginia earthquake. It involves the hypocenter locations of ~3000 earthquakes, ~400 focal mechanism solutions, statistics of the aftershock sequence, and the Coulomb stress modeling that explains the triggering mechnanism of those aftershocks. Second, I examine the S-wave attenuation at critical short hypocentral distances (< 60 km) using the aftershock data. The observed S-wave amplitudes decay as a function of hypocenter distance R according to R^-1.3 - R^-1.5, which is substantially steeper than R^-1 for a homogeneous whole space. Finally, I propose and apply a stable multi-window coda spectral ratio method to estimate corner frequencies and Brune-type stress drops for the 2011 Mineral, Virginia mainshock and aftershocks, as well as induced earthquakes in Oklahoma. The goal of this comparative study is to find out whether or not there are systematical differences in source parameters between tectonic earthquakes and induced earthquakes in the CEUS. I found generally much higher stress drops for the Mineral, Virginia sequence. However, the stress drops for those induced earthquakes in Oklahoma exhibit large varation among individual earthquake sequences, with the large mainshocks having high stress drops (20-30 MPa, Brune-type) except for the 2011 Mw 5.6 Prague, Oklahoma earthquake. And spatially varying stress drops indicates strong fault heterogeneity, which in the case of induced earthquakes may be influenced by the injection of fluids into the subsurface.
Surface Wave Propagation and Global Crustal Tomography
Liu, Kui (Virginia Tech, 2014-02-11)
In this thesis, a finite-frequency theory is developed to calculate Born sensitivity kernels for Rayleigh-wave phase and amplitude measurements that are valid in regions near seismic stations. Calculations of sensitivity kernels for inter-station measurements show that exact travelling-wave representation of Green tensor is necessary when station spacing is close to or smaller than the seismic wavelength. This finite-frequency theory will allow us to take advantage of dense seismic arrays to obtain high-resolution surface-wave tomography using inter-station measurements. The non-linear dependence of surface wave phase upon large perturbations in crustal thickness as well as finite-frequency effects in global surface-wave tomography are investigated using wave propagation simulations. Calculations show that non-linearity as well as finite-frequency effects can be accounted for by using 2D phase-velocity kernels for boundary perturbations. A 3D-reference tomographic approach is developed for iterative inversions of global crustal structure where Frechet kernels are calculated in 3D reference models. A global dataset of minor-arc and major-arc Rayleigh wave dispersion measurements at periods between 25 seconds and 100 seconds are built and global phase velocity maps based on the dataset are obtained using diffractional tomography. The phase velocity model confirms many general features associated with surface tectonics including the ocean-continent dichotomy and the signature of lithospheric cooling in oceanic plates. There are significant differences between the phase velocity model and calculations based on a current global model CRUST2.0+S20RTS in oceanic regions, Archean and Proterozoic cratons as well as orogenic belts. In addition, the high resolution phase velocity maps reveal a major change in the distribution of small scale anomalies in the Pacific at different wave periods.
Surface wave propagation in 3-D anelastic media
Ruan, Youyi (Virginia Tech, 2012-08-28)
Lateral perturbations in anelasticity (Q) and wave speed together provide important constraints on thermal and chemical structures in the mantle. In present-day tomography studies of global wave speed and anelasticity, the significance of 3-D wave speed and 3-D Q structures on surface wave travel times and amplitudes has not been well understood. In this dissertation, the effects of lateral perturbations in anelasticity (Q) and wave speed on surface wave observables are quantified based upon wave propagation simulations in 3-D earth models using a Spectral Element Method. Comparison between phase delays caused by 3-D wave speed structures and those caused by 3-D Q variations show that anelastic dispersion due to lateral perturbation in Q is important in long-period surface wave and can account for 15-20% observed phase delays. For amplitude perturbations, elastic focusing/defocusing effects associated with 3-D wave speed structures are dominant while energy dissipation is important in short-period (~ 50 s) surface waves but decreases quickly with increasing wave period. Anelastic focusing/defocusing associated with 3-D anelastic dispersion becomes more important than wave attenuation in longer period surface waves. In tomography studies, ray theory breaks down and finite frequency effects become important when the length scale of heterogenities are smaller than seismic wavelength. Finite frequency effects in 3-D earth models are investigated by comparing theoretical predictions of travel times and amplitudes with "ground truth" measurements made on synthetic seismograms generated in SEM simulations. The comparisons show that finite frequency effects are stronger in amplitudes than in phases, especially at long periods.

Browsing by Author "Zhou, Ying"

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