Browsing by Author "Morgan, Julia"
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Item Can Deep Learning Predict Complete Ruptures in Numerical Megathrust Faults?(Rice University, 2021-08-18) Blank, David; Morgan, Julia; Earth, Environmental, and Planetary SciencesThis dataset accompanies the paper, "Can Deep Learning Predict Complete Ruptures in Numerical Megathrust Faults?". The directory contains full python codes used to create Convolutional Neural Networks and Long Short Term Memory Recurrent Neural Networks, along with the raw data used for training, testing, and validation. Since stochastic initialization of weights in the training process will result in slightly different parameterization of models trained with identical hyperparameters, we have also included the trained models we presented in the paper with this dataset as well.Item Controls on Coseismic Rupture and Tsunami Potential Resulting from Great Megathrust Earthquakes: Insights from Discrete Element Simulations(2021-04-27) Wang, Xiaoyu; Morgan, JuliaUsing the numerical Discrete Element Method (DEM), I investigate potential correlations of pre-existing forearc structural features and mechanical properties to the megathrust earthquake sizes in subduction zones. My three projects provide good complements to field observation and laboratory experiments. Moreover, my results help establish a pool of candidate features that can be used for future seismic hazard assessments. My first project seeks to reproduce stress changes that occur after a large main shock, and explores the conditions that could cause stress switching both on- and offshore Tohoku. My simulations demonstrate that rapid fault weakening can produce stress change and predominant normal-fault earthquake mechanisms in the upper plate involved in the Tohoku-Oki earthquake. Several specific conditions seem to favor such stress-switching; the megathrust fault must have been frictionally strong before the main shock, and comparable values of internal and basal friction are necessary to cause the formation of the widespread normal faults within the wedge. This study also confirms that numerical simulations that incorporate dynamic upper plate extension are more appropriate for investigating stress changes associated with rapid fault weakening in the Tohoku area, than are steady-state models based on Critical Coulomb Wedge (CCW) models. Two adjacent segments of the South-Central (SC) Chile margin exhibit significant differences in earthquake magnitudes and rupture extent, during the 1960 Valdivia earthquake and 2010 Maule earthquake. My second project, informed by interpretations of the structure across these two segments, simulates the upper plate as wedges overlying megathrust faults that are partitioned into two frictional domains, modeled after dynamic Coulomb wedge models. The velocity strengthening outer wedge width strongly influences megathrust rupture extents, whereas the frictional conditions beneath the fixed strength or velocity-strengthening outer wedge can affect the size of megathrust earthquakes. My preferred models yield reasonable fits to published slip distributions, in particular, for the 2010 Maule rupture. The simulated slip distribution for the Valdivia earthquake, suggests that the margin probably experienced its highest slip close to the trench, which differs from published models. My final project focuses on how the activation of megasplay faults within the wedge can affect earthquake sizes and tsunami genesis. I model the upper plate as a wedge that is partitioned into inner (velocity-weakening) and outer (velocity-strengthening) wedges, combined with a splay fault rooting from the decollement. I examine the effects of the dip and friction along the splay fault, along with the width of outer (velocity-strengthening) wedge during seismic cycles. The splay faults can accommodate coseismic slip thus facilitating the generation of tsunamis. However, the presence of a velocity-strengthening outer wedge constrains the rupture size and tsunami generation. Moreover, my selected model, which best fits the derived slip distribution for the 2011 Tohoku earthquake, shows the reactivation of a megasplay fault moderately affects earthquake coseismic rupture and tsunami potential. In contrast, my selected model for the 2010 Maule earthquake, reasonably matches the published slip distribution for that event, suggesting that the activation of megasplay fault has minimal impact on earthquake size and tsunami potential along the SC Chile Margin.Item From mantle shear-zones to crustal detachment surfaces: Geophysical investigations of rifting at continental margins(2022-12-02) Nguyen, Luan Chan; Morgan, Julia; Levander, AlanThis dissertation contributes to the understanding of the tectonic evolution of the lithosphere as it undergoes extensional deformation during continental rifting. The study consists of two main projects carried out over two rifted margins using distinct geophysical methods to probe the subsurface at different scales. Over the Gulf of Mexico and its bounding margins, Rayleigh surface-wave tomography from the cross-correlation of the seismic ambient noise field was used to construct a 3D velocity model for the top 150 km. The model unveils a significant feature in the region beneath the extended terrain of the northwestern Gulf of Mexico. This anomalous pinch-and-swell structure in the mantle lithosphere is interpreted as geologic boudinage that reflects the deformational history of the continental lithosphere as it was stretched and deformed. Our analysis, in combination with previous findings, demonstrate that geologic boudins play an important role in enhancing localized deformation leading to the well-documented asymmetric geometry of conjugate rifted margins worldwide. We infer that boudinage development is influenced by inherited thickness of the lithosphere and by the process of mantle refertilization triggered by melt infiltration from the ascending asthenosphere during rifting. Lithospheric boudinage proves to be a lasting feature that persists long after the breakup of continents such that it provides a means to investigate other rifted margins around the world. In the second part of this project, we show that crustal variation along the US Gulf Coast margin as well as the structural asymmetry observed across its conjugate Yucatan margin are consistent with the scenarios in which rifting was accompanied by mantle shear-zones. The direction of plate motion is determined to have changed significantly during the transition from rift to drift which is attributed to the development of deep shear-zones in the western part of the rift. Our estimated thickness of the lithosphere indicates that the extended continental lithosphere over the Gulf of Mexico margins has mostly regained its thickness since the time of breakup. The topic of shear-zones, but in the brittle regime, is revisited in the context of rifting at the Galicia margin at depth of less than 12 km below sea level , where 3D active-source reflection seismic imaging clearly shows the morphology of the S reflector, a major detachment surface that facilitated the motion of the overlying crustal blocks during continental extension. The nature of the rock layer above this surface contains a record of block displacements and can yield insight rifting kinematics. Defining the physical properties of this rock layer has been a challenge in previous studies. Here, we applied the analysis of amplitude-variation-with-offset to the S reflector to determine the elastic properties of the rock layer immediately overlying S. We identified a wide distribution of areas of low elastic properties above the S reflector, consistent with the presence of a fault gouge. Our derived rock densities and Vp/Vs ratios indicate that the gouge’s composition is highly heterogeneous throughout the study area, but with systematic distributions that reflect the evolution of the fault system. Our results suggest an increasing level of mantle serpentinization within the fault gouge toward the eastern part of the Galicia margin, an indication of a longer history of mantle hydration attributed to water ingress along early-formed crustal faults. In addition, we infer from the correlation between gouge composition and thickness that heterogenuous fault strength resulting from differential serpentinization of the mantle detachment surface may have affected block displacement and overall geometry of the rifted margin.Item Influence of Geometry and Deformation History on the Slip Behavior along the South Flank of Kilauea Volcano(2022-08-04) Fortener, Holly; Morgan, JuliaThe mobile south flank of Kilauea Volcano on the island of Hawaii is a mechanically complex area. Based on field observations and seismic survey results, a collection of findings reports a pattern of behavior at the south flank: fast-slip behavior (i.e., earthquakes) at the northeastern portion and slow-slip or aseismic displacement at the central portion of the south flank. The notable differences in flank morphology and deformation history at these two areas inspire the following questions: does flank geometry influence the type of slip behavior observed at each area? Alternatively, or in concert, did the removal of the ancient landslide from the embayment, alter the stress state to favor slow-slip events in this area? We use numerical simulations to examine differences in deformation behavior of the northeastern, central, and southwestern portions of Kilauea’s south flank. Our results compare well to nature in unexpected ways, with interesting implications for future south flank slip behavior. We also explore how the sudden removal of an analogue Hilina Slump from the southwestern portion of the south flank might affect the slip behavior and compare this to present-day behavior of the central portion of the south flank, which currently experiences repeating slow-slip.Item Insights on Formation of the Gulf of Mexico by Rayleigh Surface Wave Imaging(Wiley, 2022) Nguyen, Luan C.; Levander, Alan; Niu, Fenglin; Morgan, Julia; Li, GuoliangWe used cross-correlation of ambient noise records from seismic stations in the US, Mexico, and Cuba to extract dispersion data of Rayleigh surface wave. Our derived 3D shear-wave velocity model of the greater Gulf of Mexico (GOM) region captures variations in the crustal and lithospheric structures across the continental margins of the US Gulf Coast and Yucatan, Mexico. The model shows a zone of reduced velocity in the mantle lithosphere underlying the extended continental margin of the northwestern GOM. We attributed this velocity reduction to extensional deformation and melt-induced weakening of the lithosphere during the Triassic continental rifting that preceded the seafloor spreading that formed the GOM. Melt extraction might have been hindered by the greater lithospheric thickness in the western region along the US Gulf Coast margin that resulted in the westward decrease of rift-related volcanism/magmatism reported from previous studies. The clear asymmetry between the US Gulf Coast and its conjugate Yucatan margin suggests extension along a shear-zone that focused more deformation on the North American plate prior to breakup. In contrast to the counterclockwise rotation of the Yucatan block during seafloor-spreading, our analyses using deformable plate models demonstrate that continental rifting occurred in a predominantly northwest-southeast direction. This change in plate motion is attributed to the development of mantle shear-zones in the western part of the rift. We estimated the depth of the lithosphere-asthenosphere boundary and determined that the extended continental and oceanic lithospheres have mostly regained their thickness since the time of breakup.Item Insights to slip behavior on rough faults using discrete element modeling(American Geophysical Union, 2012) Fournier, Thomas; Morgan, Julia[1] We simulate a range of fault slip behaviors using the discrete element method (DEM) to examine the controls on different slip modes on rough faults. Shear strain is imposed upon a 2-D bonded particle assemblage that contains a predefined fault. Slip modes on the fault vary from creep, to slow-slip, to stick-slip behavior, both spatially and temporally. The mode of slip is controlled largely by the local stress field along the fault, which depends on the local fault roughness. Portions of the fault that fail in relatively low normal stress regimes tend to slide continuously, whereas areas with high clamping stress produce stick-slip events. During stick-slip events, regions within the rupture zone that experience high slip are associated with physical asperities on the fault; ruptures terminate at barriers and through dissipation of the stored elastic energy. The simulated events show stress drops between 0.2–50 MPa, a slightly larger range than is inferred for natural earthquakes. Simulated events also have higher slip magnitudes than are observed during earthquakes for a given rupture length. The simulation produces many characteristics of fault behavior and is shown to be a successful avenue for future studies on the mechanics of fault slip.Item Investigating Earthquake Rupture Using Particle Dynamics Simulations(2020-11-30) Blank, David G; Morgan, JuliaThe three chapters of this thesis explore the physics that control earthquakes, particularly their precursors, using numerical models. In the first chapter, we use the discrete element method to create numerical analogs to subduction megathrusts with natural roughness and heterogeneous fault friction. Boundary conditions simulate tectonic loading, inducing fault slip. Intermittently, slip develops into complex rupture events that include foreshocks, mainshocks, and aftershocks. We probe the kinematics and stress evolution of the fault zone to gain insight into the physical processes that govern these phenomena. Prolonged, localized differential stress drops precede dynamic failure, a phenomenon we attribute to the gradual unlocking of contacts as the fault dilates prior to rupture. Slip stability in our system appears to be governed primarily by geometrical phenomena, which allow both slow and fast slip to take place at the same areas along the fault. Similarities in slip behavior between simulated faults and real subduction zones affirm that modeled physical processes are also at work in nature. In the second chapter, we develop numerical simulations of dynamic perturbations passing along a heterogeneous pre-stressed fault, to better understand the physical mechanisms that control delayed dynamic triggering of earthquakes. Our results demonstrate that weak portions of the fault that host ongoing slow slip can transfer stress in response to the perturbations, loading asperities poised for failure. We find that the magnitude of perturbation, the state of the asperity, as well as deformation of the surrounding material, jointly control the delay time between perturbation and triggered event. The slow-slip modulated delayed triggering model that we propose can account for the wide range of observed delay times in nature, including the two end-member cases of no delay and no triggering. Triggered slow slip events in nature might provide warning signs of impending earthquakes, underscoring the importance of high-resolution monitoring of active fault zones. In the third chapter, we present a new approach to characterizing precursory earthquake slip using machine learning. We use a binary classification model rooted in state-of-the-art deep learning techniques to predict whether or not complete-interface rupture is imminent along a simulated megathrust fault. The deep learning models are trained on labeled 2D space-time input features taken from the synthetic fault system. We contrast the performance of two different neural networks trained on three different types of data, in order to determine the relative predictive power of each. The neural networks are able to discriminate imminent complete rupture precursors from everything else, thus providing a relative size and time forecast. Vertical displacements along the fault demonstrate particularly good predictive power and improved performance is seen when multiple model predictions are used in tandem. The results confirm previous observations that precursory deformation scales with upcoming event size, consistent with the preslip model for earthquake nucleation. The methods we propose are adaptable and can be modified to use 3D data in the future.Item Seismic evidence for lithospheric boudinage and its implications for continental rifting(The Geological Society of America, 2022) Nguyen, Luan C.; Levander, Alan; Niu, Fenglin; Morgan, Julia; Li, GuoliangThe continental rifting that precedes the breakup of a continent and the formation of a new ocean basin is one of the key processes of plate tectonics. Although often viewed as a two-dimensional process, rifted margins exhibit significant variations along strike. We document along-strike variations developed during the ca. 200–160 Ma continental rifting that formed the margins of the Gulf of Mexico ocean basin. Rayleigh-wave ambient noise tomography reveals a zone of high and low seismic velocity resembling large scale geologic boudins in the mantle lithosphere of the northwestern Gulf of Mexico margin. These features become progressively less prominent eastward following the transition from a magma-poor to a magma-rich passive margin. We infer that mantle refertilization and thickness of the pre-rift lithosphere control deformation style and the along-strike variations in continental rifting. Our results also suggest that deformation during rifting produces long-lived features that persist long after breakup and, therefore, can be used to study rifted margins globally.