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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenSat, 13 Apr 2024 01:50:59 +0000Matching Waveform Envelopes for Earthquake Early Warning
https://resolver.caltech.edu/CaltechTHESIS:11112020-213135157
Authors: {'items': [{'email': 'beckyheeroh@gmail.com', 'id': 'Roh-Becky', 'name': {'family': 'Roh', 'given': 'Becky'}, 'orcid': '0000-0002-3905-0086', 'show_email': 'YES'}]}
Year: 2021
DOI: 10.7907/hw8k-zx98
<p>Current earthquake early warning (EEW) algorithms are continuously optimized to strive for fast, accurate source parameter estimates for the rupturing earthquake (i.e. magnitude, location), which are then used to predict ground motions expected at a site. However, they may still struggle with challenging cases, such as offshore events and complex sequences. An envelope-based two-part search algorithm is developed to handle such cases. This algorithm matches different templates to the incoming observed ground motion envelopes to find the optimal earthquake source parameter estimates.</p>
<p>The algorithm consists of two methods. Method I is the standard grid search, and it uses Cua-Heaton ground motion envelopes as its templates; Method II is the extended catalog search, and its templates are waveform envelopes from past real and synthetic earthquakes. The grid search is intended for robustness and provides approximate average solutions, whereas the extended catalog search matches envelopes considering the station’s specific site and path effects. In parallel execution, Methods I and II work together – either by confirming each other’s solutions or accepting the solution with stronger fits – to provide the best parameter estimates based on waveform-based data.</p>
<p>The main advantage of the two-part search algorithm is its ability to find parameter estimates of reduced uncertainties using the P-wave data from a single station. Many algorithms wait until multiple stations are triggered to reduce tradeoffs between the magnitude and location. This waiting time, however, is detrimental in EEW, for it jeopardizes the warning time that can be issued to nearby regions expected to experience strong shaking. The use of a single station would virtually eliminate this waiting time, maximizing the warning time without the cost in accuracy of the estimates.</p>
<p>Because EEW is a race against time, further actions are taken for more rapid estimation of the earthquake source parameters. A Bayesian approach using prior information has the potential to reduce uncertainties that arise in the initial time points due to tradeoffs between the magnitude and location. This essentially increases the confidence of the initial parameter estimates, allowing alerts to be issued faster. A KD tree nearest neighbor search is also introduced to reduce latency in the time it takes to find the best-fitting solutions. In comparison to an exhaustive, brute-force search, it cuts the searching time by only examining through a fraction of the total database.</p>
<p>An envelope-based algorithm examines the shape and relative frequency content and makes appropriate judgments, just as a human seismologist would; it also addresses the issue of data transmission latencies. Overall, this algorithm is able to interpret the complexity of earthquakes and assess the features they hold to ultimately communicate information of significant ground shaking to different regions.</p>https://thesis.library.caltech.edu/id/eprint/13998Mechanical Interactions Between Water and the Solid Earth: from Quasi-Static Geodetic Deformation to Dynamic Fault Slip
https://resolver.caltech.edu/CaltechTHESIS:05302022-071239478
Authors: {'items': [{'email': 'larochelle.stacy@gmail.com', 'id': 'Larochelle-Stacy', 'name': {'family': 'Larochelle', 'given': 'Stacy'}, 'orcid': '0000-0001-6161-5605', 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/2r5a-9277
<p>Mechanical interactions between Earth's solid interior and its hydrosphere are central to many geophysical problems of crucial societal importance: Changing conditions in the global water cycle deform the solid Earth; the groundwater storage capacity of aquifer systems is controlled by its interaction with geological materials; and crustal water - either natural occurring or added through anthropogenic activities - affects earthquakes and fault slip processes. In this thesis, we investigate some of these interactions by harnessing recent developments in the fields of satellite geodesy, statistical data analysis and elastodynamic earthquake modelling. We start by developing a procedure to identify and extract seasonal deformation signals associated with hydrological loading of the solid Earth from geodetic time series in Chapter 1. In Chapters 2 and 3, we consider the examples of the Ozarks Plateau (central United States) and Sacramento Valley (California) to establish a methodology for characterizing poroelastic deformation arising from groundwater variations with space-based geodesy. Then, in Chapter 4, we develop a model to simulate fault slip due to crustal water injections and calibrate it against a well-instrumented field experiment on a natural fault. We conclude by deriving a theoretical understanding of these fault slip simulations by considering the simple case of a fixed-length pressurized zone in Chapter 5. Overall, our work provides key insights for extracting and using different sources of hydrogeodetic signals as well as for modeling and understanding fluid-induced fault slip processes, which is becoming increasingly important in a world faced with water scarcity, a changing climate and an increased reliance on groundwater and geoenergy resources.</p>https://thesis.library.caltech.edu/id/eprint/14651Towards Accurate and Automated Detection and Quantification of Localized Methane Point Sources on a Global Scale
https://resolver.caltech.edu/CaltechTHESIS:08092021-200722718
Authors: {'items': [{'email': 'siraput.trin@gmail.com', 'id': 'Jongramrungruang-Siraput', 'name': {'family': 'Jongramrungruang', 'given': 'Siraput'}, 'orcid': '0000-0002-2477-2043', 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/ab33-7a98
<p>Methane (CH<sub>4</sub>) is the second most important anthropogenic greenhouse gas with a significant impact on radiative forcing, tropospheric air quality, and stratospheric water vapor. Because methane has a much shorter lifetime compared to carbon dioxide (CO<sub>2</sub>), reduction in methane emission is deemed a key target for climate mitigation strategies in upcoming decades. One crucial step in emission reduction is determining the location and emission rate of localized methane sources. Remote-sensing instruments using absorption spectroscopy have emerged as one promising solution for measuring atmospheric CH<sub>4</sub> concentration over large geographical areas. However, the identification and quantification of local point sources based on the observed methane column enhancement distribution has proven challenging due to uncertainties in the knowledge of local wind speed and retrieval errors arising from surface spectral interferences and instrument noise. In this thesis, it is shown how plume morphology based on a 2-D image of methane column enhancement can be used to quantify the source emission rate directly without relying on any ancillary data such as local wind speed measurements. Large eddies simulations (LES) are utilized to create realistic synthetic plume observations under various atmospheric conditions. Using this data, a deep learning model named MethaNet is trained to predict emission rates directly from 2-D methane plume images. The model achieves a level of performance for quantifying methane emission rates that is state-of-the-art for a method that does not rely on wind speed information. Obtaining methane column measurements with low precision error and bias is a key step for separating real plume enhancements from artefacts and enhancing the quantification performance. Here an instrument tradeoff analysis is presented to assess the effect of changing instrument specifications and retrieval parameters. It is shown how the retrieval errors can be mitigated with optimal spectral resolutions and a larger polynomial degree to approximate surface albedo variations in the retrieval process. The results in this thesis contribute towards building an enhanced monitoring system that can measure CH<sub>4</sub> enhancement fields and determine methane sources accurately and efficiently at scale.</p>https://thesis.library.caltech.edu/id/eprint/14319Resolving Earthquake Source Complexities in the Heterogeneous Earth
https://resolver.caltech.edu/CaltechTHESIS:05202022-003950365
Authors: {'items': [{'email': 'jiazhe868@gmail.com', 'id': 'Jia-Zhe', 'name': {'family': 'Jia', 'given': 'Zhe'}, 'orcid': '0000-0003-0652-2646', 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/na72-6395
<p>While the commonly used simple assumptions of sources and structures allows useful first-order approximation of earthquakes, they are increasingly insufficient in characterizing the complex earthquake ruptures and the seismic wave propagations. In this thesis, I present studies that address both the source and structural complexities, as well as their interactions, using flexible parameterizations and ideas.</p>
<p>For large earthquakes, I develop a subevent inversion framework to determine their spatiotemporal rupture complexities, and applied it to multiple significant earthquakes. Our method does not assume a fault geometry and kinematic history, and incorporates Bayesian analysis for uncertainty assessments. In Chapter 2, I discovered that the 2018 Fiji Magnitude 8 deep earthquake doublet actually ruptured two slabs, which demonstrates local slab temperature as the critical factor for deep earthquakes, and reveals complex interactions between slabs. In Chapter 3, I determined that the 2019 Ridgecrest Magnitude 7 sequence coseismically ruptured orthogonal faults, and has superficially complex but in-depth simple fault geometries, which illustrates the fault geometrical control of the rupture behaviors which challenges traditional seismic hazard mapping. In Chapter 4, I found the 2021 South Sandwich Island Magnitude 8 earthquake consists of deep regular ruptures connected by a slow tsunamigenic event, which highlights the tsunami potential for deep initiations of megathrust earthquakes.</p>
<p>For smaller earthquakes, I develop a Bayesian differential moment tensor inversion (diffMT) algorithm to remove the common earth structural effects, thus improving the earthquake focal mechanism resolvability. In Chapter 5, I demonstrated that diffMT reduces the moment tensor uncertainties substantially than traditional direct-inversion methods, and found that the North Korea nuclear tests in 2013-2016 are more dominated by explosive mechanism than previous understandings.</p>
<p>Towards high resolution mapping of the Earth structure, I expand the seismic tomography for high resolution basin structure by combining the wide aperture of seismic stations and high density of industrial arrays. In Chapter 6, I applied this approach on the densely populated Los Angeles Basin, and found improved mapping of small scale heterogeneities, which can potentially promote earthquake ground motion assessments.</p>
<p>In summary, I have developed research tools and applied novel ideas on complex seismic events and heterogeneous earth environments. The results illustrate the diverse controlling factors of complex earthquake ruptures, and reveal the complex interactions between earthquakes and earth structure.</p>https://thesis.library.caltech.edu/id/eprint/14588Seismic Wavefield Imaging of the Earth: the Regional, the Local, and the Remote
https://resolver.caltech.edu/CaltechTHESIS:07192021-182024547
Authors: {'items': [{'email': 'jcastillo@google.com', 'id': 'Castellanos-Jorge-Alberto-Castillo', 'name': {'family': 'Castellanos', 'given': 'Jorge Alberto Castillo'}, 'orcid': '0000-0002-0103-6430', 'show_email': 'YES'}]}
Year: 2022
DOI: 10.7907/fm32-1t74
<p>In this thesis, I use seismic wavefield methods to illuminate the interior structure and the dynamics of the Earth across different scales. First, I image the large-scale lithospheric structure at the eastern sector of the Trans-Mexican Volcanic Belt to constrain on the transition from flat to steeper subduction in central Mexico. Then, I move to a regional scale and image the dynamics of the Wallowa Mountain block in northeastern Oregon, where mantle-based stresses appear to have played an essential role in shaping the crustal structure. With the findings of this investigation, I was able to illuminate a deformation mechanism of mantle origin, which I also use here to explain other near-surface processes in different parts of the North America continent. After, I move to a local scale, where I use dense oil-industry instrumentation to image the sub-kilometer crustal structure of Long Beach, California. In the first part of this investigation, I use noise-derived surface waves to create a high-resolution shear wave velocity model of the first kilometer of the crust, which I use to numerically determine the variability in the expected ground shaking intensity of the area. In the second part, I move past the traditional surface wave analysis and use the body wave portion of the noise-derived Green's functions to create a high-resolution compressional wave velocity model beneath one of the surveys. Finally, I present a waveform-based method of analysis that shows great promise as a new way of investigating the seismic behavior and the physical conditions of isolated marine environments.</p>https://thesis.library.caltech.edu/id/eprint/14308Modeling and Parameterization of Basin Effects for Engineering Design Applications
https://resolver.caltech.edu/CaltechTHESIS:03312022-021127047
Authors: {'items': [{'email': 'peyman.ayoubi@gmail.com', 'id': 'Ayoubi-Peyman', 'name': {'family': 'Ayoubi', 'given': 'Peyman'}, 'orcid': '0000-0001-6795-4923', 'show_email': 'YES'}]}
Year: 2022
DOI: 10.7907/4e61-q346
<p>The term "Basin effects" refers to trapped and reverberating earthquake waves in soft sedimentary deposits overlying convex depressions of the basement bedrock, which significantly alter the frequency content, amplitude, and duration of seismic waves. This has played an important role in shaking duration and intensity in past earthquakes such as the M<sub>w</sub> 8.0 1985 Michoácan, Mexico, M<sub>w</sub> 6.9 1995 Kobe, Japan, and M<sub>w</sub> 7.8 2015 Gorkha, Nepal earthquakes. While the standard practice is to perform a 1D analysis of a soil column, edge-effect and surface waves are among the key contributors to the surface ground motion within a basin. This thesis studies basin effects in a 2D medium to help better understand the phenomena, better parameterize them, and suggest a path to appropriately incorporate them in ground motion prediction equations and building design codes. After the introduction in Chapter 1, I present the results in three main parts as follows:</p>
<p>In Chapter 2, we perform an extensive parametric study on the characteristics of surface ground motion associated with basin effects. We use an elastic idealized-shaped medium subjected to vertically propagating SV plane waves and examine the effects of basin geometry and material properties. We specifically study the effects of four dimensionless parameters, the width-to-depth (aspect) ratio, the rock-to-soil material contrast, a dimensionless frequency that quantifies the depth of the basin relative to the dominant incident wavelength, and a dimensionless distance that quantifies the distance of the basin edges relative to the dominant wavelength. Our results show that basin effects can be reasonably characterized using at least three independent parameters, each of which can significantly alter the resultant ground motion. To demonstrate the application of dimensional analysis applied here, we investigate the response of the Kathmandu Valley during the 2015 M<sub>w</sub> 7.8 Gorkha Earthquake in Nepal using an idealized basin geometry and soil properties. Our results show that a simplified model can capture notable ground motion characteristics associated with basin effects.</p>
<p>Chapter 3 uses the identified parameters from the previous chapter to estimate surface acceleration time-series given earthquake frequency content, basin geometry and material properties, and location inside a basin. This is of practical use when the amount of available data is limited or the fast estimation of time-series is desirable. For that, we train a neural network to estimate surface ground acceleration time-series across a basin. Three input parameters are needed for the estimation: basin-to-bedrock shear wave velocity ratio, aspect ratio of the basin, and dimensionless location. These parameters define an idealized-shaped basin and the location at which the time-series are to be computed. It will be shown that the model performs with high accuracy in comparison to the result of a full-fidelity Finite Element (FE) simulation (ground truth) and generalizes reasonably well for input parameters outside of the training set. Moreover, we will also use the model for the case of Kathmandu Valley, Nepal during the 2015 M<sub>w</sub> 7.8 Gorkha earthquake and compare the results of NN versus recordings of the mainshock, similarly to Chapter 2.</p>
<p>Once we have studied basin behavior in a homogeneous case in previous chapters, we focus on material representation inside a basin in Chapter 4. Here, we study basin effects for the cases where high-frequency response and realistic material representation are desirable. However, the lack of sufficient information about the material properties and stratigraphy of a basin prevents accurate simulation of the phenomena. To do that, we perform a stochastic analysis using the Monte Carlo technique, where a random field represents basin material. Similarly to the previous chapters, we use a 2D FE model with an idealized basin subjected to vertically propagating SV plane waves and investigate the spatial variation of surface ground motion (SGM) associated with basin effects by assuming different realizations of the correlated random field. We then study various correlation lengths, coefficients of variations, and autocorrelation functions to evaluate their contribution to SGM. We show that the coefficient of variation is the most influential parameter on SGM, followed by correlation lengths and type of autocorrelation function. Increasing the coefficient of variation not only affects the mean surface amplification, but also results in a dramatic change in the standard deviation. Correlation lengths and autocorrelation functions, on the other hand, are of less importance for the cases we examine in this study.</p>https://thesis.library.caltech.edu/id/eprint/14535Investigating the Earthquake Cycle on Multiple Temporal and Spatial Scales Using Satellites and Simulations
https://resolver.caltech.edu/CaltechTHESIS:08082022-055217161
Authors: {'items': [{'email': 'ollie.stephenson@outlook.com', 'id': 'Stephenson-Oliver-Laurent', 'name': {'family': 'Stephenson', 'given': 'Oliver Laurent'}, 'orcid': '0000-0002-5509-090X', 'show_email': 'YES'}]}
Year: 2023
DOI: 10.7907/ha9m-4p17
The motion of the Earth's tectonic plates creates a gradual accumulation of stress at their boundaries, followed by a rapid release in earthquakes, a process known as the earthquake cycle. Studying this process is important because of the hazards earthquakes pose, but presents challenges due to the multi-scale nature of the problem—stresses build up over hundreds to thousands of years, while earthquakes break narrow fault zones in a matter of seconds. In this thesis, we combine a variety of techniques to study the earthquake cycle on multiple temporal and spatial scales, including satellite-based interferometric synthetic aperture radar (InSAR) to observe the slow deformation of the Earth over wide areas, and high-performance computational simulations to model faults during earthquakes. We begin by presenting a method for removing the signal of plate-tectonic motion in large-scale InSAR measurements, allowing for better observation of small ground deformations. We then use these corrections to study the Makran subduction zone, on the Iran-Pakistan border. Our InSAR-derived ground velocity map can resolve motions at the level of millimeters per year over an area of nearly one million square kilometers, and we use it to place constraints on the degree of coupling on the subduction megathrust. Next, we show how InSAR can be combined with deep learning techniques to rapidly map earthquake damage in all weather conditions, day and night. Such products will hopefully prove useful in future disaster response. Finally, we present computational simulations of dynamic earthquake ruptures with enhanced dynamic weakening due to thermal pressurization. We apply our simplified model to the creeping section of the San Andreas Fault, which is generally thought to be a barrier to earthquake rupture. Our results show how thermal pressurization can allow earthquakes to propagate partially or completely through the creeping section for a range of physically reasonable parameters. Our work illustrates how results from multiple fields can be combined to deliver new insights into the earthquake cycle and the hazards that it poses.https://thesis.library.caltech.edu/id/eprint/14998From Tectonic Evolution to Intraplate Stress: The Role of Structural Inheritance and Long-Wavelength Loading
https://resolver.caltech.edu/CaltechTHESIS:11302023-021244845
Authors: {'items': [{'email': 'ejhightower3@gmail.com', 'id': 'Hightower-Erin-J', 'name': {'family': 'Hightower', 'given': 'Erin J.'}, 'orcid': '0000-0002-4734-5159', 'show_email': 'NO'}]}
Year: 2024
DOI: 10.7907/xc1b-ke51
<p>In this thesis, I present a multifaceted exploration of various aspects of deformation and stress in the Earth's lithosphere using a variety of methods in a range of tectonic environments. I begin by examining the evolution of a young subduction zone through a combination of gravity modeling and seismological observations. Chapter 2 details the development a linear 3-D gravity inversion method capable of modelling complex geological regions such as subduction margins. Our procedure inverts satellite gravity to determine the best-fitting differential densities of spatially discretized subsurface prisms in a least-squares sense. We use a Bayesian approach to incorporate both data error and prior constraints based on seismic reflection and refraction data. Based on these data, Gaussian priors are applied to the appropriate model parameters as absolute equality constraints. To stabilize the inversion and provide relative equality constraints on the parameters, we utilize a combination of first and second order Tikhonov regularization, which enforces smoothness in the horizontal direction between seismically constrained regions, while allowing for sharper contacts in the vertical. We apply this method to the nascent Puysegur Trench, south of New Zealand, where oceanic lithosphere of the Australian Plate has under-thrust Puysegur Ridge and Solander Basin on the Pacific Plate since the Miocene. These models provide insight into the density contrasts, Moho depth, and crustal thickness in the region. The final model has a mean standard deviation on the model parameters of about 17 kg/m<sup>-3</sup>, and a mean absolute error on the predicted gravity of about 3.9 mGal, demonstrating the success of this method for even complex density distributions like those present at subduction zones. The posterior density distribution versus seismic velocity is diagnostic of compositional and structural changes and shows a thin sliver of oceanic crust emplaced between the nascent thrust and the strike slip Puysegur Fault. However, the northern end of the Puysegur Ridge, at the Snares Zone, is predominantly buoyant continental crust, despite its subsidence with respect to the rest of the ridge. These features highlight the mechanical changes unfolding during subduction initiation.</p> <p>Chapter 3 explores the earthquake interevent time distribution. Earthquakes are commonly assumed to result from a stationary Poisson (SIP) process. We reassess the validity of this assumption using the Quake Template Matching (QTM) catalog and the relocated SCSN catalog (HYS) for Southern California. We analyze the interevent time (IET) distribution and the Schuster spectra after declustering with the Zaliapin and Ben Zion (2013) method. Both catalogs exhibit fat-tails on the IET distribution, deviating from the expected exponential distribution. The Schuster spectra of the catalogs are also inconsistent with an SIP process. The QTM catalog shows a statistically significant seasonal signal and a drift in the Schuster probability at long periods, likely due to increased seismicity following the 2010 El Mayor-Cucapah earthquake. This increase is also evident in the yearly IET distributions of the catalog. In contrast, the HYS Schuster spectrum does not show seasonality, but the yearly IET distributions exhibit a decrease in seismicity rate over the duration of the catalog, likely due to seismic network upgrades around 1990. We use synthetic catalogs to test the origin and significance of the observed deviations from the Poisson model. Variations in the QTM annual seismicity rate, around 5.6%, are too small to generate a noticeable departure from an exponential distribution, and the SIP model can not be rejected at the 5% significance level. The synthetic catalogs also suggest the fat-tail is an artefact of incomplete declustering. Overall, variations in the IET distribution for southern California are probably the result of both 1) incomplete declustering and location uncertainty, and 2) transient non-stationarity of the background rate from viscoelastic effects of large earthquakes. However, the stationary Poisson model appears adequate for describing background seismicity at the scale of Southern California and the decadal time scale of the QTM catalog.</p> <p>Chapters 4 and 5 cover the primary focus of this thesis, exploring the influence of long-wavelength loading on the stress field of continental interiors and intraplate seismicity. The continental interior of eastern North America in particular has hosted many significant historical earthquakes and is undergoing both glacial isostatic adjustment (GIA) and long-wavelength subsidence due to the sinking of the Farallon slab. The regional seismicity concentrates within ancient failed rift arms and other paleo-tectonic structures, which can act as weak zones in the crust where stress accumulates. Within some of these zones, focal mechanism stress inversion shows significant rotational deviation of the maximum horizontal stress (S<sub>Hmax</sub>) direction from the regional NE-SW trend, which may be explained by long-wavelength stress perturbations in the presence of lithospheric weakness. We focus on two sources of intraplate stress perturbation and seismicity and test the hypotheses that 1) mantle-flow induced epeirogenic subsidence and 2) GIA contribute to intraplate seismicity in eastern North America via reactivation of pre-existing faults.</p> <p>For the slab loading component of this work, we use high-resolution global, spherical finite-element flow models with CitcomS. To capture realistic temperature fields and the Farallon slab, we convert seismic tomography models to temperature using a mineralogically constrained depth-dependent scaling factor. We utilize laterally variable temperature-dependent viscosities, upon which we superimpose low-viscosity plate boundary weak zones, as well as lithospheric intraplate weak zones at the locations of failed rifts and other inherited structures in eastern North America. We parameterize the Farallon slab in terms of its buoyancy to determine the degree to which the flow induced by the sinking slab contributes to intraplate stress. Using the modeled stress tensors from instantaneous flow calculations, we compute S<sub>Hmax</sub>, the stress magnitudes, and the Coulomb failure stress on mapped faults in several major seismic zones. Slab sinking drives localized mantle flow beneath the central-eastern U.S., leading to a stress amplification of 100-150 MPa across the region that peaks over the New Madrid Seismic Zone. This stress amplification introduces a pronounced continent-wide clockwise rotation of the predicted S<sub>Hmax</sub> direction, reaching as much as 20° in some seismic zones, particularly when lithospheric weak zones are included. In the New Madrid, Central Virginia, Charlevoix, and Lower Saint Lawrence Seismic Zones, the presence of weak zones loaded by the Farallon slab at depth can explain the pattern of clockwise rotation of the observed focal mechanism derived S<sub>Hmax</sub> relative to the regional borehole derived S<sub>Hmax</sub> as reported in previous studies. However, misfits on S<sub>Hmax</sub> within many of the major seismic zones suggest other sources of stress are needed to properly reproduce the observed stress trends in some areas. We also find that in order for pre-existing lithospheric weak zones to exert appreciable control on intraplate stress under the influence of mantle flow, they must be shallow/sub-crustal and in contact with the crust. These stress perturbations and rotations ultimately bring faults in the NMSZ, the Western Quebec Seismic Zone (WQSZ), and the Lower Saint Lawrence and Charlevoix Seismic Zones closer to failure. In particular, inclusion of the Farallon slab and weak zones produces positive Coulomb failure stresses on some key faults associated with major historical earthquakes, including the Reelfoot Fault in the NMSZ and the Timiskaming fault in the WQSZ. Fault instability is even more likely when assuming weaker faults with lower coefficients of friction.</p> <p>For the glacial unloading component of this work, we use the global, spherical finite element code CitcomSVE, which models dynamic deformation of a viscoelastic and incompressible planetary mantle in response to surface loading. We supply CitcomSVE with the same seismically constrained viscosity structures computed in the CitcomS models, including those with weak zones, and load the Earth model with the ICE-6G ice history. We perform the same suite of simulations and stress analyses as in the mantle loading problem, using the stress tensor output of the corresponding CitcomS model as the tectonic background stress. We compare the mantle flow and GIA induced stresses, with focus on the present day extant glacially derived stress field. GIA induced stress perturbations are small (~10 MPa), even in the presence of lithospheric weak zones. GIA induced S<sub>Hmax</sub> alone exhibits a transition from clockwise to counterclockwise rotation moving northeast across the continent. We find that only by inclusion of the mantle flow derived background stress can we reproduce the continental scale clockwise stress rotation observed in stress data, suggesting the effect of mantle loading is more important for explaining these observations than is GIA. In the NMSZ, GIA helps promote stability on the Reelfoot Fault, in opposition to mantle flow, while promoting instability on more non-optimally oriented faults. GIA also helps localize higher Coulomb failure stress within the Charlevoix Seismic Zone and the western half of the WQSZ. In the WQSZ and LSLRS, GIA stress perturbations are large enough that even with only a small reduction in the coefficient of friction, faults that are not likely to fail under the background tectonic and geodynamic stresses alone could slip. Further investigation of the sensitivity of GIA stress to different 3D and 1D viscosity structures and the change in GIA stress with time since deglaciation is warranted to better understand how GIA affects intraplate seismicity. Ultimately, constraining how mantle flow and GIA affect stress and deformation in the presence of laterally variable viscosity is integral to quantifying how long-wavelength loading may alter the spatial distribution of seismic hazard.</p>https://thesis.library.caltech.edu/id/eprint/16254