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.
" }, { "name": "Muir, Jack Broderick", "degree": "PhD", "year": "2022", "title": "Model Parameterization and Model Selection in Geophysical Inverse Problems. Designing Inverse Problems that Respect a priori Geophysical Knowledge", "advisor": "Tsai, Victor C.; Clayton, Robert W.; Zhan, Zhongwen", "url": "https://resolver.caltech.edu/CaltechTHESIS:10202021-003229377", "creators": [ { "name": { "family": "Muir", "given": "Jack Broderick" }, "id": "Muir-Jack-Broderick", "orcid": "0000-0003-2617-3420", "display_name": "Muir, Jack Broderick" } ], "advisors": [ { "name": { "family": "Tsai", "given": "Victor C." }, "id": "Tsai-V-C", "orcid": "0000-0003-1809-6672", "role": "advisor", "display_name": "Tsai, Victor C." }, { "name": { "family": "Clayton", "given": "Robert W." }, "id": "Clayton-R-W", "orcid": "0000-0003-3323-3508", "role": "advisor", "display_name": "Clayton, Robert W." }, { "name": { "family": "Zhan", "given": "Zhongwen" }, "id": "Zhan-Zhongwen", "orcid": "0000-0002-5586-2607", "role": "co-advisor", "display_name": "Zhan, Zhongwen" } ], "committee": [ { "name": { "family": "Jackson", "given": "Jennifer M." }, "id": "Jackson-J-M", "orcid": "0000-0002-8256-6336", "role": "chair", "display_name": "Jackson, Jennifer M." }, { "name": { "family": "Tsai", "given": "Victor C." }, "id": "Tsai-V-C", "orcid": "0000-0003-1809-6672", "role": "member", "display_name": "Tsai, Victor C." }, { "name": { "family": "Clayton", "given": "Robert W." }, "id": "Clayton-R-W", "orcid": "0000-0003-3323-3508", "role": "member", "display_name": "Clayton, Robert W." }, { "name": { "family": "Zhan", "given": "Zhongwen" }, "id": "Zhan-Zhongwen", "orcid": "0000-0002-5586-2607", "role": "member", "display_name": "Zhan, Zhongwen" } ], "option_major": [ "geophys" ], "doi": "10.7907/203d-yx49", "abstract": "The vast majority of the Earth system is inaccessible to direct observation. Consequently, the structure and dynamics of the Earth can only be determined indirectly, via geophysical sensing. These methods have the mathematical form of an inverse problem, in which the data and the unknowns are linked by a physical process, such as seismic wave propagation. From the possibly noisy data, we have indirect access to the unknowns. The vast majority of geophysical inverse problems are ill-posed, and require the provision of a priori knowledge to stabilize the solution. This thesis investigates methods for designing inverse problems to better take advantage of geophysical or geological constraints, to allow better resolution or more interpretability of the solutions. Four major themes are investigated: In Chapter 2, we study the collection of a novel dataset of Rayleigh wave horizontal-to-vertical ratios to provide stronger constraints on upper-crustal structure in Southern California. In Chapters 3 and 4, we develop a method for wavefield-reconstruction of sparse seismic data, including heterogeneous networks consisting of both displacement and strain instruments. This method amounts to an inversion in data-space, and promises to unlock the potential of wavefield based methods for complex datasets. In Chapters 5 and 6, we investigate a new structural parameterization based on a combination of Gaussian processes and the level-set method, that better models discontinuous geological features such as sedimentary basins. We test our method on a variety of synthetic and real datasets, culminating in a detailed study of the northeastern Los Angeles basin, which we found to be significantly deeper and steeper than in previous models. Finally, we develop a method of model selection for noisy historical datasets, which we investigate using the case study of correcting Oldham's data misinterpretation in the 1906 paper that \"discovered\" Earth's core.
" }, { "name": "Lambert, Val\u00e8re R\u00e9gis Westbrooke", "degree": "PhD", "year": "2021", "title": "Constraining Earthquake Source Processes Through Physics-Based Modeling", "advisor": "Lapusta, Nadia", "url": "https://resolver.caltech.edu/CaltechTHESIS:05202021-190145895", "creators": [ { "name": { "family": "Lambert", "given": "Val\u00e8re R\u00e9gis Westbrooke" }, "id": "Lambert-Valere-Regis-Westbrooke", "orcid": "0000-0002-6174-9651", "display_name": "Lambert, Val\u00e8re R\u00e9gis Westbrooke" } ], "advisors": [ { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "orcid": "0000-0001-6558-0323", "role": "advisor", "display_name": "Lapusta, Nadia" } ], "committee": [ { "name": { "family": "Avouac", "given": "Jean-Philippe" }, "id": "Avouac-J-P", "orcid": "0000-0002-3060-8442", "role": "member", "display_name": "Avouac, Jean-Philippe" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "orcid": "0000-0001-6558-0323", "role": "member", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Simons", "given": "Mark" }, "id": "Simons-M", "orcid": "0000-0003-1412-6395", "role": "chair", "display_name": "Simons, Mark" }, { "name": { "family": "Zhan", "given": "Zhongwen" }, "id": "Zhan-Zhongwen", "orcid": "0000-0002-5586-2607", "role": "member", "display_name": "Zhan, Zhongwen" } ], "option_major": [ "geophys" ], "doi": "10.7907/7s93-k485", "abstract": "Determining principles and conditions governing motion along faults is crucial for assessing how earthquake ruptures start and how large they may ultimately become. This thesis aims to shed light on the physics governing earthquake source processes by (i) developing physics-based numerical models that combine geological observations and laboratory insight with theoretical developments, and (ii) using these models to examine how different physical mechanisms and conditions are reflected in a range of geophysical observations taken together, from heat-flow constraints and seismologically determined properties of earthquakes to geodetic inferences and earthquake frequency-magnitude statistics.
\r\n\r\nWe examine the behavior and observable characteristics of numerically simulated sequences of earthquakes and aseismic slip in fault models designed to reproduce well-known features of mature faults that produce large destructive earthquakes. In part, the models are consistent with the inferred low-stress, low-heat operation of mature faults, which host large earthquakes at much lower levels of stress than their expected static strength. We explore two potential explanations for such behavior, one that faults are indeed quasi-statically strong but experience dramatic weakening during earthquakes, or that faults are persistently weak, e.g., due to fluid overpressure. We find that the two classes of fault models can, in principle, be distinguished based on the amount of seismic energy radiated from earthquake ruptures. Dynamic ruptures in the form of self-healing pulses, which occur on quasi-statically strong but dynamically weak faults, result in much larger radiated energy than inferred teleseismically for megathrust events, whereas crack-like ruptures on persistently weak faults are consistent with the seismological observations. The larger radiated energy of self-healing pulses is similar to limited regional inferences for crustal strike-slip faults. Our results suggest that re-evaluating estimates of radiated energy and static stress drop would provide substantial insight into the driving physics of large earthquakes and the absolute stress conditions on faults, with potential differences between tectonic settings.
\r\n\r\nThe results also have significant implications for seismic hazard, since our modeling shows that fault models that experience efficient dynamic weakening during ruptures tend to predominantly produce large earthquakes, at the expense of smaller earthquakes. Such behavior is consistent with some mature fault segments, such as several segments of the San Andreas Fault in California that have hosted large earthquakes but are currently nearly seismically quiescent. These considerations can provide physical basis for improving earthquake early warning systems. If mature faults in California are indeed governed by enhanced dynamic weakening, then our results suggest that the likelihood of an earthquake on these faults becoming substantially larger is much higher than typical expectations based on Gutenberg-Richter statistics.
\r\n\r\nBy considering average fault stress before simulated earthquake ruptures, we find that critical stress conditions for earthquake occurrence depend on the size and style of motion (e.g. the degree of slip acceleration at the rupture front) during individual ruptures. In particular, the stress conditions required to propagate large earthquake ruptures can be considerably lower than those required for rupture nucleation, and standard notions of quasi-static fault strength based on laboratory studies. Our results demonstrate that the critical stress for earthquake occurrence is not governed by a simple condition such as a certain level of Coloumb stress, as commonly used in studies of stress interactions among faults and earthquake aftershocks patterns. More robust criteria for critical stress conditions would depend on the strength evolution during dynamic rupture and can be explored in numerical simulations.
\r\n\r\nFinally, evaluating the predictive power of numerical earthquake models for future hazards is a topic of great importance for physics-based seismic hazard assessment. Towards that end, we investigate the sensitivity of outcomes from numerical simulations of sequences of earthquakes and aseismic slip, including the long-term interaction of fault segments, to choices in numerical discretization and treatment of inertial, wave-mediated effects. In particular, we find that the rate of earthquake ruptures that manage to jump between two fault segments, a parameter routinely used in seismic hazard studies, is highly sensitive to numerical and physical modeling choices. These results suggest the need for developing different parameterization of seismic hazard than currently used, a task for which numerical modeling is well-suited.
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