From Tectonic Evolution to Intraplate Stress: The Role of Structural Inheritance and Long-Wavelength Loading

Citation

Hightower, Erin J. (2024) From Tectonic Evolution to Intraplate Stress: The Role of Structural Inheritance and Long-Wavelength Loading. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/xc1b-ke51. https://resolver.caltech.edu/CaltechTHESIS:11302023-021244845

Abstract

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^-3, 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.

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.

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_Hmax) 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.

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_Hmax, 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_Hmax 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_Hmax relative to the regional borehole derived S_Hmax as reported in previous studies. However, misfits on S_Hmax 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.

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_Hmax 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.

Thesis Files