The position of the subducting Cocos Plate beneath Mexico is unknown from the Trans-Mexican Volcanic Belt (TMVB) northward. Geochemical analysis suggests that there may be a slab tear causing mafic volcanism along the northern edge of the (TMVB). An array of 100 seismometers at 5 km spacing is currently installed in Mexico by the MesoAmerican Seismic Experiment (MASE) in conjunction with the Center for Embedded Networked Sensing (CENS) at UCLA, the Universidad Nacional Autonoma de Mexico (UNAM), and the California Institute of Technology (CIS). The data from this array is used to develop P and S wave tomographies to locate the slab. Beneath the MASE line the Cocos plate dips from the trench to a ~ 50 km depth. It is almost subhorizontal between 110 km to 275 km from the trench at a ~50 km depth. Then it continues to dip at a near constant angle.

We use data from seismic networks with unprecedented dense coverage to study the Earth's structure under Mexico.

First, we develop a three-dimensional (3-D) model of shear-wave velocity and anisotropy for the Mexico subduction zone using fundamental mode Rayleigh wave phase velocity dispersion measurements. The 3-D nature of our surface-wave-based results allows for better understanding of the interaction between the subducting slab, mantle lithosphere, and asthenosphere in the top 200 km. Our phase velocity maps reveal lateral variations at all periods consistent with the presence of flat and steep subduction. We also find that the data are consistent with two layers of anisotropy beneath Mexico: a crustal layer and a deeper layer that includes the lithosphere and asthenosphere, with the fast direction interpreted as aligned with the toroidal mantle flow around the slab edges. Our combined azimuthal anisotropy and velocity model enables us to analyze the transition from flat to steep subduction and to determine whether the transition involves a tear resulting in a gap between segments or is a continuous deformation caused by a lithospheric flexure. Our anisotropy results favor a tear, which is also consistent with the geometry of the volcanic belt.

Next, we conduct a shear wave splitting analysis that results in delay times of 1-2 s and the fast direction that coincides with the absolute plate motion for the Mesoamerican Seismic Experiment (MASE) stations as well as stations east of the MASE array. The significant difference of the anisotropy in the upper 200 km, as detected by the surface wave analysis, and the average anisotropy between the CMB and the surface, as resolved by the shear wave splitting, implies that the shear wave splitting results are dominated by a structure deeper than 200 km. Since the time delays are significantly longer for the shear wave splitting results, the deeper structure is either much larger than 200 km, or has stronger anisotropy than the top 200 km, or a combination of both. At the same time, several relatively subtle features in the shear wave splitting results reveal potential influences of the shallow structure and its deeper extensions. This includes a small change in the fast direction around the southern edge of the Trans-Mexican Volcanic Belt (TMVB), which is located above the transition from the flat to steep subduction, as well as a different pattern of fast directions west of the MASE array, the region on top of two smaller subducting slabs.

Finally, we determine phase velocities of higher modes of Rayleigh waves, in order to constrain the depth of the anisotropy revealed by the shear wave splitting. Our analysis shows that the phase velocities for a number of overtones and periods are fastest in the direction predicted by shear wave splitting, suggesting that they are affected by the same deeper structure. Remarkably, the results for different directions are consistent with the presence of azimuthal anisotropy. Inspection of obtained phase velocities together with the sensitivity kernels tentatively indicates that a layer at the 200-400 km depth is a likely candidate for the source of the anisotropy. We find that such a layer can reproduce the observed shear wave splitting delays for reasonable values of anisotropy. The 200-400 km depth likely corresponds to the bottom of the asthenosphere, and it may be affected by the plate motion, explaining why the fast shear wave splitting direction is aligned with the plate motion. This tentative estimate of the anisotropy depth is consistent with findings in Northern Australia.

Many approaches and packages have been developed for tomographic inversion of seismic data and have consistently remained in high demand in both academic and industrial geophysics for subsurface imaging. However, our survey of published implementations has shown them to suffer from shortcomings. They include: low computational efficiency leading to overreliance on gradient-based inversion methods, non-robust forward models, and overparametrization of invertible fields leading to overconfidence in parameter resolvability and susceptibility to local minima. Additionally, many suffer from poor algorithmic documentation, lacking source code availability, and complicated dependencies of many published packages, resulting in difficulty with their reproducibility, testing, and adaptation. Finally, most available packages do not incorporate different measurement sets, such as gravitational anomaly data, to better constrain seismic inversion.

To address these issues, we have developed our own implementation and integration of an improved set of algorithms for seismic and gravitational tomography and applied it to a seismic data set from the Peruvian Andes obtained in a joint experiment with Caltech between 2008 and 2013. A summary of the work completed includes the development and/or demonstration of:

An efficiently parametrized scalar field basis function incorporating 2D and 3D Natural Neighbor Interpolation specialized for mantle tomography and mapping of irregular discontinuities such as a perturbed Moho. An emphasis on perturbation continuity and smoothness makes it suitable for use with appropriately damped gradient based inversion methods.

A hybrid Eikonal solver first arrival forward model, based primarily on the Fast Marching Method (FMM)(Sethian, 1997) and utilizing key elements of Vidale Finite Difference Method (VFD)(Vidale, 1991) in order to reduce model error. We demonstrate a simple and common velocity configuration in which VFD experiences a catastrophic failure, while our variant (designated FMM-VFD) performs with well constrained and small model error.

A hybrid inversion algorithm incorporating iterating sequential application of a custom Damped Gauss-Newton (DGN) algorithm and a Markov Chain Monte Carlo (MCMC) method, which allows for problem specific balancing of inversion robustness versus efficiency. Integration of the above into a joint gravitational, local seismic, and teleseismic master inversion algorithm.

In a tangential development with potential for future integration, we developed an experimental O(n^3*log(n)) quasi-parallel and sparse implementation of an Acoustic Wave Equation solver wavefront arrival model using FMM-VFD field pre-initialization.

Full disclosure and publication of key source code, written in a maximally simplified manner with no dependencies outside the C++ standard template library, included in a fully commented form to promote its verification, adaptation, and free use by others.

Local and teleseismic arrivals, as well as the published gravitational anomaly field over Peru have been used to invert jointly for local event locations, 3D mapping of the Moho, and mantle velocity perturbation structure, showing features consistent with the subducting Nazca slab. From the obtained results, we believe to have potentially identified the region in which the subduction of the Nazca plate undergoes flattening.

SKS arrivals from OBS data of the ALBACORE experiment (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) offshore Southern California are analyzed for shear wave splitting. In a region several hundred kilometers from the Pacific Plate, splitting directions are similar to on-land directions WSW-ENE and with similar delays 1.1-1.4 seconds. Three measurements give higher delays and also W-E directions. The directions are at 45 degrees to APM of the Pacific plate suggesting that either frozen-in anisotropy from paleo-spreading dominates over APM effects in the asthenosphere or that deeper mantle shearing has occurred unrelated to APM. A toroidal flow around slab rollback presents one such possibility.

In this work we demonstrate spectrally resolved x-ray scattering from electron-plasma waves in shock-compressed deuterium and proton-heated matter. Because the spectral signature of inelastic x-ray scattering is strongly dependent on the free electron density of the system, it is used to infer ionization in dynamically heated samples. Using 2-6 ns, 500 J laser pulses from LLNL's Janus laser, we shocked liquid deuterium to pressures approaching 50 GPa, reaching compressions of 4 times liquid density. A second laser produced intense 2 keV x-rays. By collecting and spectrally dispersing forward scattered photons at 45 degrees, the onset of ionization was detected at compressions of about 3 times in the form of plasmon oscillations. Backscattered x-rays bolstered this observation by measuring the free electron distribution through Compton scattering. Comparison with simulations shows very close agreement between the pressure dependence of ionization and molecular dissociation in dynamically compressed deuterium.

In a second set of experiments, a 10 ps, 200 J Titan laser pulse was split into two beams. One created a stream of MeV protons to heat samples of boron and boron-nitride and the other pumped 4.5 keV K-alpha radiation in a titanium foil to probe the hot target. We observed scattered x-rays 300 ps after heating, noting a strong difference in average ionization between the two target materials at temperatures of 16 eV and very similar mass densities. Comparison with electron structure calculations suggests that this difference is due to a persistence of long-range ion structure in BN resulting in high-temperature band structure.

These results underscore the importance of understanding the complex electron structure of materials even at electron-volt temperatures and gigapascal pressures. Our results provide new data to guide the theoretical modeling of warm, dense matter important to understanding giant planets and inertial fusion targets.

Two hypotheses on the origin of the ‘Isabella tomographic anomaly,’ which has been interpreted as either a lithospheric delamination (Ducea and Saleeby, 1998) or a remnant of the Farallon plate attached to the Monterey micro-plate (Wang　et al., 2013), were tested. P-wave receiver functions and tomography based on teleseismic events recorded by 37 permanent stations and 4 newly installed stations were used to construct a simple geometry tomographic model of the Isabella anomaly, and to test whether the Monterey micro-plate can be connected with the location of the Isabella anomaly. A rectangular block model was used to fit the arrival-time residual pattern, and the best-fit model has its top surface located at 50km below the Great Valley. The block model dips 65º toward the Sierra Nevada with 100 km thickness. Receiver functions suggest that the remnant Farallon plate is not stalled at the base of the crust beneath the Coast Ranges as has been suggested from refraction studies (Brocher et al., 1999). Instead, the Monterey micro-plate lies in the mantle. Its top and bottom surfaces can be observed as a negative-positive dipole signal in the receiver functions. Negative-positive dipole signals at 35 km observed in the Coast Ranges' receiver functions were interpreted as the P-to-S converted phases from the Monterey micro-plate. These P-to-S converted phases from the top of the oceanic crust could be traced from the Coast Ranges to the Great Valley. The depth of the top surface of the Monterey micro-plate suggested from the receiver functions is consistent with the location of the rectangular block found from tomography, supporting the hypothesis that the Isabella anomaly is a remnant of the Farallon plate that is attached to the Monterey micro-plate. The combined results of the receiver functions and tomography study suggest that the partially subducted Monterey micro-plate extends east beyond the San Andreas fault and that the initially low angle of subduction in the west increases its dip east below the Great Valley.

Seismic scattering is the direct consequence of the changes in the elastic properties of the medium. These so-call heterogeneities play a fundamental role for the understanding of the propagation of seismic waves and their possible effect on the analysis of seismic risk. This dissertation examines the scattering properties of Middle America region at continental scale, and provides a discussion of the current methods aimed to model the effect of small-scale heterogeneities in the crust.\newline

Using data from a portable array deployed in Mexico perpendicular to the trench, we identified and modeled trapped crustal waves that were previously undocumented. When low frequency f<1Hz body phases from teleseismic events reach the trench from the ocean side, the lateral discontinuity between the continental and oceanic crust behaves as a line source that generates surface waves. This trapped mode travels with a significantly small attenuation through the crust as if it were propagating through a waveguide structure. Interestingly, conversion rates for this phase can reach up to 60% the amplitude of the incident phase in the vertical component. The scattered wave is better observed following the SP, PS, and SS phases for events in the southern hemisphere. Numerical models using a boundary element formulation support this hypothesis. Comparison between the simulations and the seismic records suggested that the structure formed by the flat-subducting slab and the crust behaves very similar to a sedimentary basin but at a larger scale. These observations could explain differences in attenuation along different paths of propagation in this area.

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