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. In a second set of experiments, a 10 ps, 200 J Titan laser pulse was split into two beams. Comparison with simulations shows very close agreement between the pressure dependence of ionization and molecular dissociation in dynamically compressed deuterium. Backscattered x-rays bolstered this observation by measuring the free electron distribution through Compton scattering. 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. A second laser produced intense 2 keV x-rays. 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. 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. In this work we demonstrate spectrally resolved x-ray scattering from electron-plasma waves in shock-compressed deuterium and proton-heated matter. This tentative estimate of the anisotropy depth is consistent with findings in Northern Australia. 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. We find that such a layer can reproduce the observed shear wave splitting delays for reasonable values of 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. Remarkably, the results for different directions are consistent with the presence of azimuthal anisotropy. 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. 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.įinally, 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. 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. 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. 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. 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. Our anisotropy results favor a tear, which is also consistent with the geometry of the volcanic belt. 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. 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 phase velocity maps reveal lateral variations at all periods consistent with the presence of flat and steep subduction. 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. We use data from seismic networks with unprecedented dense coverage to study the Earth's structure under Mexico.įirst, 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.
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