Azimuthal Anisotropy From Multimode Waveform Modeling Reveals Layering Within the Antarctica Craton

The isotropic structure of the crust and upper mantle under Antarctica has been constrained by many studies. However, the depth dependence of seismic anisotropy, a powerful tool to characterize deformation and flow, is still poorly known. Here, we modeled three-dimensional (3-D) variations in azimuthal anisotropy under Antarctica using a multimode Rayleigh waveform fitting technique. We first searched the model space with a reversible-jump Markov Chain Monte Carlo approach to find path-averaged vertically polarized shear wave velocity profiles that fit fundamental and higher mode Rayleigh waveforms. We then inverted them to obtain a 3-D velocity and azimuthal anisotropy model across the region down to 600 km depth. Our results reveal that the east-west dichotomy found in other studies is not only characterized by different wave velocities but also by different anisotropy directions, likely reflecting the different deformation histories of the two blocks. Azimuthal anisotropy was found to be present in the top 300 km only and peaks at 100 - 200 km depth under the East Antarctica craton. Additionally, depth changes in fast direction were observed within the craton between 75 km and 150 km depth, suggesting layering is present. We speculate this layering relates to the formation history of the craton.submitted to Seismic

Azimuthal Anisotropy From Multimode Waveform Modeling Reveals Layering Within the Antarctica Craton

The isotropic structure of the crust and upper mantle under Antarctica has been constrained by many studies. However, the depth dependence of seismic anisotropy, a powerful tool to characterize deformation and flow, is still poorly known. Here, we modeled three-dimensional (3-D) variations in azimuthal anisotropy under Antarctica using a multimode Rayleigh waveform fitting technique. We first searched the model space with a reversible-jump Markov Chain Monte Carlo approach to find path-averaged vertically polarized shear wave velocity profiles that fit fundamental and higher mode Rayleigh waveforms. We then inverted them to obtain a 3-D velocity and azimuthal anisotropy model across the region down to 600 km depth. Our results reveal that the east-west dichotomy found in other studies is not only characterized by different wave velocities but also by different anisotropy directions, likely reflecting the different deformation histories of the two blocks. Azimuthal anisotropy was found to be present in the top 300 km only and peaks at 100 - 200 km depth under the East Antarctica craton. Additionally, depth changes in fast direction were observed within the craton between 75 km and 150 km depth, suggesting layering is present. We speculate this layering relates to the formation history of the craton.submitted to Seismic

Seismic anisotropy inside the Earth from a model space search approach

For theoretical and computational convenience, the Earth has long been modelled as an isotropic medium for wave propagation. There is, however, considerable evidence of seismic anisotropy at different depths and different scales throughout the Earth, which can provide insight into Earth's dynamic processes. Since computational power has greatly increased during the last decades, it has now become possible to account for seismic anisotropy in tomographic modelling. Current methods to map seismic anisotropy inside the Earth are often based on the inversion of seismological data but these inversions can be highly non-unique, yielding discrepancies among the models produced in different studies. Since detection of seismic anisotropy is of great importance for the understanding of mantle mineralogy and deformation processes that take place inside the Earth, it is necessary to better understand tomographic models and the origin of the differences among them. In this thesis, we aimed to study seismic anisotropy in a more robust and systematic way than in previous studies, using a novel technique that describes better the model space and the complete ensemble of solutions, with reliable uncertainty estimates. The method employed in this thesis offers a way to characterize the model space better, and to give a complete description of resolution, trade-offs and uncertainties on the model parameters. We used the Neighbourhood Algorithm (NA), a direct search approach developed by Sambridge from which robust information on Earth's properties can be obtained without having to introduce unnecessary a priori information on the model space. The Gaussian hypothesis imposed in traditional inverse techniques can give rise to solutions that depend strongly on the starting model in highly underdetermined inverse problems, where the distribution of likely models can be strongly non-Gaussian. With the NA we can deal with ill-posed problems without assuming a priori Gaussian statistics for the model space. Another advantage of the NA is that it gives an overview of all the models compatible with the data, rather than choosing one with some subjective regularisation. In addition, likelihoods and trade-offs are obtained for the various model parameters, which gives a powerful tool to estimate true resolution and uncertainties. The NA was applied to surface wave phase velocity maps and normal mode data to establish the necessity to introduce seismic anisotropy in reference Earth models, and to study lateral variations in radial anisotropy in the upper mantle, transition zone and top of the lower mantle. A study of inner core anisotropy was also performed using recent measurements of anomalously split normal modes. It produced models of inner core anisotropy that remove a longstanding controversy arisen from the use of two different kinds of data, i.e. normal modes and travel-times

3-D synthetic modelling and observations of anisotropy effects on SS precursors: implications for mantle deformation in the transition zone

The Earth's mantle transition zone (MTZ) plays a key role in the thermal and compositional interactions between the upper and lower mantle. Seismic anisotropy provides useful information about mantle deformation and dynamics across the MTZ. However, seismic anisotropy in the MTZ is difficult to constrain from surface wave or shear wave splitting measurements. Here, we investigate the sensitivity to anisotropy of a body wave method, SS precursors, through 3-D synthetic modelling and apply it to real data. Our study shows that the SS precursors can distinguish the anisotropy originating from three depths: shallow upper mantle (80–220 km), deep upper mantle above 410 km, and MTZ (410–660 km). Synthetic resolution tests indicate that SS precursors can resolve ≥3 per cent azimuthal anisotropy where data have an average signal-to-noise ratio (SNR = 7) and sufficient azimuthal coverage. To investigate regional sensitivity, we apply the stacking and inversion methods to two densely sampled areas: the Japan subduction zone and a central Pacific region around the Hawaiian hotspot. We find evidence for significant VS anisotropy (15.3 ± 9.2 per cent) with a trench-perpendicular fast direction (93° ± 5°) in the MTZ near the Japan subduction zone. We attribute the azimuthal anisotropy to the grain-scale shape-preferred orientation of basaltic materials induced by the shear deformation within the subducting slab beneath NE China. In the central Pacific study region, there is a non-detection of MTZ anisotropy, although modelling suggests the data coverage should allow us to resolve at least 3 per cent anisotropy. Therefore, the Hawaiian mantle plume has not produced detectable azimuthal anisotropy in the MTZ

Internal deformation of the subducted Nazca slab inferred from seismic anisotropy

Within oceanic lithosphere a fossilized fabric is often preserved originating from the time of plate formation. Such fabric is thought to form at the mid-ocean ridge when olivine crystals align with the direction of plate spreading1, 2. It is unclear, however, whether this fossil fabric is preserved within slabs during subduction or overprinted by subduction-induced deformation. The alignment of olivine crystals, such as within fossil fabrics, can generate anisotropy that is sensed by passing seismic waves. Seismic anisotropy is therefore a useful tool for investigating the dynamics of subduction zones, but it has so far proved difficult to observe the anisotropic properties of the subducted slab itself. Here we analyse seismic anisotropy in the subducted Nazca slab beneath Peru and find that the fast direction of seismic wave propagation aligns with the contours of the slab. We use numerical modelling to simulate the olivine fabric created at the mid-ocean ridge, but find it is inconsistent with our observations of seismic anisotropy in the subducted Nazca slab. Instead we find that an orientation of the olivine crystal fast axes aligned parallel to the strike of the slab provides the best fit, consistent with along-strike extension induced by flattening of the slab during subduction (A. Kumar et al., manuscript in preparation). We conclude that the fossil fabric has been overprinted during subduction and that the Nazca slab must therefore be sufficiently weak to undergo internal deformation

Initial results from the InSight mission on Mars

NASA’s InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018. It aims to determine the interior structure, composition and thermal state of Mars, as well as constrain present-day seismicity and impact cratering rates. Such information is key to understanding the differentiation and subsequent thermal evolution of Mars, and thus the forces that shape the planet’s surface geology and volatile processes. Here we report an overview of the first ten months of geophysical observations by InSight. As of 30 September 2019, 174 seismic events have been recorded by the lander’s seismometer, including over 20 events of moment magnitude Mw = 3–4. The detections thus far are consistent with tectonic origins, with no impact-induced seismicity yet observed, and indi- cate a seismically active planet. An assessment of these detections suggests that the frequency of global seismic events below approximately Mw = 3 is similar to that of terrestrial intraplate seismic activity, but there are fewer larger quakes; no quakes exceeding Mw = 4 have been observed. The lander’s other instruments—two cameras, atmospheric pressure, temperature and wind sensors, a magnetometer and a radiometer—have yielded much more than the intended supporting data for seismometer noise characterization: magnetic field measurements indicate a local magnetic field that is ten-times stronger than orbital estimates and meteorological measurements reveal a more dynamic atmosphere than expected, hosting baroclinic and gravity waves and convective vortices. With the mission due to last for an entire Martian year or longer, these results will be built on by further measurements by the InSight lander