We report observations of Rayleigh waves that orbit around Mars up to three times following the S1222a marsquake. Averaging these signals, we find the largest amplitude signals at 30 s and 85 s central period, propagating with distinctly different group velocities of 2.9 km/s and 3.8 km/s, respectively. The group velocities constraining the average crustal thickness beneath the great circle path rule out the majority of previous crustal models of Mars that have a >200 kg/m3 density contrast across the dichotomy. We find that the thickness of the martian crust is 42-56 km on average, and thus thicker than the crusts of the Earth and Moon. Together with thermal evolution models, a thick martian crust suggests that the crust must contain 50-70% of the total heat production to explain present-day local melt zones in the interior of Mars.

We present the first continental-scale seismic model of the lithosphere and underlying mantle beneath Southeast Asia obtained from adjoint waveform tomography (often referred to as full-waveform inversion or FWI), using seismic data filtered at periods from 20 - 150s. Based on >3,000h of analyzed waveform data gathered from ~13,000 unique source-receiver pairs, we image isotropic P-wave velocity, radially anisotropic S-wave velocity and density via an iterative non-linear inversion that begins from a 1-D reference model. At each iteration, the full 3-D wavefield is determined through an anelastic Earth, accommodating effects of topography, bathymetry and ocean load. Our data selection aims to maximize sensitivity to deep structure by accounting for body-wave arrivals separately. SASSY21, our final model after 87 iterations, is able to explain true-amplitude data from events and receivers not included in the inversion. The trade-off between inversion parameters is estimated through an analysis of the Hessian-vector product. SASSY21 reveals detailed anomalies down to the mantle transition zone, including multiple subduction zones. The most prominent feature is the (Indo-)Australian plate descending beneath Indonesia, which is imaged as one continuous slab along the 180-degree curvature of the Banda Arc. The tomography confirms the existence of a hole in the slab beneath Mount Tambora and locates a high S-wave velocity zone beneath northern Borneo that may be associated with subduction termination in the mid-late Miocene. A previously undiscovered feature beneath the east coast of Borneo is also revealed, which may be a signature of post-subduction processes, delamination or underthrusting from the formation of Sulawesi.

A seismic investigation on Saturn’s moon Enceladus could determine the thickness of the ice shell, along with variations from the mean thickness, by recovering phase and group velocities, and through the frequency content of surface waves. Here, we model the Enceladus ice shell with uniform thicknesses of 5 km, 20 km, and 40 km, as well as with ice topography ranging from 5-40 km. We investigate several approaches for recovering the mean ice shell thickness. We show that surface wave dispersions could be used to determine the mean ice shell thickness. Flexural waves in the ice only occur if the shell is thinner than a critical value < 20 km. Rayleigh waves dominate only in thicker ice shells. The frequency content of Crary waves depends on the ice shell thickness.

Ultrasound Computer Tomography (USCT) is an emerging technique for breast cancer screening. Ultrasonic waves are propagated through the tissue and recorded by a set of transducers that are surrounding the breast. The experiment collects transmission and reflection data, which are used to obtain quantitative images of acoustic properties of the tissue (see Figure). This information is useful to characterize the breast tissue, and improves the specificity of standard imaging modalities. However, providing a diagnostic tool with high accuracy and clinically affordable time-to-solution (goal: ~15 min/patient) still remains a challenge. The goal of this work is to show that, despite the vast scale differences, experiments in seismology and USCT share many similarities. In both fields, the relative wave speed variations are comparable and the number of propagated wavelengths in the domain has the same order of magnitude. Because the wave equation is scale invariant, the cross-fertilization between both fields will benefit imaging methods on all scales. In this study, we present methods from seismic tomography that we have recently introduced to USCT: 1) We employ a linearized finite-frequency traveltime tomography approach for speed-of-sound reconstruction. Using the cross-correlation traveltime misfit functional, we compute analytically the sensitivity kernels using adjoint techniques. Our method can operate almost in real time while still including finite-frequency effects. It also can retrieve useful 3D information from 2D acquisition systems. 2) Similar to exploration geophysics, both speed-of-sound and reflectivity images are important for the interpretation. Here, we suggest a framework that combines full-waveform inversion for speed-of-sound and reverse time migration for reflectivity. 3) We apply the Sequential Optimal Experimental Design (SOED) method to optimize the position and number of transducers, in terms of accuracy and cost, to image both reflection and transmission information. Using the Bayesian approach, we define the quality of a design as the average of the posterior variances of the parameters. SOED provides cost-benefit curves that quantifies the information gain versus the computational cost. These are useful to control the trade-off between accuracy and practicality.

We present probabilistic centroid-moment tensor solutions inferred from the combination of Hamiltonian Monte Carlo sampling and a 3-D full-waveform inversion Earth model of the Japanese archipelago. While the former provides complete posterior probability densities, the latter allows us to exploit waveform data with periods as low as 15 s. For the computation of Green’s functions, we employ spectral-element simulations through the radially anisotropic and visco-elastic model, leading to substantial improvements of data fit compared to layered models. Focusing on Mw 4.8 - Mw 5.3 offshore earthquakes with a significant non-double-couple component, we simultaneously infer the centroid location, time and moment tensor without any a priori constraints on the faulting mechanism. Furthermore, we perform the inversions across several period bands, varying the minimum period between 15 s and 50 s. Accounting for 3-D Earth structure at shorter periods can increase the double-couple component of an event, compared to the GCMT solution, by tens of percent. This suggests that at least some of the non-double-couple events in the GCMT catalog might result from unmodeled Earth structure. We also observe that significant changes in source parameters, and the double-couple component in particular, may be related to only small waveform changes, thereby accentuating the importance of a reliable Earth model. Posterior probability density distributions become increasingly multimodal for shorter-period data that provide tighter constraints on source parameters. This implies, in our specific case, that stochastic approaches to the source inversion problem are required for periods below ~ 20 s to avoid trapping in local minima.