We detected surface waves from two meteorite impacts on Mars. By measuring group velocity dispersion along the impact-lander path, we obtained a direct constraint on crustal structure away from the InSight lander. The crust north of the equatorial dichotomy had a shear wave velocity of approximately 3.2 kilometers per second in the 5- to 30-kilometer depth range, with little depth variation. This implies a higher crustal density than inferred beneath the lander, suggesting either compositional differences or reduced porosity in the volcanic areas traversed by the surface waves. The lower velocities and the crustal layering observed beneath the landing site down to a 10-kilometer depth are not a global feature. Structural variations revealed by surface waves hold implications for models of the formation and thickness of the martian crust.

The largest seismic event ever detected on Mars occurred on 4 May 2022, and was recorded by the Seismic Experiment for Interior Structure (SEIS) Very Broadband (VBB) seismometer ) of NASA's InSight mission (Banerdt et al., 2020). The event, labeled S1222a, occurred at 23:23:06.57 UTC (sol 1222) and has a moment magnitude M w estimated at 4.7 ± 0.2 (Kawamura et al., 2022). This event is thus five times larger than the second largest event recorded (Horleston et al., 2022) and had enough energy to allow for the first direct observation of multi-orbiting Rayleigh waves on Mars (Panning et al., 2022). Based on their calculated back azimuth of 101° (with a range of 96°-112°) and epicentral distance (37° ± 1.6°), the Mars Quake Service (MQS) determined that this quality A (InSight Marsquake Service, 2020) event originated at 7.63°S, 170.67°E, near the North-South dichotomy, east of the landing site, and south of Cerberus Fossae (Figure 1).Most remarkably, S1222a generated surface waves clearly observable on all three components of the ground motion (Figure 2). While Rayleigh waves have been previously detected on Mars with impact events S1000a and S1094b (Kim et al., 2022), no Love waves had been seen before. Since Rayleigh and Love waves are predominantly sensitive to different elastic parameters (governing the speed of vertically and horizontally polarized shear waves traveling horizontally, respectively), this gives us an opportunity to study the presence of large-scale radial seismic anisotropy on Mars between the lander and the event epicenter. Radial anisotropy, which is a type of transverse isotropy with a vertical symmetry axis, manifests as a difference in the velocities of vertically (V SV

SUMMARY We investigated the likelihood of radial anisotropy in the shallow and deep upper mantle, including the mantle transition zone (MTZ) under the Indian Ocean. Seismic anisotropy can be an indicator of mantle deformation through lattice preferred orientation of anisotropic crystals in the mantle. It has thus the potential to illuminate Earth’s dynamic interior, but previous seismic tomography studies have not achieved consensus on the existence of radial anisotropy below ∼250 km depth. We developed a fully nonlinear transdimensional hierarchical Bayesian Markov Chain Monte Carlo approach to invert fundamental and higher mode surface wave dispersion data and applied it to a subset of a global Love and Rayleigh wave data set. We obtained posterior model parameter distributions for shear wave velocity (VS) and radial anisotropy ξ under the Indian Ocean. These posterior model distributions were used to calculate the probability of having radial anisotropy at different depths. We demonstrated that separate inversions of Love and Rayleigh waves yield models compatible with the results of joint inversions within uncertainties. The obtained pattern of VS anomalies agrees with most previous studies. They display negative anomalies along ridges in the uppermost mantle, but those are stronger than for regularized inversions. The Central Indian Ridge and the Southeastern Indian Ridge present velocity anomalies that extend to ∼200 km depth, whereas the Southwestern Indian Ridge seems to have a shallower origin. Weaker, laterally variable velocity perturbations were found at larger depths. The anisotropy models differ more strongly from regularized inversion results and their uncertainties were rather large. We found that anisotropy models from regularized inversions also depend on the chosen parametrization, which is consistent with the existence of a large model null-space. Apart from a fast horizontally polarized shear wave signal in the top 100 km, likely reflecting the horizontal plate motion due to asthenospheric deformation, no clear relation to surface geology was detected in the anisotropy models. We found that, although the anisotropy model uncertainties are rather large, and lateral variations are present, the data generally prefer at least 1 per cent anisotropy in the MTZ with fast vertically polarized shear waves, within errors. Incorporating group velocity data did not help better constrain deep structure by reducing parameter trade-offs. We also tested the effect of prior constraints on the 410- and 660-km topography and found that the undulations of these discontinuities had little effect on the resulting models in our study region.