Seismic anisotropy layering in the Martian lowlands crust

Abstract

The largest seismic event ever detected on Mars occurred on May 4, 2022, likely situated just north of the hemispherical dichotomy, east of the landing site, and south of Cerberus Fossae. This event was unique in that it generated both Love and Rayleigh waves, including fundamental and higher modes, providing us with a rare opportunity to determine whether seismic radial anisotropy is present on Mars. We performed non-linear waveform modeling and used a Niching Genetic Algorithm to find acceptable velocity models. Our analysis revealed that seismic anisotropy is necessary in the top 40 km, with the fast direction for seismic wave propagation being horizontal, similar to previous results solely based on fundamental mode surface wave group velocity dispersion. Our new models display layering with varying degrees of anisotropy. We found anisotropic parameter ξ = 1.0-1.2 between 5 and 20 km depth and ξ = 1.2-1.3 at 25–30 km depth. No significant anisotropy was detected below 35 km. While the origin of the anisotropy is still being debated, it is characteristic of a medium with a vertical symmetry axis and could result from both magmatic events and impacts. We propose that the anisotropy layering reflects different stages in the formation history of the Martian crust.