Crustal Anisotropy in the Martian Lowlands From Surface Waves. Issue 24 (14th December 2022)

• Record Type: Journal Article
• Authors: Beghein, C.; Li, J.; Weidner, E.; Maguire, R.; Wookey, J.; Lekić, V.; Lognonné, P.; Banerdt, W.
• Is Part Of: Geophysical research letters. Volume 49:Issue 24(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-12-14
• Subjects: Mars -- seismic waves -- surface waves -- seismic anisotropy -- crust -- inversion; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2022GL101508
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: The largest seismic event ever recorded on Mars, with a moment magnitude of 4.7 ± 0.2, is the first event to produce both Love and Rayleigh wave signals. We measured their group velocity dispersion between about 15 and 40 s period and found that no isotropic depth‐dependent velocity model could explain the two types of waves wave simultaneously, likely indicating the presence of seismic anisotropy. Inversions of Love and Rayleigh waves yielded velocity models with horizontally polarized shear waves traveling faster than vertically polarized shear waves in the top 10–25 km. We discuss the possible origins of this signal, including the preferred orientation of anisotropic crystals due to shear deformation, alignment of cracks, layered intrusions due to an impact, horizontal layering due to the presence of a large‐scale sediment layer on top of the crust, and alternation of sedimentation and basalt layers deposits due to large volcanic eruptions. Plain Language Summary: The largest marsquake recorded so far sent energy across the red planet in the form of different kinds of waves, including two types of waves trapped near the surface. This was the first time both types of so‐called surface waves were detected on Mars. Combining measurements from these two types of surface waves allowed us to determine the speed of other types of waves, that is, waves that travel horizontally and that make rocks move perpendicular to the direction of propagation. We found that these waves move faster in the crust between 10 and 25 km depth when the rocks oscillate in a direction sub‐parallel to the planet surface than if the rocks vibrate in the vertical direction. This wave speed dependence can tell us about deformation mechanisms inside the crust. We found that either an alternation of volcanic rocks and sediments layers due to volcanic eruptions or internal layering within the crust due to an impact are the preferred explanations for our observations. Key Points: Rayleigh and Love waves were detected on Mars Seismic wave speed is directionally dependent over large‐scales within Mars crust The shear wave horizontal velocity is faster than the vertical velocity at 10–25 km depth in the lowlands between the event and the lander