Crustal Anisotropy in the Martian Lowlands From Surface Waves

Abstract

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

Investigation of Martian Regional Crustal Structure Near the Dichotomy Using S1222a Surface‐Wave Group Velocities

“…However, compared to the anisotropic estimate from Beghein et al. (2022), our homogenization presents lower $${\left({V}_{SH}/{V}_{SV}\right)}^{2}$$ ratios (Figure S5 in Supporting Information ). Thus, further investigation of the crustal structure in this region is required to determine if the structure is isotropic‐layered or anisotropic and which geological activity may have led to this structure.…”

Different Martian Crustal Seismic Velocities Across the Dichotomy Boundary From Multi‐Orbiting Surface Waves

“…On the 1222nd Martian day (sol) of the InSight mission, the largest marsquake (S1222a) ever recorded occurred with a magnitude of 4.7 ± 0.2, which corresponds to energy five times greater than the second‐largest event (InSight Marsquake Service, 2022; Kawamura et al., 2022). For the first time, both Rayleigh and Love waves were observed on a planet other than Earth (Beghein et al., 2022; Kawamura et al., 2022; Xu et al., 2022).…”

Constraints on the martian crust away from the InSight landing site

“…Constraining the crustal structure at other locations is, however, crucial to fully understand the formation and evolution of the crust, and to improve global models of crustal thickness based on gravity data 17 . Although surface waves have been recently detected for the first time from two meteorite impacts 18 , 19 and the largest marsquake ever recorded 20 , surface waves constrain the average structures along the event-lander path 19 , 21 , 22 and are less sensitive to subsurface discontinuities. The present work aims at providing seismic constraints on the crustal structure at a location far from the InSight lander.…”

“…Although surface waves have been recently detected for the first time from two meteorite impacts 18,19 and the largest marsquake ever recorded 20 , surface waves constrain the average structures along the event-lander path 19,21,22 and are less sensitive to subsurface discontinuities. The present work aims at providing seismic constraints on the crustal structure at a location far from the InSight lander.…”