Stratification of Heterogeneity in the Lithosphere of Mars From Envelope Modeling of Event S1222a and Near Impacts: Interpretation and Implications for Very‐High‐Frequency Events

Abstract

Seismic attenuation is a key process characterizing the state of planetary interiors. When the first seismic data collected in the framework of the Apollo missions were analyzed, it was promptly realized by seismologists that the long duration of lunar seismograms was indicative of a very low level of elastic energy dissipation in the lunar lithosphere as compared to the Earth (Latham et al., 1969). The inferred very high intrinsic shear quality factor Q μ ≈ 3,500 has later been attributed to the generally dry conditions on the Moon and more specifically to the absence of volatiles in the porosity of the lunar crust (Tittman, 1977;Tittmann et al., 1980). Absorption of energy through inelastic processes is one essential component of seismic attenuation. The other key mechanism is scattering which finds its origin in the presence of small-scale heterogeneities in the lithosphere (Aki, 1969;Dainty et al., 1974). The development of the coda following ballistic arrivals is a prominent manifestation of the central role played by scattering in shaping high-frequency seismograms (>1 Hz). In planetology, the scattering

Focal Mechanism Determination of Event S1222a and Implications for Tectonics Near the Dichotomy Boundary in Southern Elysium Planitia, Mars

“…Third, scattered wave analysis by Menina et al. (2023) suggests a thick crust (e.g., 60 km) in the source region of S1222a, consistent with a source in the southern highlands. There are no mapped reverse faults in the southern highlands within the source uncertainty bounds that align with the orientation of the preferred thrust fault solution, so if S1222a was located in the southern highlands, it may not have taken place on a fault with a clear surface expression.…”

“…Second, the average crustal thickness inferred from surface waves along the S1222a path is ∼60 km which is inconsistent with crustal propagation primarily in the northern lowlands, where thinner crust is expected (Kim et al, 2023). Third, scattered wave analysis by Menina et al (2023) suggests a thick crust (e.g., 60 km) in the source region of S1222a, consistent with a source in the southern highlands. There are no mapped reverse faults in the southern highlands within the source uncertainty bounds that align with the orientation of the preferred thrust fault solution, so if S1222a was located in the southern highlands, it may not have taken place on a fault with a clear surface expression.…”

Structure of the Martian Crust Below InSight From Surface Waves and Body Waves Generated by Nearby Meteoroid Impacts

“…In contrast, the diagram for S0986c is more patchy than for S1034a. This effect may be due to a scattering effect of surface waves in a diffuse crust (Menina et al., 2023), since S0986c is almost twice as far away as S1034a. It could also be caused by a less energetic impact which is consistent with the crater diameter estimations of respectively 6.1 and 9.2 m for S0986c and S1034a (Daubar et al., 2023).…”