Empirical H/V spectral ratios at the InSight landing site and implications for the martian subsurface structure

Abstract

Summary The H/V spectral ratio inversion is a traditional technique for deriving the local subsurface structure on Earth. We calculated the H/V from the ambient vibrations at different wind levels at the InSight landing site, on Mars, and also computed the H/V from the S-wave coda of the martian seismic events (marsquakes). Different H/V curves were obtained for different wind periods and from the marsquakes. From the ambient vibrations, the recordings during low-wind periods are close to the instrument self-noise level. During high-wind periods, the seismic recordings are highly contaminated by the interaction of the lander with the wind and the martian ground. Therefore, these recordings are less favorable for traditional H/V analysis. Instead, the recordings of the S-wave coda of marsquakes were preferred to derive the characteristic H/V curve of this site between 0.4 and 10 Hz. The final H/V curve presents a characteristic trough at 2.4 Hz and a strong peak at 8 Hz. Using a full diffuse wavefield approach as the forward computation and the Neighbourhood Algorithm as the sampling technique, we invert for the 1D shear-wave velocity structure at the InSight landing site. Based on our inversion results, we propose a strong site effect at the InSight site to be due to the presence of a shallow high-velocity layer (SHVL) over low-velocity units. The SHVL is likely placed below a layer of coarse blocky ejecta and can be associated with Early Amazonian basaltic lava flows. The units below the SHVL have lower velocities, possibly related to a Late Hesperian or Early Amazonian epoch with a different magmatic regime and/or a greater impact rate and more extensive weathering. An extremely weak buried low velocity layer (bLVL) between these lava flows explains the data around the 2.4 Hz trough, whereas a more competent bLVL would not generate this latter feature. These subsurface models are in good agreement with results from hammering experiment and compliance measurements at the InSight landing site. Finally, this site effect is revealed only by seismic events data and explains the larger horizontal than vertical ground-motion recorded for certain type of marsquakes.

Mars Seismology

“…Below this, velocities increase again, but remain lower than in the Amazonian lavas, which could indicate a more physically or chemically weathered basaltic unit. These layered models down to 100 m depth are compatible with the lower-resolution compliance data (Carrasco et al 2022).…”

“…Assuming Rayleigh waves as the main component of the ambient background wavefield, the H/V inversion revealed several layers with altering high and low velocities. Subsequent analysis focused on the S-wave coda of marsquakes (Carrasco et al 2022), since the ambient wavefield could be biased by amplitudes close to the instrument self-noise for periods of low wind and the influence of lander modes for higher winds (Mahvelati & Coe, 2021, Xio & Wang 2022. In the 0.4-10 Hz bandwidth, an additional H/V peak around 8 Hz was found and inverted with a diffusive wave field approach, i.e.…”

High‐Frequency Receiver Functions With Event S1222a Reveal a Discontinuity in the Martian Shallow Crust

“…To carry out this task, different approaches have been employed. The seismic studies have revealed that the uppermost 100–200 m beneath the landing site are composed of two low‐velocity zones (∼0–20 m and ∼30–80 m) and two high‐velocity zones (∼20–30 m, and ∼80–200 m) (Carrasco et al., 2022; Hobiger et al., 2021; Kenda et al., 2020). The low‐velocity zones are interpreted as brecciated regolith and sediments, while the high‐velocity zones represent basaltic layer based on in situ geological investigations (Warner et al., 2022).…”

Are high frequency marsquakes caused by meteoroid impacts? Implications for a seismically determined impact rate on Mars

“…The high corner frequency (several Hz) indicates a short source duration, consistent with a hypervelocity impact. The tell-tale signature of the VF events in the seismic record is a strong horizontal motion at frequencies above 5 Hz (Clinton et al 2021), which has been explained by a local resonance effect (Carrasco et al 2023) (for a spectral comparison of a HF and VF event see Fig. S2).…”