Site Effect Study based on Magnitude 4~5 InSight Marsquakes

The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission has discovered over thirteen hundreds seismic events since landing on Mars in 2018, shedding the light on the internal structure of Mars (InSight Marsquake Service, 2022). These events have been applied in various Martian seismology studies such as interiors determination and scattering estimation (Lognonné et al., 2020). Site effect study, which is commonly conducted on Earth to estimate the seismic hazard and invert the surficial subsurface structure, has been implemented to Martian ambient noise and event records (Carrasco et al., 2022; Xiao et al., 2022). Nevertheless, a recently detected big event S1222a that reach the magnitude of about 5.0 would further aid to the site effect study beneath the landing site. The amplification of ground accelerations is thought to be related to the subsurface layers and mostly event-independent. Therefore, site effects from different events are expected to share similar features and help to obtain a better result with lower uncertainty compared to result from single event.

 

The main focus of this study is to investigate the subsurface resonances excited by seismic events in high frequencies above 1.0 Hz. We adopted the classical horizontal to vertical spectral ratio (HVSR) method (Nakamura, 1989) and coherency analyses and explored the optimal parameters for these time-frequency analyses in order to balance both time and frequency resolutions. We used the ambient noise to serve as a baseline in order to distinguish the effects of seismic events and environmental contaminations. Results from ambient noise show that the HVSR spectrogram and coherogram are consistent to each other and both show diurnal variation controlled by the local meteorological conditions. Several H/V peaks are found in the averaged H/V curve, most likely originating from the wind-induced lander vibrations. In contract, we calculated the H/V curves within the P-wave and S-wave windows provided by InSight Marsquake Service (2022) to compare with the results from ambient noise. S1222a stands out to exhibit abundant high frequency content well above the noise level, while other events present energy close to noise level for frequencies above 5.0 Hz. Besides, we identified the excitation of lander modes by seismic events, which is most obvious for the magnitude ~5 S1222a event perhaps due to its very high signal-to-noise ratio. All the analyses above indicate the existence of site effects and both subsurface and lander mechanical resonances could be excited. It is thus important to study how to separate the subsurface resonances and further use them for sublayer inversion.

 

Previous studies from InSight hammering experiments and jointly seismic-geodetic inversion obtained the physical properties of surficial regolith (Lognonné et al., 2020). Together with these results, it is possible to better constrain the very shallow layers beneath the landing site. Since the site effects would basically have influence on all observation of InSight seismic data, this study also has implication to environmental contamination removal, sublayer velocity inversion, and any other seismic studies that could be affected by strong site effects.

 

References

[1] Carrasco, S., et al. (2022). Empirical H/V spectral ratios at the InSight landing site and implications for the martian subsurface structure. Geophysical Journal International, submitted.

[2] Lognonné, P., et al. (2020). Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data. Nature Geoscience, 13(3), 213-220. https://doi.org/10.1038/s41561-020-0536-y

[3] InSight Marsquake Service (2022). Mars Seismic Catalogue, InSight Mission; V10 2022-04-01. ETHZ, IPGP, JPL, ICL, Univ. Bristol. https://doi.org/10.12686/a16

[4] Nakamura, Y. (1989). A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface. Railway Technical Research Institute Quarterly Report, 30(1), 25–33.

[5] Xiao, W., et al. (2022). Characteristics of Horizontal to Vertical Spectral Ratio of InSight Seismic Data from Mars. Journal of Geophysical Research, under review.