On May 4th 2022, the seismometer on Mars observed the largest marsquake (S1222a) during its operation. One of the most specific features of S1222a is the long event duration lasting more than 8 hours from the occurrence, in addition to the clear appearance of body and surface waves. As demonstrated on Earth, by modeling a long-lasting and scattered surface wave with the radiative transfer theory, we estimated the scattering and intrinsic quality factors of Mars (Qs and Qi). This study especially focused on the frequency range between 0.05 - 0.09 Hz, where Qs and Qi have not been constrained yet. Our results revealed that Qi = 1000 - 1500 and Qs = 30 - 500. By summarizing the Martian Qi and Qs estimated so far and by comparing them with those of other celestial bodies, we found that, overall, the Martian scattering and absorption properties showed Earth-like values.

A document by Sebastián Carrasco. Click on the document to view its contents.

Seismic noise recorded at the surface of Mars has been monitored since February 2019, using the seismometers of the InSight lander. The noise on Mars is 500 times lower than on Earth at night and it increases during the day. We analyze its polarization as a function of time and frequency in the band 0.03-1Hz. We use the degree of polarization to extract signals with stable polarization whatever their amplitude. We detect polarized signals at all frequencies and all times. Glitches correspond to linear polarized signals which are more abundant during the night. For signals with elliptical polarization, the ellipse is in the horizontal plane with clockwise and anti-clockwise motion at low frequency (LF). At high frequency (HF), the ellipse is in the vertical plane and the major axis is tilted with respect to the vertical. Whereas polarization azimuths are different in the two frequency bands, they are both varying as a function of local time and season. They are also correlated with wind direction, particularly during the day. We investigate possible aseismic and seismic origin of the polarized signals. Lander or tether noise are discarded. Pressure fluctuation transported by environmmental wind may explain part of the HF polarization but not the tilt of the ellipse. This tilt can be obtained if the source is an acoustic emission in some particular case. Finally, in the evening when the wind is low, the measured polarized signals seems to correspond to a diffuse seismic wavefield that would be the Mars microseismic noise.

Mars is the first extraterrestrial planet with seismometers (SEIS) deployed directly on its surface in the framework of the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission. The lack of strong Marsquakes, however, strengthens the need of seismic noise studies to additionally constrain the Martian structure. Seismic noise autocorrelations of single-station recordings permit the determination of the zero-offset reflection response underneath SEIS. We present a new autocorrelation study which employs state-of-the-art approaches to determine a robust reflection response by avoiding bias from aseismic signals which are recorded together with seismic waves due to unfavorable deployment and environmental conditions. Data selection and segmentation is performed in a data-adaptive manner which takes the data root-mean-square amplitude variability into account. We further use the amplitude-unbiased phase cross-correlation and work in the 1.2-8.9 Hz frequency band. The main target are crustal scale reflections, their robustness and convergence. The strongest signal appears at 10.6 s, and, if interpreted as P-wave reflection, would correspond to a discontinuity at about 24 km depth. This signal is a likely candidate for a reflection from the base of the Martian crust due to its strength, polarity, and stability. Additionally we identify, among the stable signals, a signal at about 6.85 s that can be interpreted as a P-wave reflection from the mid-crust at about 9.5 km depth.

The SEIS seismometer deployed at the surface of Mars in the framework of the NASA-InSight mission has been continuously recording the ground motion at Elysium Planitia for more than one martian year. In this work, we investigate the seasonal variation of the near surface properties using both background vibrations and a particular class of high-frequency seismic events. We present measurements of relative velocity changes over one martian year and show that they can be modeled by a thermoelastic response of the Martian regolith. Several families of high-frequency seismic multiplets have been observed at various periods of the martian year. These events exhibit repeatable waveforms with an emergent character and a coda that is likely composed of scattered waves. Taking advantage of these properties, we use coda waves interferometry to measure relative travel-time changes as a function of the date of occurrence of the quakes. While in some families a stretching of the coda waveform is clearly observed, in other families we observe either no variation or a clear contraction of the waveform. Measurements of velocity changes from the analysis of background vibrations above 5Hz are consistent with the results from coda wave interferometry. We identify a frequency band structure in the power spectral density, that can be tracked over hundreds of days. This band structure is the equivalent in the frequency domain of an autocorrelogram and can be efficiently used to measure relative travel-time changes as a function of frequency. The observed velocity changes can be adequately modeled by the thermoelastic response of the regolith to the time-dependent incident solar flux at the seasonal scale. In particular, the model captures the time delay between the surface temperature variations and the velocity changes in the sub-surface. Our observations could serve as a basis for a joint inversion of the seismic and thermal properties in the first meters below InSIght.

Monitoring changes of seismic properties at depth can provide a first order insight into Earth's dynamic evolution. Coda wave interferometry is the primary tool for this purpose. This technique exploits small changes of waveforms in the seismic coda and relates them to temporal variations of attenuation or velocity at depth. While most existing studies assume statistically homogeneous scattering strength in the lithosphere, geological observations suggest that this hypothesis may not be fulfilled in active tectonic or volcanic areas. In a numerical study we explore the impact of a non-uniform distribution of scattering strength on the spatio-temporal sensitivity of coda waves. Based on Monte Carlo simulation of the radiative transfer process, we calculate sensitivity kernels for three different observables, namely travel-time, decorrelation and intensity. Our results demonstrate that laterally varying scattering properties can have a profound impact on the sensitivities of coda waves. Furthermore, we demonstrate that the knowledge of the mean intensity, specific intensity and energy flux, governed by spatial variation of scattering strength, is key to understanding the decorrelation, travel-time and intensity kernels, respectively. A number of previous works on coda wave sensitivity kernels neglect the directivity of energy fluxes by employing formulas extrapolated from the diffusion approximation. In this work, we demonstrate and visually illustrate the importance of the use of specific intensity for the travel-time and scattering kernels, in the context of volcanic and fault zone setting models. Our results let us foresee new applications of coda wave monitoring in environments of high scattering contrast.

The SEIS seismometer of the InSight mission was deployed on the ground of Elysium Planitia, on 19 December 2018. Interferometry techniques can be used to extract information on the internal structure from the autocorrelation of seismic ambient noise and coda of seismic events. In a single-station configuration, the zero-offset global reflection of the ground vertically below the seismometer can be approximated by the stacked ZZ autocorrelation function (ACF) for P-waves and the stacked EE and NN ACFs for S-waves, assuming a horizontally layered medium and homogeneously distributed and mutually uncorrelated noise sources. We analyze continuous records from the very broadband seismometer (SEIS-VBB), and correct for potential environmental disturbances through systematic preprocessing. For each Sol (martian day), we computed the correlations functions in 24 windows of one martian hour in order to obtain a total correlation tensor for various Mars local times. In addition, a similar algorithm is applied to the Marsquake waveforms in different frequency bands. Both stability analysis and inter-comparison between background noise and seismic event results suggest that the background seismic noise at the landing site is reliably observed only around 2.4 Hz, where an unknown mechanism is amplifying the ground shaking, and only during early night hours, when the noise induced by atmospheric disturbances is minimum. Seismic energy arrivals are consistently observed across the various data-sets. Some of these arrivals present multiples. These observations are discussed in terms of Mars’ crustal structure.

High resolution maps of seismic attenuation parameters in Taiwan have been obtained by using a modified ”Multiple Lapse Time Window Analysis’ (MLTWA). At most of the stations in porous sedimentary and highly faulted areas in Taiwan, the conventional modeling of MLTWA based on the scalar theory of radiative transfer in a half-space with isotropic scattering fails to explain the spatio-temporal distribution of the whole S wavetrain. Using Monte Carlo simulations of wave transport, we demonstrate that this anomalous energy distribution in the coda may be modelled by multiple anisotropic scattering of seismic waves. In addition to the scattering quality factor Qsc, we introduce a parameter g (independant of Qsc) which determines the angular redistribution of energy upon scattering (scattering anisotropy). We determine the attenuation parameters Qsc-1, Qi-1 and g in three frequency bands (1-2, 2-4 and 4-8Hz). Overall, Taiwan is more attenuating than most orogens with a mean effective scattering loss (Qsc*)-1=Qsc-1(1-g) about 0.025 and a mean intrinsic absorption Qi-1 about 0.009 at 1.5Hz. Scattering loss (Qsc*)-1 varies over more than one order of magnitude across Taiwan while absorption fluctuations are about 30%. The more attenuating zones are the Coastal Range and the Coastal Plain where scattering dominates over absorption at low frequency, and inversely at high frequency. These regions are also characterized by strong backscattering (g<-0.85) at 1.5Hz and rather high VP/VS ratio. We speculate that the observed strong back-scattering at low frequency is related to strong impedance fluctuations in the crust induced by the presence of fluids.