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SUMMARY The horizontal-to-vertical (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 favourable 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 1-D 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.

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Random inhomogeneities in the earth can highly influence the characteristics of propagating seismic waves. They exist at all scales and can become an important source of epistemic uncertainty in the ground motion estimation. Despite several works have evaluated these effects, few of them have verified the accuracy of their numerical solutions or controlled the propagation regime they were simulating. In this work we present a comprehensive study of SH wave propagation in 2D random media, which covers from ballistic to diffusive behaviors. In order to understand and identify the interaction of these regimes, we analyzed the coherent and incoherent components of the wavefield. The random media consist in correlated density and velocity fluctuations described by von Kármán autocorrelation function with a Hurst coefficient of 0.25 and a correlation length a=500 m. The Birch correlation coefficient which relates density to velocity fluctuations takes 4 possible values between 0.5 and 1, and the standard deviation of the perturbations is either 5% or 10%. Spectral element simulations of SH wave propagation excited by a plane wave are performed for normalized wavenumbers (ka) up to 5. By measuring the amplitude decay of the coherent wave we obtain the scattering attenuation, which is then compared with theoretical predictions from the mean field theory. Similarly, mean intensities from synthetic waveforms are also compared with those from radiative transfer theory. Both sets of comparisons show excellent agreement between numerical and theoretical predictions. Addionally, we perform statistical analyses on the fluctuations of the ballistic peak which exhibits a transition from log-normal to exponential distribution. These two types of distribution characterize the ballistic and diffusive behaviors, respectively, which means that after certain propagation distances the quasi-ballistic peak is composed mainly by multiply-diffused components. Such critical distance is of the order of the scattering mean free path and offers an alternative method to measure this parameter. Finally, we pay particular attention on the attenuation of the quasi-ballistic peak, which in the forward scattering regime appears to decay exponentially over a length scale known as the transport mean free path.

NASA InSight mission revealed that Mars is seismically active today and identified a region with high activity called Cerberus Fossae. Since then, the origin of marsquakes has been an important question in Mars seismology. On 2022/05/04, InSight seismometer detected the largest marsquake during the mission, named S1222a. The source was estimated to be outside the Cerberus Fossae and it would be important to constrain the source parameters of this remarkable event. In this study, we estimate the source parameters of S1222a through spectral analyses. Since we found that seismic spectra of S1222a are heavily contaminated, we performed Empirical Green’s Function analyses. We found that the corner frequencies are 0.34-0.54 Hz for the P wave and 0.17-0.24 Hz for the S wave. These values are slightly higher than those of seismic events at Cerberus Fossae, which implies a faster rupture at the source and different origin for S1222a.

Since its deployment at the surface of Mars, the Seismic Experiment for Interior Structure (SEIS) instrument of the InSight mission has detected hundreds of small-magnitude seismic events. In this work, we highlight some features of two specific families: high-frequency (HF) and very-high-frequency (VF) events. We characterize the shape of the energy envelopes of HF and VF events with two parameters: (1) the delay time td between the onset and the peak of the dominant arrival; and (2) the quality factor Qc, which quantifies the energy decay rate in the coda. We observe that the envelope of HF and VF events is frequency independent. As a consequence, a single delay time suffices to characterize envelope broadening in the 2.5–7.5 Hz band. The typical coda decay time is also frequency independent, as attested by the close to linear increase of Qc with frequency. Finally, we use elastic radiative transfer theory to perform a series of inversion of seismogram envelopes for the attenuation properties of the Martian lithosphere. The good fit between synthetic and observed envelopes confirms that multiple scattering of elastic waves released by internal sources is a plausible explanation of the events characteristics. We quantify scattering and attenuation properties of Mars and highlight the differences and similarities with the Earth and the Moon. The albedo, that is, the contribution of scattering to the total attenuation, derived from VF events is very high, which we interpret as a signature of a mostly dry medium. Our results also suggest a stratification of the scattering and attenuation properties.

Since its deployment at the surface of Mars in 2019, the SEIS seismometer (Seismic Experiment for Interior Structure) of the NASA-InSight mission (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) has detected hundreds of small magnitude seismic events. But recently, the seismic event S1222a of magnitude 4.9 which primarily contains energy above 0.1 Hz was reported by the Mars Quake Service. It is characterized by two temporally separated arrivals with a gradual beginning, a broad maximum and a very long decay lasting up to 1 hour. In this work, we propose to use seismic coda analysis on this specific event to explore the attenuation properties of Martian crust and to provide an Earth-Mars-Moon comparison. Fig 1: Envelope characteristics of the S1222a Broadband event. The data were collected on the VBB sensors with sampling rates of 20 samples per second. (a) Base 10 logarithm of energy envelopes normalized by the noise level as a function of time and fit of the coda decay (black dashed lines). The gray dashed line indicates signal-to-noise ratio (SNR) = 2. The different colors correspond to different frequency bands indicated in inset. The time t = 0 s corresponds to the first arrival. tP and tS refer to the arrival times of the first and second wave packets (red dashed lines). (b) Frequency dependence of Qc in the 0.4 –7.5 Hz frequency band. The color bar indicates the SNR measured on the horizontal components. The red and black dashed line shows the power-law fit Qc = 900f0.36and Qc = 655f0.94 respectively (f is the frequency.).We employ basic multiple-scattering concepts to examine and to compare the energetic characteristics of the Earth, Martian and Lunar crust. We measured the coda quality factor Qc, that characterizes the rate of decay of the seismogram envelope, to have a first view of the attenuation characteristics [1] (see fig 1.a). We performed a fit of the frequency dependence of Qc using a parameterization of the form Qc = Q0 fn. Based on linear least-squares applied to the logarithm of Qc (see fig.1.b), we obtain two very distinct regimes of Qc as a function of the frequency f. In the [0.4 Hz - 1.5 Hz] range Q0 = 900 and n 0.36, when at higher frequency, Q0 = 655 and n 0.94 This result is specific to Mars and could be related to the stratification of the attenuation properties in the Martian crust. The quasi-linear frequency increase of Qc seen in [2.5 Hz – 7.5 Hz] range suggests that the coda decay is governed by a frequency independent attenuation time. Similar values of Q0 and n, in this frequency range, were reported by using a Very High Frequency group events [2] (see Fig 2 in orange and red). Fig 2: Coda attenuation as a function of the frequency for the Earth-cratons regions (green), the Earth-tectonics regions (bleu), the Moon (gray), Mars ( α= 0 in red and α= 1 in orange) and the S1222a BB event (black). In the case of the regional earthquakes on Earth, the frequency dependence of Qc is radically different between tectonic areas and shields. Q0varies from 100 to 400 in tectonic areas (blue) [e.g. 3] and from 600 to 1000 in shields areas (green) [e.g. 4], where the frequency exponent n is equal to 1 and 0.1-0.5 respectively. [5] report coda Qc values of 2400 at 0.5 Hz and possibly as large as 10000 at 7 Hz on the Moon (gray). This result suggests that Q0 is a factor of 2 to 4 larger than on Mars when the frequency exponent n is more likely in the range of 0.4 – 0.6. To summarize, our Qc measurements exhibit two different regimes of frequency dependence. At higher frequency, the Q0 and n valuesare very close to the one found on Very High Frequency group events. The unique combination of high Q0 with a frequency exponent n =1 also distinguishes very clearly Mars from the Earth. By the time of the meeting, we hope to present the first results of envelope modeling based on elastic radiative transport in a stratified medium to quantify the contribution of the scattering and the absorption to the total attenuation. References: [1] Aki, K. (1969). Analysis of the seismic coda of local earthquakes as scattered waves. Journal of geophysical research, 74(2), 615-631. [2] Menina et al. (2021), Energy Envelope and Attenuation Characteristics of High‐Frequency (HF) and Very‐High‐Frequency (VF) Martian Events, Bulletin of the Seismological Society of America (2021) 111 (6): 3016–3034. [3] Mayeda, K., Koyanagi, S., Hoshiba, M., Aki, K., & Zeng, Y. (1992). A comparative study of scattering, intrinsic, and coda Q− 1 for Hawaii, Long Valley, and central California between 1.5 and 15.0 Hz. Journal of Geophysical Research: Solid Earth, 97(B5), 6643-6659.[4] Mitchell, B, J et al. (2008). A continent-wide map of 1-Hz Lg coda Q variation across Eurasia and its relation to lithospheric evolution. Journal of Geophysical Research: Solid Earth, 113(B4).[5] Gillet, K., et al (2017). Scattering attenuation profile of the Moon: Implications for shallow moonquakes and the structure of the megaregolith. Physics of the Earth and Planetary Interiors, 262, 28-40.

After more than 3 years of observations, NASA InSight mission has encountered a magnitude 5+ seismic event on 2022/05/04, which is the largest seismic event ever detected on Mars. This gigantic event enables us to study spectral feature of the seismic signal as well as some local effects that may be visible in the data. In this study, we will study the seismic spectra to understand the source time function of the event, attenuation feature of the crust/upper mantle of Mars. To better constrain these features, we will also evaluate the effect of local structure or the site effect on the seismic spectrum. Spectral feature of S1222a shows two different features at low and high frequencies. While we observe typical decay in spectra at low frequencies (1Hz). The low frequency can be well explained by a classical model with a seismic source time function and attenuation (e.g. Aki and Richards, 2022). On the other hand, the limited frequency band prevents us from distinguishing between different attenuation models and a source time function. To better understand and constrain the seismic source parameters and martian attenuation, it would be mandatory to better understand also the high frequency component of the spectrum. The energy enhancement at high frequencies does not follow predicted models and implies there is additional effect from local structure. In this study, we will overcome this problem with two approaches. One is to use other events that has similar distance with S1222a to evaluate the site effect. Since the site effect reflects the effect of locas structure around the InSight landing site, this will be commonly seen for all events. Indeed, marsquekes that are classified as Very High Frequency(VF) Type events show characteristic spectral feature where high frequency energy is enhanced especially on the horizontal components (Clinton et al., 2021). This feature was investigated through seismic coda analyses (van Driel et al., 2021; Menina et al., 2021), however, their origin is yet to be understood. While S1222a is classified as Broadband(BB) type event, its seismic coda features show significant similarities with VF type events (Menina et al., this issue). Thus, in this study, we use other VF type events to evaluate the site effect observed in seismic spectra and will use this to correct the S1222a spectrum to remove the site effect. Secondly, we will also use the noise data to constrain the site effect, this will mainly be studied in Xiao et al. (this issue). This will give us another independent estimates on the site effect and we will also use this information to correct the S1222a spectrum. With corrected spectrum, we will better constrain the source time function, more specifically its corner frequency, and attenuation structure or seismic Q. The corner frequency will give us a way to estimate the stress drop related to the seismic event and the slip velocity. This will give us some unique opportunities to study seismic excitation on Mars. Attenuation will also enable us to give constrains on the elastic/inelastic features of the crust and upper mantle. The elastic features can be understood as scattering of seismic energy and we compare our results with other studies that also focus on this effect (e.g. Menina et al., this issue). Inelastic effect is closely related with thermal structure of Mars and/or inclusion of light elements such as volatiles. By estimating the attenuation on Mars, we discuss its implication to the Martian shallow structure.Figure P spectrum of S1222a event and comparison with different attenuation models. The red line shows the data and green, blue, magenda, light blue and yellow lines show model spectra with different attenuation models. The figure shows a clear change in spectral feature of S1222a below and above 1Hz. Comparison with modeled spectra shows that we cannot distinguish between different attenuation models if we limit our analyses to frequencies below 1 Hz and understanding the energy enhancement at high frequencies are mandatory to fully understand the spectral feature of this event.

International audience; Meteoroid impacts shape planetary surfaces by forming new craters and alter atmospheric composition. During atmospheric entry and impact on the ground, meteoroids excite transient acoustic and seismic waves. However, new crater formation and the associated impact-induced mechanical waves have yet to be observed jointly beyond Earth. Here we report observations of seismic and acoustic waves from the NASA InSight lander’s seismometer that we link to four meteoroid impact events on Mars observed in spacecraft imagery. We analysed arrival times and polarization of seismic and acoustic waves to estimate impact locations, which were subsequently confirmed by orbital imaging of the associated craters. Crater dimensions and estimates of meteoroid trajectories are consistent with waveform modelling of the recorded seismograms. With identified seismic sources, the seismic waves can be used to constrain the structure of the Martian interior, corroborating previous crustal structure models, and constrain scaling relationships between the distance and amplitude of impact-generated seismic waves on Mars, supporting a link between the seismic moment of impacts and the vertical impactor momentum. Our findings demonstrate the capability of planetary seismology to identify impact-generated seismic sources and constrain both impact processes and planetary interiors.

Coda wave interferometry is an important tool to gain insights into the dynamic evolution of the Earth. A limitation of the majority of current studies employing this technique, is the neglect of variations in scattering strength in the lithosphere. Geological observations indicate that scattering properties can strongly vary laterally, especially in complex geological settings, e.g. in the vicinity of active tectonic or volcanic areas.In presented work we explore the implications of non-uniform distribution of scattering strength on the spatio-temporal sensitivity of coda waves. In the first part, we numerically derive 2-D coda wave sensitivity kernels based on Monte Carlo simulations of the radiative transfer process, considering lateral heterogeneity of the crust. The kernels are calculated for three different observables, namely travel-time, decorrelation and intensity. Our results illustrate that laterally varying scattering properties can have a profound impact on the sensitivities of coda waves.In a second part, we validate the kernels. Firstly, synthetic lapse-time based travel-time changes are calculated using the kernels for non-uniform media. Using these synthetic observations, we conduct damped least-squares inversions to localise changes in space for both a fault zone and volcanic setting. We compare the accuracy of localisation of the medium changes between inversions carried out with kernels for uniform and non-uniform media. Our results demonstrate that superior localisation of the seismic anomaly is obtained when considering local scattering information by employing kernels for non-uniform media. This holds for the fault zone as well as the volcanic setting. The stability of the results is verified by conducting inversions where 10dB white noise is added to the synthetic time-shift observations.

CiteAndres Barajas, Ludovic Margerin, Michel Campillo, Coupled body and surface wave sensitivity kernels for coda-wave interferometry in a three-dimensional scalar scattering medium, Geophysical Journal International, Volume 230, Issue 2, August 2022, Pages 1013–1029, https://doi.org/10.1093/gji/ggac091; International audience; Analysis of long recordings of ambient seismic noise has shown to be effective for estimation of seismic responses between points located on the surface. This includes both the ballistic and the coda part of the waveforms. Passive image interferometry is used to analyse perturbations in the reconstructed coda, to detect and locate changes in the medium. This method has been shown to be effective in monitoring variations in seismic velocity produced by a wide range of phenomena. However, localization of the sources of these changes is still an open problem for a 3-D half-space, given the difficulties of integrating body and surface waves within the same framework. In this study, we approach this problem by developing the sensitivity kernels of a scalar model that integrates the body and surface scalar waves. First, we establish a parallel between the penetration depth of the surface waves for the elastic and scalar cases, which equips the latter with a natural scaling with frequency that is otherwise not included in the model. Next, using a variational approach, we quantify how a velocity perturbation in the medium affects the propagation velocity of the surface waves. Based on these results, we extend the sensitivity theory to include the body and surface waves as modes of propagation and detection, as restricted to a 1-D depth-dependent perturbation description, for simplicity. The obtained kernel can be expressed as the sum of a surface and a body waves sensitivity kernels, which are inter-dependent through a set of traveltime distributions. These distributions are estimated with Monte Carlo simulations based on the radiative transfer equations of the system, with the source and the receiver located in the same position at the surface. The sensitivity at depth is in good agreement with previous results based on full wavefield elastic simulations in 3-D inhomogeneous half-space. The temporal evolution of the body and surface waves sensitivity is quantified, as well as the contribution of all the possible modes of propagation and detection to each of these sensitivities. We show how the position of the source affects the sensitivity between the two types of waves. We find that the efficacy of energy conversion from surface to body waves is controlled by the ratio between the surface wave penetration depth and the mean free path, a feature that has not been reported in previous studies. This means that configurations that share this ratio have the same sensitivity as long as all the spatial and temporal variables (e.g. elapsed time, depth) are non-dimensionalized with the mean free path and the mean free time, respectively.

SUMMARYThe horizontal to vertical spectral ratio (HVSR) of seismic ambient noise has been proven to be a fast and efficient method for characterizing the 1-D resonance frequency of the local subsurface in a practical framework. Over the last decades, theories have been developed in order to extend the exploitation of HVSR beside the frequency of its first peak, notably the diffuse field assumption (DFA) which links the HVSR to the Green’s function of the local medium assuming the diffuseness of the seismic ambient noise wavefield. However, the underlying assumption of the seismic ambient noise being a diffuse, equipartitioned field may not be satisfied under certain circumstances. In order to exploit the contribution of scattering in forging diffuse wave fields, we leverage the advantages of coda waves and present a novel procedure for computing the HVSR, using the coda part of ambient noise correlations. We applied this technique to data gathered at the plio-quaternary sedimentary basin of Argostoli, Greece. Results on this data set show the potential of the method to improve the temporal stability of the HVSR measurements compared to the classical computation, and the fit with the theoretical HVSR curve derived from the DFA theory. These results suggest that this procedure could help in extracting physical information from the HVSR and thus could lead to an extended use of these measurements to characterize the mechanical properties of the medium.

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We investigate the scattering attenuation characteristics of the Martian crust and uppermost mantle to understand the structure of the Martian interior. We examine the energy decay of the spectral envelopes for 21 high-quality Martian seismic events from sols 128 to 500 of InSight operations. We use the model of Dainty, Toksöz, et al. (1974) to approximate the behavior of energy envelopes resulting from scattered wave propagation through a single diffusive layer over an elastic half-space. Using a grid search, we mapped the layer parameters that fit the observed InSight data envelopes. The single diffusive layer model provided better fits to the observed energy envelopes for high-frequency (HF) and very-high-frequency (VF) than for the low-frequency and broadband events. This result is consistent with the suggested source depths (Giardini et al., 2020) for these families of events and their expected interaction with a shallow scattering layer. The shapes of the observed data envelopes do not show a consistent pattern with event distance, suggesting that the diffusivity and scattering layer thickness is nonuniform in the vicinity of InSight at Mars. Given the consistency in the envelope shapes between HF and VF events across epicentral distances and the trade-offs between the parameters that control scattering, the dimensions of the scattering layer remain unconstrained but require that scattering strength decreases with depth and that the rate of decay in scattering strength is the fastest near the surface. This is generally consistent with the processes that would form scattering structures in planetary lithospheres.

The scattering of seismic waves is the signature of random heterogeneities, present in the lithospheric structure of a terrestrial planet. It is the result of refraction and reflection of the seismic waves generated by a quake, when they cross materials with different shear rigidity, bulk modulus, and density and therefore different seismic wave velocities, compared to the ambient space. On Earth, the seismic waves show relatively weak scattering, identified in later arriving coda waves that follow the main arrivals of body waves and decay with time. In contrast, seismic wave scattering is much more significant on the Moon, where the high heterogeneous structure of the lunar megaregolith, produced through millions of years of impact bombardment, is a structure that creates an extreme scattering environment. The landing of the NASA InSight mission on Mars in 2018, which carried and deployed a seismometer for the first time on the Martian ground, offered a pristine dataset for the investigation and analysis of the characteristics of the scattering attenuation of the Martian crust and uppermost mantle which is important for understanding the structure of the Martian interior. Lognonné et al. (2020) used a methodology based on the radiative transfer model (Margerin et al., 1998) to offer the first constraints for the scattering and attenuation in the Martian crust. In this study, we performed a further examination based on more and newer events of the Martian Seismic Catalog (InSight Marsquake Service, 2021). The Marsquake Catalog contains events that are categorized according to the frequency content of the seismic signal (Clinton et al., 2021). In this study, we used 19 events of 4 different families, namely the Low Frequency, Broadband, High Frequency, and Very High Frequency events, for our investigation. We focused our investigation on the characteristics of the S-coda waveforms and for this reason, we worked on the respective energy envelopes. We manually picked the envelopes, defining the time window of the S-coda waves, as well the frequency range for each event, directly from the spectrograms of the events' signals, using an appropriately developed visual tool. We used a modeling approach (Dainty et al., 1974) that was developed for the computation of the energy envelopes of shallow events (Lunar impacts) and a diffusive, highly scattering layer, sitting over an elastic half-space. The energy envelope depends on the thickness of the diffusive layer, the range of the seismic ray, the diffusivity and the attenuation in the top layer, and the seismic wave velocity in the underneath elastic half-space. We analyzed all the tradeoffs between the terms of the modeling equation, namely the geometrical relationship of the velocity contrast between the diffusive layer and the elastic half-space with the seismic ray range and the diffusive layer thickness, the diffusivity with the diffusive layer thickness, and between the diffusivity and the velocity contrast of the two examined layers. The presence of the aforementioned tradeoffs made the definition of a unique model a very hard task, as the information for the azimuthal characteristics of the signal is not available for the examined events. This is a limitation that exists in seismology only while working with one station, with the InSight seismometer being the only station on a planet, and the amplitude of the seismic signal is not big enough to perform a specific polarization analysis and derive the azimuthal origin of the recorded signal. For this reason, we reviewed the fit between the modeling and the data, depending on the frequency content of the events. The Low Frequency and the Broadband events, which have a frequency content mainly below the tick noise detected at 1 Hz, could not satisfy the modeling approach of a simple diffusive layer. The spectral envelopes of the S-coda waves of these events are decaying very rapidly, which suggests an origin in a more elastic environment. This is in agreement with previous studies (Giardini et al., 2020) that suggest that these events are generated deeper in the Martian mantle. For this reason, we applied another approach to these signals, with an energy envelope equation designed for deep moonquakes (Dainty et al.,1974), but it was not either capable to fit the examined data envelopes, suggesting the absence of a very thick megaregolith structure on Mars. Based on the results of the High Frequency (HF) and Very High Frequency (VF) events we observed a range of possible paths and diffusivities that can satisfy the data and we investigated the tradeoffs between the parameters of a modeling equation that control the shape of the energy envelope for the events. The analysis of these tradeoffs does not permit us to make any assumptions about the depth of the diffusive region in the Martian crust and the upper mantle as their azimuthal characteristics are unknown and therefore it is not feasible to tell if the difference in the result analysis reflects vertical or lateral variations of the uppermost diffusive layer in the Martian lithosphere. The results of this study illustrate one of the challenges in working with single-station seismic data where event location information, including distance, azimuth, and depth are crucial for understanding the lateral variation in seismic properties of a planet. The existence of a seismic network on the planetary scale will improve the ability of phase peaking and location identification of the events and therefore it will give additional constraints for a similar analysis. References Clinton, J. F. et al. (2021). The Marsquake catalogue from InSight, sols 0–478.Phys Earth Planet In, 310:106595. Dainty, A. M. et al. (1974). Seismic scattering and shallow structure of the Moon in Oceanus Procellarum.The Moon,9(1-2):11–29. Giardini, D. et al. (2020). The seismicity of Mars.Nat Geosci, 13(3):205–212.InSight Marsquake Service (2021). Mars Seismic Catalogue, InSight Mission; V5 2021-01-04. Lognonné, P. et al. (2020). Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data.NatGeosci, 13(3):213–220. Margerin, L. et al. (1998). Radiative transfer and diffusion of waves in a layered medium: new insight into coda Q.GeophysJ Int, 134(2):596–612.

Mars is the first extraterrestrial planet with seismometers (Seismic Experiment for Interior Structure, SEIS) deployed directly on its surface in the framework of the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) 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 a P-wave reflection, would correspond to a discontinuity at about 21 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 6.15 s that can be interpreted as the P-wave reflection from the mid-crust at about 9.5 km depth., This is the InSight contribution number 144 and IPGP contribution number 4215. French authors are supported by ANR MAGIS (ANR-19-CE31-0008-08) and by CNES SEIS/InSight Phase E2. MS thanks SANIMS (RTI2018-095594-B-I00), Generalitat de Catalunya (2017SGR1022), and invitations for research stays at IPGP.

Since early February 2019, the SEIS (Seismic Experiment for Interior Structure) seismometer deployed at the surface of Mars in the framework of the InSight mission has been continuously recording the ground motion at Elysium Planitia. In this study, we take advantage of this exceptional data set to put constraints on the crustal properties of Mars using seismic interferometry (SI). To carry out this task, we first examine the continuous records from the very broadband seismometer. Several deterministic sources of environmental noise are identified and specific preprocessing strategies are presented to mitigate their influence. Applying the principles of SI to the single-station configuration of InSight, we compute, for each Sol and each hour of the martian day, the diagonal elements of the time-domain correlation tensor of random ambient vibrations recorded by SEIS. A similar computation is performed on the diffuse waveforms generated by more than a hundred Marsquakes. A careful signal-to-noise ratio analysis and an inter-comparison between the two datasets suggest that the results from SI are most reliable in a narrow frequency band around 2.4 Hz, where an amplification of both ambient vibrations and seismic events is observed. The average autocorrelation functions (ACFs) contain well identifiable seismic arrivals, that are very consistent between the two datasets. Interpreting the vertical and horizontal ACFs as, respectively, the P- and S- seismic reflectivity below InSight, we propose a simple stratified velocity model of the crust, which is mostly compatible with previous results from receiver function analysis. Our results are discussed and compared to recent works from the literature., This study is InSight contribution number 164. The authors acknowledge both "Universite Federale de Toulouse Midi Pyrenees" and the "Region Occitanie" for funding the PhD grant of Nicolas Compaire. The French authors acknowledge the French Space Agency CNES and ANR (ANR-14-CE36-0012-02 and ANR-19-CE31-0008-08) for funding the InSight Science analysis. We acknowledge NASA, CNES, their partner agencies and Institutions (UKSA, SSO, DLR, JPL, IPGP-CNRS, ETHZ, IC, MPS-MPG) and the flight operations team at JPL, SISMOC, MSDS, IRIS-DMC and PDS for providing SEED SEIS data.

SUMMARYScattered seismic coda waves are frequently used to characterize small scale medium heterogeneities, intrinsic attenuation or temporal changes of wave velocity. Spatial variability of these properties raises questions about the spatial sensitivity of seismic coda waves. Especially the continuous monitoring of medium perturbations using ambient seismic noise led to a demand for approaches to image perturbations observed with coda waves. An efficient approach to localize spatial and temporal variations of medium properties is to invert the observations from different source–receiver combinations and different lapse times in the coda for the location of the perturbations. For such an inversion, it is key to calculate the coda-wave sensitivity kernels which describe the connection between observations and the perturbation. Most discussions of sensitivity kernels use the acoustic approximation in a spatially uniform medium and often assume wave propagation in the diffusion regime. We model 2-D multiple non-isotropic scattering in a random elastic medium with spatially variable heterogeneity and attenuation using the radiative transfer equations which we solve with the Monte Carlo method. Recording of the specific energy density of the wavefield that contains the complete information about the energy density at a given position, time and propagation direction allows us to calculate sensitivity kernels according to rigorous theoretical derivations. The practical calculation of the kernels involves the solution of the adjoint radiative transport equations. We investigate sensitivity kernels that describe the relationships between changes of the model in P- and S-wave velocity, P- and S-wave attenuation and the strength of fluctuation on the one hand and seismogram envelope, traveltime changes and waveform decorrelation as observables on the other hand. These sensitivity kernels reflect the effect of the spatial variations of medium properties on the wavefield and constitute the first step in the development of a tomographic inversion approach for the distribution of small-scale heterogeneity based on scattered waves.

Seismic noise recorded at the surface of Mars has been monitored since February 2019, using the InSight seismometers.This noise can reach -200 dB and is 500 times lower than on Earth at night and it increases of 30 dB 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 independent of their amplitude and type of polarization. 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 below 0.3 Hz (LF). Above 0.3 Hz (HF) and except in the evening, the ellipse is in the vertical plane and the major axis is tilted. While polarization azimuths are different in the two frequency bands, they both vary as a function of local hour and season. They are also correlated with wind direction, particularly during the daytime.We investigate possible aseismic and seismic origins of the polarized signals. Lander or tether noise can be discarded. Pressure fluctuations transported by 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 coming from high altitude at critical angle. Finally, in the evening when the wind is low, the polarized signals may correspond to the seismic wavefield of the Mars background noise.

The InSight seismometer SEIS recorded tens of high-frequency (1.5-5Hz; HF) and Very-high frequency (1.5-15Hz, VF) Martian events. They are characterized by two temporally separated arrivals with a gradual beginning, a broad maximum and a very long decay. This observation is consistent with a long-range propagation of seismic P and S waves in a heterogeneous crust (Van Driel et al., accepted). To examine this hypothesis, first, we employ basic multiple-scattering concepts on the two groups of events. Then, we propose a full envelope modeling based on elastic radiative transport in a half-space. The model parametrization and the radiative transfer equations are presented in (Lognonné, P., et al. (2020) and Margerin, L., (2017)). We find that both HF and VF signals are depolarized and verify Gaussian statistics, at the exception of the ballistic primary and secondary arrivals. These properties agree with a multiple-scattering origin. For VF events, the energy partitioning ratio V2/H2 between horizontal and vertical components is frequency dependent. We observe that V2/H2 is maximum at the so-called ‘2.4Hz resonance’ (~2) and decreases rapidly at frequencies higher than 5Hz (~0.1) then i remains relatively low up to frequencies of 15Hz at least. HF events do not exhibit a decrease of V2/H2 at high frequencies however further analysis reveals a strong correlation between energy partitioning and signal-to-noise (S/N) ratio for HF events. This observation suggests that a part of the difference between the HF and VF events can to some extent be explained by noise contamination. The generally low V2/H2 ratio of VF events is reminiscent of the response of unconsolidated layers, as observed at Pinyon Flats Observatory on Earth (Margerin, L., et al. (2009)). Unlike earthquakes and moonquakes observed in the same frequency band, the delay time measured from onset to peak of the secondary arrival of HF and VF events is frequency-independent. This suggests that the spectrum of heterogeneity of the Martian crust is smooth. We observe that, for HF and VF events, the delay time is weakly dependent on hypocentral distance. This observation cannot be reconciled with the predictions of multiple-scattering theories in a statistically homogeneous medium however it suggests a stratification of heterogeneity in the Martian lithosphere. The coda quality factor Qc of VF events is high and shows a linear increase with frequency. Qc of HF events is higher but it may be overestimated due to the noise contamination. The linear frequency dependence of Qc is strongly reminiscent of the leakage effect in a crustal scattering waveguide and suggests that part of the observed coda attenuation may be of structural origin. The full envelope modeling of the S0334a VF event results shows that the estimated value of the diffusivity (≃ 619 km2/s) is almost 6 times greater than for the S0128a VF event (≃ 90 km2/s). This observation again suggests a stratification of heterogeneity. In future works, we will perform the full envelope modeling of all the VF selected events at different frequencies to constrain a 1D attenuation and diffusion model of the Martian crust.

International audience; The seismometer deployed on the surface of Mars as part of the InSight mission (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) has recorded several hundreds of marsquakes in the first 478 sols after landing. The majority of these are classified as high-frequency (HF) events in the frequency range from approximately 1 to 10 Hz on Mars' surface. All the HF events excite a resonance around 2.4 Hz and show two distinct but broad arrivals of seismic energy that are separated by up to 450 s. Based on the frequency content and vertical-to-horizontal energy ratio, the HF event family has been subdivided into three event types, two of which we show to be identical and only appear separated due to the signal-to-noise ratio. We show here that the envelope shape of the HF events is explained by guided Pg and Sg phases in the Martian crust using simple layered models with scattering. Furthermore, the relative travel times between these two arrivals can be related to the epicentral distance, which shows distinct clustering. The rate at which HF events are observed varies by an order of magnitude over the course of one year and cannot be explained by changes of the background noise only. The HF content and the absence of additional seismic phases constrain crustal attenuation and layering, and the coda shape constrains the diffusivity in the uppermost shallow layers of Mars.

The InSight mission landed on Mars on November 26th, 2018 and its seismometer, the Seismic Experiment for Interior Structure (SEIS), has recorded continuous Martian seismic data since February 2019, consisting of mainly ambient seismic noise but also hundreds of seismic events. We used the SEIS data to study the horizontal-to-vertical spectral ratios from both the ambient seismic noise (nHV) and the seismic events (eHV), for frequencies above 0.6 Hz, in order to get further constraints on the first tens of meters at the Insight landing site. The nHV curve was obtained by using data segments of 50 s over more than 400 Sols. The preferred nHV curve is observed during the northern spring and summer at low wind levels and it is a mostly flat curve with a prominent trough around ~2.4 Hz. Outside of these time periods, the nHV curve is contaminated with artificial peaks likely related to lander modes. On the other hand, the eHV curve was created using 336 seismic events with quality either A, B or C, as defined by the Marsquake Service. For each seismic event, we computed the signal-to-noise ratio (SNR) at each frequency and only frequencies with SNR>3 were used to obtain the final eHV curve. In addition to the 2.4 Hz trough, the final eHV curve shows a strong peak around 8 Hz, which is not observed from the ambient noise data possibly due to a lack of seismic energy in this frequency band able to excite it. A preliminary inversion of the eHV curve, considering the fundamental mode of the Rayleigh wave only, shows that the 2.4 Hz trough and the 8 Hz peak can be explained by a shear-wave velocity model increasing from the surface to a depth of 5-8 m (likely the boundary between the regolith and coarse ejecta), in good agreement with previous analysis based on compliance observations, hammering measurements and satellite images. At this depth, a discontinuity leading to a higher velocity layer is observed, which is followed by a deeper low-velocity layer about 20 m thick. The modeling assuming body waves only or a full diffuse seismic wavefield is currently under investigation., EGUsphere

Seismic noise recorded at the surface of Mars has been monitored since February 2019, using the InSight seismometers. This noise can reach -200 dB. It is 500 times lower than on Earth at night and it increases of 30 dB during the day. We analyze its polarization as a function of time and frequency in the band 0.03-1 Hz. We use the degree of polarization to extract signals with stable polarization independent of their amplitude and type of polarization. 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 below 0.3 Hz. In the 0.3-1Hz high frequency band (HF) and except in the evening, the ellipse is in the vertical plane and the major axis is tilted. While polarization azimuths are different in the two frequency bands, they both vary as a function of local hour and season. They are also correlated with wind direction, particularly during the daytime. We investigate possible aseismic and seismic origins of the polarized signals. Lander or tether noise can be discarded. Pressure fluctuations transported by 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 coming from high altitude at critical angle. Finally, in the evening when the wind is low, the measured polarized signals may correspond to the seismic wavefield of the Mars background noise. Plain Language Summary Seismic noise at the surface of Mars was unknown until the first measurements by the seismometers from the InSight mission in January 2019. On Earth, the microseismic noise (0.05-1 Hz) is composed dominantly of surface waves generated by the numerous sources related to ocean wave activities. On Mars, because there is no ocean, seismic noise is down to 500 times lower than on Earth reaching -200 dB in acceleration at night. In order to determine the nature of the Mars noise, we analyze its polarization with a statistical method and show that it is different to that on Earth. In the low frequency band 0.03-0.3 Hz, we detect signals with elliptical polarization in the horizontal plane. In the high frequency band 0.3-1Hz (HF), signals have elliptical polarization in the vertical plane. The polarization ellipse azimuth gives the direction toward the source. On Mars, these azimuths are varying as a function of local hour and season and they are correlated with wind direction during the daytime. The HF polarized signals may be explained by local effects of pressure fluctuations and/or by acoustic emission coming from high altitudes in particular conditions. It is only in the evening when the wind is low, that the measured polarized signals may correspond to propagating seismic waves that would be the Mars seismic background noise., This is the InSight contribution number 143 and IPGP contribution number 4183. We acknowledge NASA, CNES, their partner agencies and institutions (UKSA, SSO, DLR, JPL, IPGP-CNRS, ETHZ, IC, MPS-MPG) and the flight operations team at JPL, SISMOC, MSDS, IRIS-DMC, and PDS for providing SEED SEIS data. French authors are supported by ANR MAGIS (ANR-19-CE31-0008-08) and by CNES for SEIS science support. A. Horleston is supported by the UK Space Agency through grant #ST/R002096/1. M. Schimmel is supported by the SANIMS project (RTI2018-095594-B-I00, MICINN, Spain).

SUMMARY Seismic coda waves are commonly used in estimation of subsurface Q values and monitoring subsurface changes. Coda waves mainly consist of multiply scattered body and surface waves. These two types of waves interact with each other in the multiple scattering process, which thus leads to a spatiotemporal evolution of the body and surface wave energies. One cannot characterize the evolution because one has not fully understood the multiple scattering of the two types of waves. Thus one commonly assumes only one type of waves exists or ignores their interaction while studying the coda waves. However, neglecting the interaction leads to an incorrect energy evolution of the two types of waves and consequently biases the Q estimation or interpretation of coda wave changes for monitoring. To better understand the interaction between these waves during multiple scattering and to model the energy evolution correctly, we propose a Monte Carlo algorithm to model the multiple scattering process. We describe the physics of the scattering for the two types of waves and derive scattering properties like cross sections for perturbations in elastic properties (e.g. density, shear modulus and Lamé parameters). Our algorithm incorporates this knowledge and thus physically models the body- and surface wave energy evolution in space and time. The energy partitioning ratios between surface and body waves provided by our algorithm match the theoretical prediction based on equipartition theory. In the equipartition state, our simulation results also match Lambert’s cosine law for body waves on the free surface. We discuss how the Rayleigh-to-body-wave scattering affects the energy partitioning ratios. Our algorithm provides a new tool to study multiple scattering and coda waves in elastic media with a free surface.

vEGU21: Gather Online | 19–30 April 2021, Since early February 2019, 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. In this work, we take advantage of this exceptional dataset to put constraints on the crustal properties of Mars using seismic interferometry (SI). This method use the seismic waves, either from background vibrations of the planet or from quakes, that are scattered in the medium in order to recover the ground response between two seismic sensors. Applying the principles of SI to the single-station configuration of SEIS, we compute, for each Sol (martian day) and each local hour, all the components of the time-domain autocorrelation tensor of random ambient vibrations in various frequency bands. A similar computation is performed on the diffuse waveforms generated by more than a hundred Marsquakes. For imaging application a careful signal-to-noise ratio analysis and an inter-comparison between the two datasets are applied. These analyses suggest that the reconstructed ground responses are most reliable in a relatively narrow frequency band around 2.4Hz, where an amplification of both ambient vibrations and seismic events is observed. The average Auto-Correlation Functions (ACFs) from both ambient vibrations and seismic events contain well identifiable seismic arrivals, that are very consistent between the two datasets. We interpret the vertical and horizontal ACFs as the ground reflection response below InSight for the compressional waves and the shear waves respectively. We propose a simple stratified velocity model of the crust, which is most compatible with the arrival times of the detected phases, as well as with previous seismological studies of the SEIS record. The hourly computation of the ACFs over one martian year also allows us to study the diurnal and seasonal variations of the reconstructed ground response with a technique call Passive Image Interferometry (PII). In this study we present measurements of the relative stretching coefficient between consecutive ACF waveforms and discuss the potential origins of the observed temporal variations.

Several seismic experiments were deployed on the Moon by the astronauts during the Apollo missions. The experiments began in 1969 with Apollo 11, and continued with Apollo 12, 14, 15, 16 and 17. Instruments at Apollo 12, 14, 15, 16 and 17 remained operational until the final transmission in 1977. These remarkable experiments provide a valuable resource. Now is a good time to review this resource, since the InSight mission is returning seismic data from Mars, and seismic missions to the Moon and Europa are in development from different space agencies. We present an overview of the seismic data available from four sets of experiments on the Moon: the Passive Seismic Experiments, the Active Seismic Experiments, the Lunar Seismic Profiling Experiment and the Lunar Surface Gravimeter. For each of these, we outline the instrumentation and the data availability. We show examples of the different types of moonquakes, which are: artificial impacts, meteoroid strikes, shallow quakes (less than 200 km depth) and deep quakes (around 900 km depth). Deep quakes often occur in tight spatial clusters, and their seismic signals can therefore be stacked to improve the signal-to-noise ratio. We provide stacked deep moonquake signals from three independent sources in miniSEED format. We provide an arrival-time catalog compiled from six independent sources, as well as estimates of event time and location where available. We show statistics on the consistency between arrival-time picks from different operators. Moonquakes have a characteristic shape, where the energy rises slowly to a maximum, followed by an even longer decay time. We include a table of the times of arrival of the maximum energy 𝑡�max and the coda quality factor 𝑄�𝑐� . Finally, we outline minimum requirements for future lunar missions to the Moon. These requirements are particularly relevant to future missions which intend to share data with other agencies, and set out a path for an International Lunar Network, which can provide simultaneous multi-station observations on the Moon. ISSN:1572-9672 ISSN:0038-6308

To get started, read DataPaper_ElectronicSup1.docx. This repository is a collection of resources for lunar seismology, including: S1: Location Parameters S2 - Deep Moonquake Stacks S3 - Lunar Catalog from Various Sources S4 - Extensive Lunar Catalog from Nakamura et al. 1981 and updates S5 - Attenuation Parameters from Gillet et al. (2017) S6 - Low-Level Requirements for Future Seismic Missions to the Moon S7 - Jupyter Notebook to plot the transfer functions S8 - Block Diagrams S9 - Artificial Impacts S10 - Current Data Availability S11 - Response Files, Replaced ImpactParameters.xlsx with ImpactParameters.csv which is easier to import and use. Also corrected an error in the date of the Apollo 15 Lunar Module (A15/LM) impact.

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 ni...

NASA InSight (the Interior Exploration using Geodesy and Heat Transport) has placed the first broadband seismometer (SEIS) on the Martian surface and now continuously monitoring Martian seismic activity. Since the first detection of a marsquake in March 2019, SEIS detected more than 200 marsquakes and Mars has been revealed to be a seismically active planet. The dataset can now be used to perform the seismic investigation of the Mars interior and interpret this in a comparative manner by referring to the examples from the Earth and the Moon.In this study, we investigate the seismic attenuation on Mars and compare this with the Earth and the Moon. Attenuation can be described as a combination of inelastic absorption and elastic diffusion of energy. Such properties will give important constraints on the composition of the Mars interior and also its thermal state. Another interesting aspect will be to discuss the water content with respect to the attenuation. Given the large variety of water content for the Earth, the Moon and Mars, the attenuation feature will be likely to differ significantly between these planets and satellite. Here we use the seismic dataset obtained by InSight SEIS and construct a 1D structure of seismic attenuation on Mars. Then we refer to the values obtained for the Earth and the Moon to discuss the possible implication on their differences and similarities. The presentation aims to summarize the results from different approaches taken by the authors. The approach includes; 1) spectral analyses of seismic signals and spectral decay fitting, 2) seismic coda analyses with coda rise time and decay, 3) numerical coda simulation with diffusion theory on seismic energy. With these approaches we will be constraining seismic quality factor Q and diffusivity D for different depth range. Different approaches have sensitivities to different depth and prarameters and we aim to provide our view on the martian attenuation and diffusion to date by summarizing the obtained results.

NASA’s InSight mission arrived on Mars in November 2018 and deployed the first very broad-band seismometer, SEIS, on the planet’s surface. SEIS has been collecting data continuously since early February 2019, by now recording more than 400 events of different types. InSight aims at enhancing our understanding of the internal structure and dynamics of Mars, including better constraints on its crustal thickness. Various models based on topography and gravity observed from the orbit currently vary in average crustal thickness from 30 km to more than 100 km, with important implications for Mars’ thermal evolution, and the partitioning of silicates and heat-producing elements between different layers of Mars.We present P-to-S and S-to-P receiver functions, which are available for 4 and 3 marsquakes, respectively, up to now. Out of all of the marsquakes recorded to date, these are the only ones with clear enough P- or S-arrivals not dominated by scattering to make them suitable for the analysis. All of the quakes are located at comparatively small epicentral distances, between 25° and 40°. We observe three consistent phases within the first 10 seconds of the P-to-S receiver functions. The S-to-P receiver functions also show a consistent first phase. Later arrivals are harder to pinpoint, which could be due to the comparatively shallow incidence of the S-waves at the considered distances, which prevents the generation of converted waves. Identification of later multiple phases in the P-to-S receiver functions likewise remains inconclusive. To obtain better constraints on velocity, we also calculated apparent velocity curves from the P-to-S receiver functions, but these provide meaningful results for only one event so far, implying a large uncertainty. Due to difficulties in clearly identifying multiples, the receiver functions can currently be explained by either two crustal layers and a thin (25-30 km) crust or three crustal layers and a thicker (40-45 km) crust at the landing site. This model range already improves the present constraints by providing a new maximum value of less than 70 km for the average crustal thickness. Information from noise autocorrelations as a complementary method, identification of P-reverberations and S-precursors in the event recordings, and more extensive modeling, ultimately including 3D-effects, are considered to further our understanding of the waveforms and tighten the constraints on the crust.

Monitoring of temporal seismic velocity changes can provide us with information on the mechanical state of the Earth’s crust due to processes of stress build-up and release. In current work, we use the Dense Array of North Anatolia [1], which has been continuously recording from May 2012 until October 2013, to analyse the spatio-temporal variations of seismic velocity changes in the North Anatolian Fault zone (NAF). We compute daily ambient-noise cross-correlation functions for all 63 three-component stations in the frequency band between 0.1 – 1 Hz.To retrieve spatial distribution of seismic velocity changes in such an inhomogeneous fault zone, we go beyond the simple linear travel-time shifts approximation and homogeneous sensitivity kernel. We therefore invert for the travel-time shifts at different lag-times. Furthermore, we use sensitivity kernels for media with inhomogeneous scattering properties. The scattering properties for the sensitivity kernels are derived from the data: a scattering mean free path inside the fault zone (northern strand of NAF) of ∼ 10 km and ∼ 150 km outside the fault zone, the attenuation coefficient inside and outside the fault zone are 80 and 100 respectively. [1] DANA. Dense array for north anatolia. International Federation of Digital Seismograph Networks doi:10.7914/SN/YH2012, 2012.

The ambient seismic noise has proven to be a powerful tool to assess velocity changes within the ground using coda-wave interferometry (CWI). CWI is based on the analysis of small waveform changes in the coda of the signals. Localizing and imaging the source that generates changes can be done with the help of sensitivity kernels which contain information on how each part of the surrounding medium contributes to the overall waveform perturbation that is recorded at a receiver. Although progress has been made in the theory of sensitivity kernels in the case of a full elastic space, the inclusion of a free surface has proven to be difficult. Indeed, the free surface couples body waves and surface waves, which affects the sensitivity of coda waves with respect to the full-space case. Furthermore, one expects the depth sensitivity of coda waves to be strongly dependent on the relative contribution of surface and body waves, which depends on the lapse-time, source-receiver distance and scattering properties of the medium. Using the Monte-Carlo method, we compute traveltime-sensitivity kernels in a 3D scalar problem that includes body and surface waves, based on a recent theoretical model that integrates both through a mixed boundary condition. From these results, we assess the impact of the depth of a velocity perturbation on the recorded signals at the surface. Our results will be compared with previous numerical approaches from the literature.

SUMMARY Intrinsic absorption and scattering properties provide us with information about the physical state and heterogeneity of the Earth’s crust. These properties are usually obtained by observing the energy decay of naturally occurring earthquakes, leading to sparse spatial sampling and therefore average scattering values over a large region. This study uses ambient noise cross-correlations to analyse the energy decay and scattering properties over a part of the North Anatolian Fault (NAF; Turkey) from the continuous records of the 73 stations of the DANA temporary array in the frequency band 0.1–0.5 Hz. The region covered by the stations has rapidly varying geological characteristics and is highly faulted around the northern strand of the NAF. We measured in the noise correlations the space–time evolution of the energy of the coda waves. We first perform measurements in separate subregions. The local scattering and attenuation properties are obtained by global optimization of a 2-D solution of the radiative transfer equation for surface waves. We found that the mean free path and attenuation coefficient are considerably varying laterally with strong scattering observed in the region lying along the northern strand of NAF. The optimization provides well-constrained values for the scattering mean free path on the order of 10 km in the fault region. The mean free path is much larger (>100 km) in the neighbouring regions. We compare our global observations with a phonon based Monte Carlo simulation of scattered energy in a laterally variable scattering model. These simulations confirm the large contrast of heterogeneity between NAF and the surrounding crust and provide further constraints on the lateral extent of NAF. When sources are located inside the fault zone, we find a signature of the actual non-uniform scattering properties, observed as a concentration of energy in the fault zone for a limited amount of time. This in turn suggests that lateral variations of scattering properties should be taken into account in future monitoring studies.

Banerdt, William B. et al., NASA’s InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018. It aims to determine the interior structure, composition and thermal state of Mars, as well as constrain present-day seismicity and impact cratering rates. Such information is key to understanding the differentiation and subsequent thermal evolution of Mars, and thus the forces that shape the planet’s surface geology and volatile processes. Here we report an overview of the first ten months of geophysical observations by InSight. As of 30 September 2019, 174 seismic events have been recorded by the lander’s seismometer, including over 20 events of moment magnitude M = 3–4. The detections thus far are consistent with tectonic origins, with no impact-induced seismicity yet observed, and indicate a seismically active planet. An assessment of these detections suggests that the frequency of global seismic events below approximately M = 3 is similar to that of terrestrial intraplate seismic activity, but there are fewer larger quakes; no quakes exceeding M = 4 have been observed. The lander’s other instruments—two cameras, atmospheric pressure, temperature and wind sensors, a magnetometer and a radiometer—have yielded much more than the intended supporting data for seismometer noise characterization: magnetic field measurements indicate a local magnetic field that is ten-times stronger than orbital estimates and meteorological measurements reveal a more dynamic atmosphere than expected, hosting baroclinic and gravity waves and convective vortices. With the mission due to last for an entire Martian year or longer, these results will be built on by further measurements by the InSight lander., With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737)

The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018 and fully deployed its seismometer by the end of February 2019. The mission aims to detect, characterize and locate seismic activity on Mars, and to further constrain the internal structure, composition and dynamics of the planet. Here, we present seismometer data recorded until 30 September 2019, which reveal that Mars is seismically active. We identify 174 marsquakes, comprising two distinct populations: 150 small-magnitude, high-frequency events with waves propagating at crustal depths and 24 low-frequency, subcrustal events of magnitude Mw 3–4 with waves propagating at various depths in the mantle. These marsquakes have spectral characteristics similar to the seismicity observed on the Earth and Moon. We determine that two of the largest detected marsquakes were located near the Cerberus Fossae fracture system. From the recorded seismicity, we constrain attenuation in the crust and mantle, and find indications of a potential low-S-wave-velocity layer in the upper mantle. © 2020, The Author(s), under exclusive licence to Springer Nature Limited., We acknowledge NASA, CNES and its partner agencies and institutions (UKSA, SSO, DLR, JPL, IPGP-CNRS, ETHZ, IC and MPS-MPG) and the flight operations team at JPL, SISMOC, MSDS, IRIS-DMC and PDS for providing SEIS data. The Swiss co-authors were jointly funded by (1) the Swiss National Science Foundation and French Agence Nationale de la Recherche (SNF-ANR project 157133 ‘Seismology on Mars’), (2) the Swiss National Science Foundation (SNF project 172508 ‘Mapping the internal structure of Mars’), (3) the Swiss State Secretariat for Education, Research and Innovation (SEFRI project ‘MarsQuake Service-Preparatory Phase’) and (4) ETH Research grant no. ETH-06 17-02. Additional support came from the Swiss National Supercomputing Centre (CSCS) under project ID s922. The Swiss contribution in the implementation of the SEIS electronics was made possible by funding from the federal Swiss Space Office (SSO) and contractual and technical support from the ESA-PRODEX office. The French Team acknowledge the French Space Agency CNES, which has supported and funded all SEIS-related contracts and CNES employees, as well as CNRS and the French team universities for personal and infrastructure support. Additional support was provided by ANR (ANR-14-CE36-0012-02 and ANR-19-CE31-0008-08) and, for the IPGP team, by the UnivEarthS Labex programme (ANR-10-LABX-0023), IDEX Sorbonne Paris Cité (ANR-11-IDEX-0005-0). SEIS-SP development and delivery were funded by the UK Space Agency. A portion of the work was carried out at the InSight Project at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The MPS SEIS team acknowledges funding for development of the SEIS leveling system by the DLR German Space Agency. We thank gempa GmbH for software development related to the MQS tools. This paper is InSight contribution number 102.

In this work, I propose to model the propagation of high-frequency seismic waves in the heterogeneous Earth by means of a coupled system of radiative transfer equations for P and S waves. The model describes the propagation of both coherent and diffuse waves in a statistically isotropic heterogeneous medium and takes into account key phenomena such as scattering conversions between propagation modes, scattering anisotropy and absorption. The main limitation of the approach lies in the neglect of the shear wave polarization information. The canonical case of a medium with uniform scattering and absorption properties is studied in details. Using an adjoint formalism, Green’s functions (isotropic point source solutions) of the transport equation are shown to obey a reciprocity relation relating the P energy density radiated by an S source to the S energy density radiated by a P source. A spectral method of calculation of the Green’s function is presented. Application of Fourier, Hankel and Legendre transforms to time, space and angular variables, respectively, turns the equation of transport into a numerically tractable penta-diagonal linear system of equations. The implementation of the spectral method is discussed in details and validated through one-to-one comparisons with Monte Carlo simulations. Numerical experiments in different propagation regimes illustrate that the ratio between the correlation length of heterogeneities and the incident wavelength plays a key role in the rate of stabilization of the P-to-S energy ratio in the coda. The results suggest that the rapid stabilization of energy ratios observed in the seismic coda is a signature of the broadband nature of crustal heterogeneities. The impact of the texture of the medium on both pulse broadening and generation of converted S wave arrivals by explosion sources is illustrated. The numerical study indicates that smooth media enhance the visibility of ballistic-like S arrivals from P sources.

International audience; An international team of researchers gathered, with the support of the Interna-tional Space Science Institute (ISSI), (1) to review seismological investigations of the lunarinterior from the Apollo-era and up until the present and (2) to re-assess our level of knowl-edge and uncertainty on the interior structure of the Moon. A companion paper (Nunn et al.in Space Sci. Rev., submitted) reviews and discusses the Apollo lunar seismic data with theaim of creating a new reference seismic data set for future use by the community. In thisstudy, we first review information pertinent to the interior of the Moon that has becomeavailable since the Apollo lunar landings, particularly in the past ten years, from orbitingspacecraft, continuing measurements, modeling studies, and laboratory experiments. Fol-lowing this, we discuss and compare a set of recent published models of the lunar interior,including a detailed review of attenuation and scattering properties of the Moon. Commonfeatures and discrepancies between models and moonquake locations provide a first esti-mate of the error bars on the various seismic parameters. Eventually, to assess the influenceof model parameterisation and error propagation on inverted seismic velocity models, aninversion test is presented where three different parameterisations are considered. For thispurpose, we employ the travel time data set gathered in our companion paper (Nunn et al. inSpace Sci. Rev., submitted). The error bars of the inverted seismic velocity models demon-strate that the Apollo lunar seismic data mainly constrain the upper- and mid-mantle struc-ture to a depth of ∼1200 km. While variable, there is some indication for an upper mantlelow-velocity zone (depth range 100–250 km), which is compatible with a temperature gradi-◦ent around 1.7 C/km. This upper mantle thermal gradient could be related to the presenceof the thermally anomalous region known as the Procellarum Kreep Terrane, which containsa large amount of heat producing elements.

SUMMARYTo describe the energy transport in the seismic coda, we introduce a system of radiative transfer equations for coupled surface and body waves in a scalar approximation. Our model is based on the Helmholtz equation in a half-space geometry with mixed boundary conditions. In this model, Green’s function can be represented as a sum of body waves and surface waves, which mimics the situation on Earth. In a first step, we study the single-scattering problem for point-like objects in the Born approximation. Using the assumption that the phase of body waves is randomized by surface reflection or by interaction with the scatterers, we show that it becomes possible to define, in the usual manner, the cross-sections for surface-to-body and body-to-surface scattering. Adopting the independent scattering approximation, we then define the scattering mean free paths of body and surface waves including the coupling between the two types of waves. Using a phenomenological approach, we then derive a set of coupled transport equations satisfied by the specific energy density of surface and body waves in a medium containing a homogeneous distribution of point scatterers. In our model, the scattering mean free path of body waves is depth dependent as a consequence of the body-to-surface coupling. We demonstrate that an equipartition between surface and body waves is established at long lapse-time, with a ratio which is predicted by usual mode counting arguments. We derive a diffusion approximation from the set of transport equations and show that the diffusivity is both anisotropic and depth dependent. The physical origin of the two properties is discussed. Finally, we present Monte Carlo solutions of the transport equations which illustrate the convergence towards equipartition at long lapse-time as well as the importance of the coupling between surface and body waves in the generation of coda waves.

International audience; Coda-wave interferometry is a technique which exploits tiny waveform changes in the coda to detect temporal variations of seismic properties in evolving media. Observed waveform changes are of two kinds: traveltime perturbations and distortion of seismograms. In the last 10 yr, various theories have been published to relate either background velocity changes to traveltime perturbations, or changes in the scattering properties of the medium to waveform decorrelation. These theories have been limited by assumptions pertaining to the scattering process itself-in particular isotropic scattering, or to the propagation regime-single-scattering and/or diffusion. In this manuscript, we unify and extend previous results from the literature using a radiative transfer approach. This theory allows us to incorporate the effect of anisotropic scattering and to cover a broad range of propagation regimes, including the contribution of coherent, singly scattered and multiply scattered waves. Using basic physical reasoning, we show that two different sensitivity kernels are required to describe traveltime perturbations and waveform decorrelation, respectively, a distinction which has not been well appreciated so far. Previous results from the literature are recovered as limiting cases of our general approach. To evaluate numerically the sensitivity functions, we introduce an improved version of a spectral technique known as the method of `rotated coordinate frames', which allows global evaluation of the Green's function of the radiative transfer equation in a finite domain. The method is validated through direct pointwise comparison with Green's functions obtained by the Monte Carlo method. To illustrate the theory, we consider a series of scattering media displaying increasing levels of scattering anisotropy and discuss the impact on the traveltime and decorrelation kernels. We also consider the related problem of imaging variations of scattering properties based on intensity perturbations observed in the coda. The impact of anisotropy is particularly pronounced for the scattering and decorrelation sensitivity kernels, which probe spatial/temporal changes in the scattering properties of the medium. Compared to the isotropic case, scattering anisotropy strongly increases the sensitivity of coda waves in the vicinity of the single-scattering ellipse, which may have important implications for imaging applications. In addition to demonstrating the impact of non-isotropic scattering on the sensitivity kernels of coda waves, our work offers a practical solution to model this process accurately.

International audience; In wave physics, the geometrical limit is defined as a propagation regime where the scattering cross-section σ of an object becomes independent of its internal structure and tends to twice its geometrical cross section σg as the frequency goes to infinity. This is a result that is particularly well documented in the field of optics. Following the classification of Wu and Aki (1985b), we study the high-frequency scattering limit for velocity-type and impedance-type elastic perturbations. Although velocity-type scatterers do follow the geometrical limit of σ → 2σg, the scattering cross section of any impedance-type scatterer depends on both its density and elastic properties at all frequencies. These results are illustrated using the example of a spherical inclusion that exhibits a small contrast of properties with its environment. We derive simple asymptotic formulas that show good agreement with exact solutions of the boundary value problem (BVP). Our results confirm the distinct behavior of velocity- type versus impedance-type perturbations at all frequencies.

In this work, we map the absorption properties of the French crust by analyzing the decay properties of coda waves. Estimation of the coda quality factor $$Q_{c}$$ in five non-overlapping frequency-bands between 1 and 32 Hz is performed for more than 12,000 high-quality seismograms from about 1700 weak to moderate crustal earthquakes recorded between 1995 and 2013. Based on sensitivity analysis, $$Q_{c}$$ is subsequently approximated as an integral of the intrinsic shear wave quality factor $$Q_{i}$$ along the ray connecting the source to the station. After discretization of the medium on a 2-D Cartesian grid, this yields a linear inverse problem for the spatial distribution of $$Q_{i}$$ . The solution is approximated by redistributing $$Q_{c}$$ in the pixels connecting the source to the station and averaging over all paths. This simple procedure allows to obtain frequency-dependent maps of apparent absorption that show lateral variations of $$50\%$$ at length scales ranging from 50 km to 150 km, in all the frequency bands analyzed. At low frequency, the small-scale geological features of the crust are clearly delineated: the Meso-Cenozoic basins (Aquitaine, Brabant, Southeast) appear as strong absorption regions, while crystalline massifs (Armorican, Central Massif, Alps) appear as low absorption zones. At high frequency, the correlation between the surface geological features and the absorption map disappears, except for the deepest Meso-Cenozoic basins which exhibit a strong absorption signature. Based on the tomographic results, we explore the implications of lateral variations of absorption for the analysis of both instrumental and historical seismicity. The main conclusions are as follows: (1) current local magnitude $$M_{L}$$ can be over(resp. under)-estimated when absorption is weaker(resp. stronger) than the nominal value assumed in the amplitude-distance relation; (2) both the forward prediction of the earthquake macroseismic intensity field and the estimation of historical earthquake seismological parameters using macroseismic intensity data are significantly improved by taking into account a realistic 2-D distribution of absorption. In the future, both $$M_{L}$$ estimations and macroseismic intensity attenuation models should benefit from high-resolution models of frequency-dependent absorption such as the one produced in this study.