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We have modeled the high‐frequency seismogram envelopes of the large event S1222a and four recently identified near impacts recorded by the InSight mission by introducing a stratification of velocity and attenuation into a multiple‐scattering approach. We show that a simple conceptual model composed of a strongly diffusive, weakly attenuating layer overlying a transparent medium captures the essential features of the observed envelopes. The attenuation profiles reveal that the minimal extension of heterogeneities at depth is of the order of 20 km in the vicinity of InSight and 60 km on the path to S1222a. We interpret this result as an indication that the Martian crust as a whole is at the origin of the strong scattering. Our heterogeneity model suggests that the sources of a number of distant Very‐high‐Frequency seismic events are shallow and located to the south or in close vicinity of the Martian dichotomy. Plain Language Summary: The seismometers deployed at the surface of Mars in the framework of the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport mission have recently recorded seismic waves generated by meteorite impacts and a very large Marsquake. These exceptional data offer the opportunity to study how seismic waves propagate in the interior of Mars and more particularly how they attenuate. This is an important topic because attenuation characterizes the physical state of planetary interiors. There are two basic mechanisms at the origin of seismic attenuation: "absorption," which is highly sensitive to the presence of fluids—such as water—in the porosity of the rocks and "scattering," which is caused by the geological heterogeneity at length scales ranging from tens of meters to kilometers. Using advanced modeling techniques which allow for the separate quantification of the two processes, we have determined that scattering is the dominant seismic attenuation mechanism on Mars, that originates from a heterogeneous and dry crust. In the light of this result, we have revised previous interpretations of specific seismic events with a predominantly high‐frequency content and propose that they originate from the vicinity of a major geological feature known as the Martian dichotomy. Key Points: Envelope modeling of event S1222a and near impacts reveals a strong stratification of scattering properties in the lithosphere of MarsThe whole crust acts as a diffusive layer with a minimal thickness of 20 km below InSight and 60 km at the location of S1222aDistant Very‐high‐Frequency events are shallow quakes or impacts originating from the south or the vicinity of the Martian dichotomy [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: The correlation of seismic records is the basis of seismic interferometry methods. These methods use seismic waves, either from ambient vibrations of the planet or from quakes, that are scattered in the medium in order to recover information about the structure between two seismic sensors. The method is implemented to compute the auto‐correlation functions of the three components of the ground motion recorded by the SEIS seismometer. The comparison of the results obtained from earthquake data to the ones obtained from ambient vibrations demonstrates that the ambient seismic vibration is clearly above the self‐noise of SEIS during early night hours around a specific frequency (2.4 Hz). The seismic vibrations appear to be amplified at this frequency by an unknown mechanism. Some seismic energy arrivals appear consistently in the auto‐correlation functions, at specific propagation times, independent of the data sets and processing parameters tested. These arrivals are interpreted as vertically propagating seismic waves which are reflected on top of crustal layers. Their propagation times can be used to constrain a model of Mars crustal structure. Key Points: Autocorrelation functions (ACFs) of SEIS ambient vibrations and seismic events are computed and validated by intercomparisonThe stability of autocorrelations at 2.4 Hz resonance favor an excitation by a diffuse seismic wavefieldVarious arrivals are observed in ACFs and interpreted as seismic reflections on internal discontinuities [ABSTRACT FROM AUTHOR]

We report measurements of the attenuation of short period seismic waves in the Moon based on the quantitative analysis of envelope records of lunar quakes. Our dataset consists of waveforms corresponding to 62 events, including artificial and natural impacts, shallow moonquakes and deep moonquakes, recorded by the four seismometers deployed during Apollo missions 12, 14, 15 and 16. To quantify attenuation and distinguish between elastic (scattering) and inelastic (absorption) mechanisms we measure the time of arrival of the maximum of energy t max and the coda quality factor Q c . The former is controlled by both scattering and absorption, while the latter is an excellent proxy for absorption. Consistent with the strong broadening of seismogram envelopes in the Moon, we employ diffusion theory in spherical geometry to model the propagation of seismic energy in depth-dependent scattering and absorbing media. To minimize the misfit between predicted and observed t max for deep moonquakes and impacts, we employ a genetic algorithm and explore a large number of depth-dependent attenuation models quantified by the scattering quality factor Q sc or equivalently the wave diffusivity D , and the absorption quality factor Q i . The scattering and absorption profiles that best fit the data display very strong scattering attenuation ( Q sc ≤ 10 ) or equivalently very low wave diffusivity ( D ≈ 2 km 2 /s) in the first 10 km of the Moon. These values correspond to the most heterogeneous regions on Earth, namely volcanic areas. Below this surficial layer, the diffusivity rises very slowly up to a depth of approximately 80 km where Q sc and D exhibit an abrupt increase of about one order of magnitude. Below 100 km depth, Q sc increases rapidly up to approximately 2000 at a depth of about 150 km, a value similar to the one found in the Earth’s mantle. By contrast, the absorption quality factor on the Moon Q i ≈ 2400 is about one order or magnitude larger than on Earth. Our results suggest the existence of an approximately 100 km thick megaregolith, which is much larger than what was previously thought. The rapid decrease of scattering attenuation below this depth is compatible with crack healing through viscoelastic mechanisms. Using our best attenuation model, we invert for the depth of shallow moonquakes based on the observed variation of t max with epicentral distance. On average, they are found to originate from a depth of about 50 km ± 20 km, which suggests that these earthquakes are caused by the failure of deep faults in the brittle part of the Moon. [ABSTRACT FROM AUTHOR]

We have studied the partition of shear, compressional and kinetic energies in the coda of 10 earthquakes recorded on a dense array, located at Pinyon Flats Observatory (PFO), California. Deformation energies are estimated by measuring finite differences of the wavefield components. We have thoroughly studied the validity and stability of this technique for the PFO data and obtained reliable measurements in the 5–7 Hz frequency band. We observe a clear stabilization of the ratio between shear and compressional energies in the coda, with similar values for all 10 earthquakes under study. We interpret this observation as a signature of equipartition. The average ratio is about 2.8, which is smaller by a factor 2.5 than the expected value, around 7.2, for equipartitioned elastic waves at the surface of a homogeneous Poisson half-space. The ratio between the vertical and horizontal kinetic energies also exhibits stabilization in the coda and can be measured from 5 to 25 Hz. The ratio shows an abrupt transition from 0.1 in the 5–10 Hz band, to about 0.8 in the 15–25 Hz band. These measured values are again in sharp contrast with the theoretical prediction, around 0.56, for equipartitioned elastic waves at the surface of a Poisson half-space. To explain these observations, we have developed a theory of equipartition in a layered elastic half-space. Using a rigorous spectral decomposition of the elastic wave equation, we define equipartition as a white noise distributed over the complete set of eigenfunctions. This definition is shown to agree with the standard physical concepts in canonical cases. The theory predicts that close to the resonance frequency of a low-velocity layer, the ratio between the shear and compressional energies strongly decreases. Using a detailed model of the subsurface at PFO, this counter-intuitive result is found to be in good qualitative and quantitative agreement with the observations. Near the resonance frequency of the low-velocity structure, the drop of the energy ratios and is controlled by the change of ellipticity of the Rayleigh wave and the large contribution of the fundamental Love mode. At higher frequencies, the interplay between Rayleigh and Love modes trapped in shallow low-velocity layers is responsible for the abrupt increase of the kinetic energy ratio . Our study demonstrates that the partition of energy in the seismic coda contains information on the local geological structure. [ABSTRACT FROM AUTHOR]

Random field cross-correlation is a new promising technique for seismic exploration, as it bypasses shortcomings of usual active methods. Seismic noise can be considered as a reproducible, stationary in time, natural source. In the present paper we show why and how cross-correlation of noise records can be used for geophysical imaging. We discuss the theoretical conditions required to observe the emergence of the Green's functions between two receivers from the cross-correlation of noise records. We present examples of seismic imaging using reconstructed surface waves from regional to local scales. We also show an application using body waves extracted from records of a small-scale network. We then introduce a new way to achieve surface wave seismic experiments using cross-correlation of unsynchronized sources. At a laboratory scale, we demonstrate that body wave extraction may also be used to image buried scatterers. These works show the feasibility of passive imaging from noise cross-correlation at different scales. [ABSTRACT FROM AUTHOR]

Abstract: Most theoretical investigations of seismic wave scattering rely on the assumption that the underlying medium is statistically isotropic. However, deep seismic soundings of the crust as well as geological observations often reveal the existence of elongated or preferentially oriented scattering structures. In this paper, we develop mean field and radiative transfer theories to describe the attenuation and multiple scattering of a scalar wavefield in an anisotropic random medium. The scattering mean free path is found to depend strongly on the propagation direction. We derive a radiative transfer equation for statistically anisotropic random media from the Bethe–Salpeter formalism and propose a Monte Carlo method to solve this equation numerically. At longer times, the energy density is shown to obey a tensorial diffusion equation. The components of the diffusion tensor are obtained in closed form and excellent agreement is found between Monte Carlo simulations and analytical solutions of the diffusion equation. The theory has important potential implications for lithospheric models where scatterers are for example flat structures preferentially aligned along the surface. In this simple geometry, analytical expressions of the Coda Q parameter can be given explicitly in the diffusive regime. Our results suggest that pulse broadening and coda decay are controlled by different parameters, related to the eigenvalues of the diffusion tensor. These eigenvalues can differ by more than one order of magnitude. This theory could be applied to probe the anisotropy of length scales in the lithosphere. [Copyright &y& Elsevier]

Understanding the internal structure of an asteroid has important implications for interpreting its evolutionary history, for understanding its continuing geological evolution, and also for asteroid deflection and in-situ space resource utilisation. Given the strong evidence that asteroids are seismically active, an in-situ passive seismic experiment could provide information about the asteroid surface and interior properties. Here, we discuss the natural seismic activity that may be present on Didymoon, the secondary component of asteroid (65803) Didymos. Our analysis of the tidal stresses in Didymoon shows that tidal quakes are likely to occur if the secondary has an eccentric orbit. Failure occurs most easily at the asteroid poles and close to the surface for both homogeneous and layered internal structures. Simulations of seismic wave propagation in Didymoon show that the seismic moment of even small meteoroid impacts can generate clearly observable body and surface waves if the asteroid's internal structure is homogeneous. The presence of a regolith layer over a consolidated core can result in the seismic energy becoming trapped in the regolith due to the strong impedance contrast at the regolith-core boundary. The inclusion of macro-porosity (voids) further complexifies the wavefield due to increased scattering. The most prominent seismic waves are always found to be those traveling along the surface of the asteroid and those focusing in the antipodal point of the seismic source. We find also that the waveforms and ground acceleration spectra allow discrimination between the different internal structure models. Although the science return of a passive seismic experiment would be enhanced by having multiple seismic stations, one single seismic station can already vastly improve our knowledge about the seismic environment and sub-surface structure of an asteroid. We describe several seismic measurement techniques that could be applied in order to study the asteroid internal structure with one three-component seismic station. [ABSTRACT FROM AUTHOR]

We present theory and numerical simulations to model seismic wave propagation in the Earth crust. We compare them to observations made in Mexico. [Copyright &y& Elsevier]