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For the first time, we measured the ellipticity of direct Rayleigh waves at intermediate periods (15–35 s) on Mars using the recordings of three large seismic Martian events, including S1222a, the largest event recorded by the InSight mission. These measurements, together with P‐to‐s receiver functions and P‐wave reflection times, were utilized for performing a joint inversion of the local crustal structure at the InSight landing site. Our inversion results are compatible with previously reported intra‐crustal discontinuities around 10 and 20 km depths, whereas the preferred models show a strong discontinuity at ∼37 km, which is interpreted as the crust‐mantle interface. Additionally, we support the presence of a shallow low‐velocity layer of 2–3 km thickness. Compared to nearby regions, lower seismic wave velocities are derived for the crust, suggesting a higher porosity or alteration of the whole local crust. Plain Language Summary: As never before on Mars, we measured the characteristics of seismic waves traveling along the Martian surface that carry information about the crustal structure at the InSight site. We combined these measurements with two other local‐scale independent observations to derive a consolidated model for the crust underneath the InSight lander. Our results suggest a Martian crust with 4 layers and, particularly, one thin layer of about 2 km thickness close to the surface. The crust‐mantle discontinuity was found at ∼37 km depth, where the sharpest change in seismic wave velocity is observed. Overall, the seismic wave velocities of the local Martian crust at the InSight site are lower than those derived in other regions on Mars, which suggests a higher porosity or local alteration. Key Points: Rayleigh waves ellipticity was measured between periods 15–35 s at the InSight landing site using large seismic events, including S1222aA 4‐layer crust, including a shallow low‐velocity layer, is required to explain the ellipticity, receiver functions and P‐wave lag timesLow crustal velocities are derived for the InSight site, which may be due to high porosity or heavy alteration at local scale [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Seismic waves radiated by small crustal earthquakes are prone to multiple mode conversions caused by reflection and transmission at interfaces, and scattering by small-scale heterogeneities in the bulk of the medium. The goal of this study is to clarify the complex interplay between volume scattering and interface reflections in crustal waveguides and how it will impact the crustal energy propagation. To carry out this task, we have incorporated a rigorous description of wave polarization in the context of Monte–Carlo simulations of the multiple-scattering process by introducing a five-dimensional Stokes vector. To shed light on the wave content of the regional short-period seismic wavefield, we investigate the asymptotic partitioning of seismic energy onto P, SV and SH polarizations in the coda, as well as the angular distribution of energy flux in the waveguide. In full elastic space, equipartition theory predicts that (1) the energy ratio between P- and S-wave energies tends to β 3 / (2 α 3) , (2) an equal distribution of energy among SV- and SH-waves and (3) that energy fluxes are isotropic. In the presence of interfaces, we find that the isotropy of the wavefield is systematically broken and that energy ratios are shifted to the detriment of P-waves and in favor of SV-waves in a non-absorbing medium. This implies that a residual polarization is preserved in the waveguide. Through an extensive parametric study, we illustrate in detail how the anisotropy of the wavefield, the partitioning ratios and the shear wave polarization depend on the crustal attenuation parameters. The role of the initial polarization at the source has also been examined. In the case of an explosion and a shear dislocation with equal magnitude, we find that the energy level in the coda can differ by more than one order of magnitude when the effect of crustal scattering becomes very weak compared to reflections or transmissions at interfaces. When comparing different shear dislocation mechanisms, we find that the energy level in the coda can differ by up to 60%. While equipartition, depolarization and coda normalization remain fundamental guides to our understanding of the coda, their application requires a good a priori knowledge of the attenuation properties of the crust. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. Key Points: Estimation of high‐frequency reflection response using phase autocorrelations of seismic noise recorded on Mars at the InSight landing sitePresentation of a new data processing method that avoids aseismic signal bias and stability analysis of the reflection responseA signal at 10.6 s lag time is a possible candidate for a reflection from the base of the crust due to its strength, polarity, and stability [ABSTRACT FROM AUTHOR]

Scattered 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. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: The high‐frequency events are the most commonly observed class of marsquakes by the InSight mission. As the frequency content and signal shape over time is different from seismic events (i.e., events that excite elastic waves traveling in the subsurface such as earthquakes, impacts, or explosions) observed both on Earth and the Moon, these were not immediately recognized as signals of seismic origin. This paper shows that these signals can be explained by distant shallow small quakes together with wave propagation effects in the Martian crust. This interpretation opens the possibility to use these signals to probe the material properties of the crust and raises the question which physical process causes these events. Key Points: InSight's seismometers have recorded several hundreds of events at frequencies between 1 and 10 HzThe envelopes of these events can be explained by seismic waves guided in the crust over significant distancesThis observation helps to constrain the elastic properties of the shallow structure [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Radiative transfer theory is applied to the measurment of average values of the transport mean free path and absorption length (l[SUBa], related to the intrinsic quality factor) in the lithosphere beneath France by comparing simulated and observed coda envelopes of regional records. The lithosphere is modeled as a flat layer representing the crust overlying a half-spaces representing the mantle, with different velocities and scattering properties. We infer the average values of l[SUBa] (assumed to be constant over the whole medium) and transport mean free paths in the crust (l*[SUBC]) and mantle (l*[SUBM]) by minimizing a misfit function between observed and modeled L[SUBg] coda envelopes. This comparison is conducted in two different time windows, the whole L[SUBg] coda (group velocity smaller than 2.6 km s[SUP-1]) and the early coda (v[SUBg] > 2.6 km s[SUP-1] and lapse time smaller than 300 s), when coda envelopes have decay rates that depend on distance. In the early coda, a single set of parameters L[SUBa] and l*[SUBc] minimizes the misfit function. Using the whole L[SUBg] coda time window, we find that many models fit the data equally well. We explain this difference by the common decay rate of envelopes independent of distance over most of the coda window. The measurement of the attenuation of the coherent L[SUBg] wave complements these results to constrain l[SUBa] and l*[SUBc]. We find that a l*[SUBc] ∼250 km l[SUBa] ∼ 150 km explains the attenuation of both the direct L[SUBg] wave train and the entire L[SUBg] coda with both time and distance at 3 Hz. However, the trade-off between the two parameters is too strong to conclude reliable on the dominance of intrinsic on scattering attenuation. [ABSTRACT FROM AUTHOR]

Seismic attenuation parameters in Taiwan have been obtained by using a modified "MultipleLapse Time Window Analysis" approach in order to take into account the scatteringanisotropy. MLTWA method exploits the spatio-temporal dependence of the ratio betweencoherent and incoherent seismic intensity to determine the scattering and absorptionproperties of a heterogeneous medium. In addition to the mean free path l (or the scatteringquality factor Qsc), the seismic wave propagation is controlled by a parameter g(independent of l) which determines the angular redistribution of energy upon scattering(scattering anisotropy). Preferentially forward (resp. backward) scattering correspondsto g > 0 (resp. g < 0), with g = 0 for isotropic scattering. We use Monte-Carlosimulations to compute Green functions and sensitivity kernels for Qsc, Qi (absorptionquality factor) and g in media with an heterogeneous crust overlying a transparentmantle. We analyse seismic records of local earthquakes with magnitude greater than 3.5which occurred between 1994 and 2016. For each stations, we collect events withina radius of 100 km and a maximum depth of 40 km. About 350000 waveformswith a S/N ratio greater than 4 in the coda are selected. Seismic energy in three15 s time windows from the S-wave arrival are measured in 3 frequency bands(1-2, 2-4,4-8 Hz). To account for site amplification and source amplitude, wenormalize observed energy by the average energy in a late coda window at 65 s fromthe origin time of the earthquakes. For every station, we analyse the normalizedenergy in the three time widows as a function of the hypocentral distance. In order todetermine the attenuation parameters Qsc−1, Qi−1 and g with related errors, weimplement a Levenberg-Marquardt algorithm. To guide the inversion, we use thesensitivity kernels for the attenuation parameters computed with our Monte-Carloapproach. Overall, Taiwan is more attenuating than most orogens with a mean effective scatteringloss (Qsc∗)−1 = Qsc−1(1 − g) about 0.012 and a mean intrinsic absorption Qi−1 about0.009 at 1.5 Hz. Our maps reveals strong lateral variations of attenuation with scale lengthfrom 10 to 100 km. Scattering loss (Qsc∗)−1 varies over more than one order of magnitudeacross Taiwan (between 0.0022 and 0.16 at 1.5 Hz). By contrast, absorption fluctuations areabout 30%. The more attenuating zones are the Coastal Range and the Costal Plain wherescattering dominates over absorption at low frequency, and inversely at high frequency. Theseregions are also characterized by strong backscattering (g < −0.85) at 1.5 Hz and ratherhigh V P∕V S ratio. Scattering becomes much more isotropic at high frequency.We speculate that the observed strong backscattering at low frequency is relatedto strong impedance fluctuations in the crust induced by the presence of fluids. [ABSTRACT FROM AUTHOR]

Authors:T. Kawamura(1), N. C. Schmerr(2), L. Margerin(3), M. van Driel(4), R. Garcia(5), F. Karakostas(1,2), Benoit Tauzin (6), M. Drilleau(1), P. Lognonné(2),Affiliations:1 Institut de Physique du Globe de Paris2 University of Maryland3 CNRS / Université de Toulouse 4 ETH Zurich5 ISAE-SUPAERO6 Université de LyonTitle:Measuring the Scattering and Atteunation of Seismic Waves in Mars with the InSight Seismometers. Abstract: The amplitudes of seismic waves decrease over distance due frequency dependent energy loss via scattering and attenuation. Scattering, or extrinsic attenuation, is dependent upon the size, strength, and abundance of elastic heterogeneities within the medium. Intrinsic attenuation results from dissipation due to anelastic processes and internal friction between molecules and defects within crystal lattices. Both forms of attenuation are described using the seismic quality factor, Q. Scattering can arise from compositional heterogeneities, mechanical boundaries (such as fractures, cracks, veins, cavities), and other major non-radial changes in material properties of an object. Intrinsic attenuation is tied to grain boundary processes, crystal defects, fluid-filled cracks, and other dissipative viscoelastic processes. Attenuation is vital for understanding the detectability, travel times, and propagation properties of seismic waves, as well as the properties of the interior of a world. The Interior Exploration using Seismic Investigations Geodesy and Heat Transport (InSight) Seismic Experiment for Interior Structure (SEIS) has placed a very broadband (VBB) and short period (SP) seismometers on Mars that allows us to investigate the fundamental geophysical process of seismic attenuation at Mars, which is expected to have properties falling between the Earth and the Moon. In the Earth, attenuation by seismic scattering is found to be relatively weak, and is typically confined to small-scale compositional heterogeneities in the crust, lithosphere, and mantle. Intrinsic attenuation varies throughout the Earth, and is especially high where fluids, elevated temperatures, and changes in grain size are present. On the Moon, seismic scattering is pervasive in the highly fractured and dry crust and uppermost mantle, and intrinsic attenuation is large in the deep interior. At present, the total Q, layering in Q, and relative proportion of seismic scattering to intrinsic attenuation within Mars remains unconstrained. [ABSTRACT FROM AUTHOR]

Investigating spatio-temporal changes in crustal structure is a key to understanding the dynamic processes in the earth's crust associated with large earthquakes and volcanic activities. The dense seismic network in the northeast part of Japan (Tohoku region) allows us to apply seismic interferometry to continuous records of ambient noise data for studying crustal changes related to the 2008 M w 6.9 Iwate-Miyagi Nairiku earthquake and the 2011 M w 9.0 Tohoku-oki earthquake. We track changes in both seismic scattering properties and velocities over five years and spatially localize them based on the coda wave decorrelation and travel time sensitivity kernels calculated from the solution of the radiative transfer equation. The 2008 Iwate-Miyagi Nairiku earthquake caused significant changes in both seismic scattering properties and seismic velocities in the vicinity of the epicenter subjected to dynamic or/and static stress perturbations. Changes in scattering properties associated with the 2011 Tohoku-Oki earthquake around some active volcanoes in the Tohoku region are likely related to geofluid migrations triggered by strong vibrations during the large earthquake. The decrease in seismic velocity exists mainly along the eastern coastal region and extends to the central Quaternary volcanic chains under the combined dynamic and static stress changes by the earthquake. The waveform correlation is possibly sensitive to the fluid content and might be a useful indicator of fluid migration in the crust. This is the first comprehensive demonstration and comparison of the spatio-temporal changes of seismic scattering properties and velocities at long-term and regional scales under the influence of two large earthquakes. • Large earthquakes caused changes in subsurface structure in northeastern Japan. • Fluid migration causes co-seismic changes in scattering properties near volcanoes. • Both dynamic and static stress affect co-seismic velocity drops. [ABSTRACT FROM AUTHOR]