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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]

On 4 May 2022, the seismometer on Mars observed the largest marsquake (S1222a) during its operation. One of the most specific features of S1222a is the long event duration lasting more than 8 hr, in addition to the clear appearance of body and surface waves. As demonstrated on Earth, by modeling a long‐lasting and scattered surface wave with the radiative transfer theory under the isotropic scattering condition, we estimated the scattering and intrinsic quality factors of Mars (Qs and Qi). This study especially focused on the frequency range between 0.05–0.09 Hz, where Qs and Qi have not been constrained yet. Our results revealed that Qi = 1,000–1,500 and Qs = 30–500. By summarizing the Martian Qi and Qs estimated so far and by comparing them with those of other celestial bodies, we found that, overall, the Martian scattering and absorption properties showed Earth‐like values. Plain Language Summary: Since February 2019, NASA's InSight (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport) has been conducting quasi‐continuous seismic observation for more than three years. The seismic data from Mars has contributed significantly to a better understanding of the interior structure and the seismicity of the red planet. On 4 May 2022 (1222 Martian days after landing), another key event occurred, called S1222a. The event showed the largest seismic moment release (magnitude 4.7) and extremely long duration (>8 hr) with intense seismic scattering. As demonstrated on Earth, the long‐lasting scattered waves are useful for retrieving information about the structural heterogeneity within a planet. In this study, by applying the radiative transfer theory—which considers the energy transportation from the seismic source to the observation point—to Mars, we evaluated the energy decay rate due to seismic scattering and energy absorption by a medium. By comparing our results with those of other solid bodies, we found that the Martian scattering and absorption features were closer to the terrestrial ones than to the lunar ones. Key Points: We modeled the scattering effect of the largest marsquake (S1222a) using radiative transfer theory on a spherical MarsThe inversion revealed that the intrinsic and scattering quality factors below 0.1 Hz are 1,000–1,500 and 30–500, respectivelyWe summarized the Martian quality factors derived so far and found that they are relatively Earth‐like rather than Moon‐like [ABSTRACT FROM AUTHOR]

Monitoring changes of seismic properties at depth can provide a first‐order insight into Earth's dynamic evolution. Coda wave interferometry is the primary tool for this purpose. This technique exploits small changes of waveforms in the seismic coda and relates them to temporal variations of attenuation or velocity at depth. While most existing studies assume statistically homogeneous scattering strength in the lithosphere, geological observations suggest that this hypothesis may not be fulfilled in active tectonic or volcanic areas. In a numerical study we explore the impact of a non‐uniform distribution of scattering strength on the spatio‐temporal sensitivity of coda waves. Based on Monte Carlo simulation of the radiative transfer process, we calculate sensitivity kernels for three different observables, namely travel‐time, decorrelation, and intensity. Our results demonstrate that laterally varying scattering properties can have a profound impact on the sensitivities of coda waves. Furthermore, we demonstrate that the knowledge of the mean intensity, specific intensity, and energy flux, governed by spatial variation of scattering strength, is key to understanding the decorrelation, travel‐time, and scattering kernels, respectively. A number of previous works on coda wave sensitivity kernels neglect the directivity of energy fluxes by employing formulas extrapolated from the diffusion approximation. In this work, we demonstrate and visually illustrate the importance of the use of specific intensity for the travel‐time and scattering kernels, in the context of volcanic and fault zone setting models. Our results let us foresee new applications of coda wave monitoring in environments of high scattering contrast. Plain Language Summary: To monitor the evolution of the dynamic Earth, seismologists use a part of the seismic record called "coda," which is composed of waves that have bounced multiple times off heterogeneities of the crust. The coda is extremely sensitive to weak perturbations of propagation properties induced by Earth's tectonic and volcanic activity. The correct physical modeling of coda waves is therefore key to unravel the rich information encoded in their waveforms. A limitation of current seismological monitoring techniques is the neglect of strong lateral variations of coda waves propagation properties documented by geological observations. Our work focuses specifically on this aspect. We provide a complete theoretical and numerical framework to model and understand the spatial and temporal sensitivity of coda waves to medium perturbations in complex geological settings. Using simple but realistic models of a fault zone and a volcano, we illustrate the profound impact of non‐uniform scattering properties on the coda wave sensitivity, which in turn determines the ability of seismologists to correctly retrieve the magnitude and location of physical changes in the crust. Our results let us foresee new applications of coda wave monitoring in environments of high scattering contrast, such as volcanic and fault zone settings. Key Points: A non‐uniform distribution of scattering strength can have a profound impact on the spatio‐temporal sensitivity of coda wavesWe illustrate this using Monte Carlo simulations for models with either a volcanic, fault zone, or two half‐spaces settingThe mean intensity, specific intensity, and energy flux, is key to understanding the decorrelation, travel‐time, and scattering kernels, respectively [ABSTRACT FROM AUTHOR]

Velocity variations obtained from ambient seismic noise are sensitive to many factors. We aimed to disentangle these processes in a 10-year-long recording of seismic noise from a single station in the Pollino region, in southern Italy. This region is characterized by aquifers and by a relatively short period of high seismic activity that included slow slip events and a MW5.0 earthquake that occurred on October 25, 2012. We apply two models that estimate the water level inside an aquifer, which show a good correlation with the measured δv/v, showing that the velocity variations are inversely proportional to the pore pressure inside the aquifer. Our interpretation is further confirmed by geodetic measurements that show that in a direction parallel to the strike angle of the fault rupture, the expansion-contraction displacement of the zone follows the same patterns observed in the models and in the velocity variations, as a result of the pressure generated by the water on its interior. Going one step further, we analyze the nature of the small discrepancies between the measured and modeled velocity variations. These correlate well with the rainfall and with the vertical geodetic measures, which indicates an elastic response of the zone to the loading generated by the rainwater. Comparisons between these variables allow us to clearly identify the period of the seismic activity in the zone, which is represented by the characteristic drop in the seismic velocity in the period from the beginning of 2012 to mid-2013. [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]

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]