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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 critical zone (CZ) is the region of the Earth's surface that extends from the bottom of the weathered bedrock to the tree canopy and is important because of its ability to store water and support ecosystems. A growing number of studies use active source shallow seismic refraction to explore and define the size and structure of the CZ across landscapes. However, measurement uncertainty and model resolution at depth are generally not evaluated, which makes the identification and interpretation of CZ features inconclusive. To reliably resolve seismic velocity with depth, we implement a Transdimensional Hierarchical Bayesian (THB) framework with reversible‐jump Markov Chain Monte Carlo to generate samples from the posterior distribution of velocity structures. We also perform 2D synthetic tests to explore how well THB traveltime inversion can resolve different subsurface velocity structures. We find that THB recovers both sharp changes in velocity as well as gradual velocity increases with depth. Furthermore, we explore the velocity structure in a series of ridge‐valley systems in northern California. The posterior velocity model shows an increasing thickness of low velocity material from channels to ridgetops along a transect parallel to bedding strike, implying a deeper weathering zone below ridgetops and hillslopes than below channels. The THB method enhances the ability to reliably image CZ structure, and the model uncertainty estimates it yields provides an objective way to interpret deep CZ structure. The method can be applied across other near‐surface studies, especially in the presence of significant surface topography. Key Points: An open‐source Transdimensional Hierarchical Bayesian (THB) inversion code can resolve seismic structure and measurement uncertainty from active source seismic refraction surveysComprehensive synthetic tests demonstrate the reliability of using seismic refraction for critical zone researchA field survey with THB inversion shows changes of weathering structure thickness in a ridge‐channel system landscape [ABSTRACT FROM AUTHOR]

Receiver functions are sensitive to sharp seismic velocity variations with depth and are commonly used to constrain crustal thickness. The H – κ stacking method of Zhu & Kanamori is often used to constrain both the crustal thickness (H) and |${V_P}$| / |${V_S}$| ratio (⁠|$\kappa $|⁠) beneath a seismic station using P-to-s converted waves (Ps). However, traditional H –κ stacks require an assumption of average crustal velocity (usually |${V_P}$|⁠). Additionally, large amplitude reverberations from low velocity shallow layers, such as sedimentary basins, can overprint sought-after crustal signals, rendering traditional H – |$\ \kappa $| stacking uninterpretable. We overcome these difficulties in two ways. When S -wave reverberations from sediment are present, they are removed by applying a resonance removal filter allowing crustal signals to be clarified and interpreted. We also combine complementary Ps receiver functions, Sp receiver functions, and the post-critical P -wave reflection from the Moho (SPmp) to remove the dependence on an assumed average crustal |${V_P}$|⁠. By correcting for sediment and combining multiple data sets, the crustal thickness, average crustal P -wave velocity and crustal |${V_P}$| / |${V_S}$| ratio is constrained in geological regions where traditional H – |$\ \kappa $| stacking fails, without making an initial P -wave velocity assumption or suffering from contamination by sedimentary reverberations. [ABSTRACT FROM AUTHOR]

Knowing the location of large-scale turbulent eddies during catastrophic flooding events improves predictions of erosive scour. The erosion damage to the Oroville Dam flood control spillway in early 2017 is an example of the erosive power of turbulent flow. During this event, a defect in the simple concrete channel quickly eroded into a 47m deep chasm. Erosion by turbulent flow is difficult to evaluate in real time, but near-channel seismic monitoring provides a tool to evaluate flow dynamics from a safe distance. Previous studies have had limited ability to identify source location or the type of surface wave (i.e., Love or Rayleigh wave) excited by different river processes. Here we use a single three-component seismometer method (frequency-dependent polarization analysis) to characterize the dominant seismic source location and seismic surface waves produced by the Oroville Dam flood control spillway, using the abrupt change in spillway geometry as a natural experiment. We find that the scaling exponent between seismic power and release discharge is greater following damage to the spillway, suggesting additional sources of turbulent energy dissipation excite more seismic energy. The mean azimuth in the 5-10 Hz frequency band was used to resolve the location of spillway damage. Observed polarization attributes deviate from those expected for a Rayleigh wave, though numerical modeling indicates these deviations may be explained by propagation up the uneven hillside topography. Our results suggest frequencydependent polarization analysis is a promising approach for locating areas of increased flow turbulence. This method could be applied to other erosion problems near engineered structures as well as to understanding energy dissipation, erosion, and channel morphology development in natural rivers, particularly at high discharges. [ABSTRACT FROM AUTHOR]

Large low shear velocity provinces (LLSVPs), whose origin and dynamic implication remain enigmatic, dominate the lowermost mantle. For decades, seismologists have created increasingly detailed pictures of the LLSVPs through tomographic models constructed with different modeling methodologies, data sets, parametrizations and regularizations. Here, we extend the cluster analysis methodology of Lekic et al., to classify seismic mantle structure in five recent global shear wave speed (VS) tomographic models into three groups. By restricting the analysis to moving depth windows of the radial profiles of VS, we assess the vertical extent of features. We also show that three clusters are better than two (or four) when representing the entire lower mantle, as the boundaries of the three clusters more closely follow regions of high lateral VS gradients. Qualitatively, we relate the anomalously slow cluster to the LLSVPs, the anomalously fast cluster to slab material entering the lower mantle and the neutral cluster to 'background' lower mantle material. We obtain compatible results by repeating the analysis on recent global P-wave speed (VP) models, although we find less agreement across VP models. We systematically show that the clustering results, even in detail, agree remarkably well with a wide range of local waveform studies. This suggests that the two LLSVPs consist of multiple internal anomalies with a wide variety of morphologies, including shallowly to steeply sloping, and even overhanging, boundaries. Additionally, there are indications of previously unrecognized meso-scale features, which, like the Perm anomaly, are separated from the two main LLSVPs beneath the Pacific and Africa. The observed wide variety of structure size and morphology offers a challenge to recreate in geodynamic models; potentially, the variety can result from various degrees of mixing of several compositionally distinct components. Finally, we obtain new, much larger estimates of the volume/mass occupied by LLSVPs--8.0 per cent ±0.9 (µ ± 1s) of whole mantle volume and 9.1 per cent ±1.0 (µ ± 1s) of whole mantle mass--and discuss implications for associating the LLSVPs with the hidden reservoir enriched in heat producing elements. [ABSTRACT FROM AUTHOR]