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.

Temperature distribution at depth is of key importance for characterizing the crust, defining its mechanical behavior and deformation. Temperature can be retrieved by heat flow measurements in boreholes that are sparse, shallow, and have limited reliability, especially in active and recently active areas. Laboratory data and thermodynamic modeling demonstrate that temperature exerts a strong control on the seismic properties of rocks, supporting the hypothesis that seismic data can be used to constrain the crustal thermal structure. We use Rayleigh wave dispersion curves and receiver functions, jointly inverted with a transdimensional Monte Carlo Markov Chain algorithm, to retrieve the VS and VP/VS within the crust in the Italian peninsula. The high values (>1.9) of VP/VS suggest the presence of filled‐fluid cracks in the middle and lower crust. Intracrustal discontinuities associated with large values of VP/VS are interpreted as the α−β quartz transition and used to estimate geothermal gradients. These are in agreement with the temperatures inferred from shear wave velocities and exhibit a behavior consistent with the known tectonic and geodynamic setting of the Italian peninsula. We argue that such methods, based on seismological observables, provide a viable alternative to heat flow measurements for inferring crustal thermal structure.

(1) Max-Planck Institute for Solar System Science, Planets and Comets, Göttingen, Germany (joshir@mps.mpg.de), (2) Institute of Geology and Mineralogy, University of Cologne, Cologne, Germany (brigitte.knapmeyer-endrun@uni-koeln.de), (3) Institute of Geophysics, ETH Zürich, Switzerland (vandriel@erdw.ethz.ch), (4) Institute of Geophysics, ETH Zürich, Switzerland (savas.ceylan@erdw.ethz.ch), (5) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA (mark.p.panning@jpl.nasa.gov)

The 50th Lunar and Planetary Science Conference in The Woodlands, Texas, March 18–22, 2019.

Introduction: Global anomalies in the interior of a planet reveal information about its evolution. For example, the location and shape of Large Low Shear Velocity Provinces (LLSVPs) on Earth have been used as constraints for mantle evolution models [1, 2]. However, LLSVPs have first been detected using seismic data, which is not available in sufficient quantity for other planets [3]. Instead, gravity data must be used to search for equivalent features in planetary interiors [4]. However, gravity inversions are non-unique, and the epistemic uncertainty (uncertainty due to model assumptions) is rarely quantified [5]. We describe here a gravity inversion method that addresses these issues and provides a measure of confidence in gravity-derived interior models. Method. The approach advocated here is to conduct a Transdimensional Hierarchical Bayesian Object-Oriented Gravity Inversion (THeBOOGIe). The central idea of this method is to use gravity data of a planetary body as input in a Markov chain Monte Carlo (McMC) algorithm that generates many models of the interior density distribution, each parameterized as a collection of finite-size objects. Confidence in the shape or magnitude of density anomalies depends on how consistent these objects are throughout the different models.

To confirm the accuracy of our forward matrix provide set a point for comparison, we first performed a damped least squares inversion of the residual travel times relative to AK135. The inversion is performed on a 3◦× 3◦ grid, and smoothing is applied at a level determined by L-curve test. The resulting model is shown in Figure S1. The isotropic model exhibits strong hemispheric structure in addition to regional variations on the scale of ∼30◦, particularly in the western hemisphere. The heterogeneity in the eastern hemisphere varies from ∼0.3 %dV/V to ∼0.8 %dV/V, while the variation in the west is stronger, about -1.0 %dV/V to 0.1 %dV/V. It should be noted that the locations of near-zero velocity perturbation in the west correspond to the locations without adequate ray coverage. The isotropic hemisphere boundaries each vary strongly with latitude. The boundary beneath Africa has a westward bulge of 3–5◦ at 30◦S, while the Pacific boundary trends westward from 210◦ to 180◦ between the northern and southern ends of the boundary. As a result of lateral smoothing, the transitions between the hemispheres appear to take place over 10–15◦ in longitude. The anisotropic part (b + c) has no discernible hemispheric structure. The damping limits anisotropy primarily to regions with rays traveling in both the axial and equatorial

Introduction: Geophysical evidence from the Galileo mission hints that Europa’s ice shell is underlain by a global water ocean (e.g., [13]). However, the thickness of the ice shell and the connectivity between the ocean and surface is still unclear. Seismology offers a promising means of probing Europa’s ice shell structure since tidally induced ice fracturing events provide a natural source of seismic energy to illuminate the subsurface (e.g., [4 6]). A future seismic lander mission to Europa will likely employ a variety of techniques to image the interior, including body wave, surface wave, and normal mode seismology. Here, we use numerical simulations of seismic wave propagation on Europa in order to investigate the potential of using surface wave dispersion measurements to constrain the ice shell thickness. Numerical simulations of wave propagation: We simulate seismic wave propagation through thermodynamically self-consistent models of Europa’s interior [7] at frequencies up to 1 Hz using the spectral-element method (SEM) code AxiSEM [8] (Fig. 1). The modeling suggests that Mw 3 or greater events, which may be fairly common on Europa (e.g., [9]), are likely to produce Rayleigh waves that could be observed globally by commonly employed seismic instruments.

Low-velocity layers within the crust can indicate the presence of melt and lithologic differences with implications for crustal composition and formation. Seismic wave conversions and reverberations across the base of the crust or intracrustal discontinuities, analysed using the receiver function method, can be used to constrain crustal layering. This is commonly accomplished by inverting receiver functions jointly with surface wave dispersion. Recently, the proliferation of model-space search approaches has made this technique a workhorse of crustal seismology. We show that reverberations from shallow layers such as sedimentary basins produce spurious low-velocity zones when inverted for crustal structure with surface wave data of insufficiently high frequency. Therefore, reports of such layers in the literature based on inversions using receiver function data should be re-evaluated. We demonstrate that a simple resonance-removal filter can suppress these effects and yield reliable estimates of crustal structure, and advocate for its use in receiver-function based inversions.