The Atlantic Coastal Plain is comprised of thick, unconsolidated sediments that span much of the East Coast of the United States. Seismic waves passing through this shallow, loosely compacted layer of sedimentary rock display increased ground motion amplitudes as measured on broadband seismometers deployed during the Southeastern Suture of the Appalachian Margin Experiment (SESAME) from 2010–2014. This amplification can lead to greater damage during earthquakes, making site response analysis, and constraints on the associated shallow shear velocity structure, important for informing ground motion modeling in the region. Site amplification is most noticeable in the horizontal direction and, therefore, can be quantified at single seismic stations by dividing the horizontal spectrum by the vertical spectrum of ambient seismic noise. The resulting horizontal-to-vertical spectral ratio (HVSR) provides a simple estimate of the fundamental frequency of a given site; however, more holistic modeling of the complete HVSR curve to constrain shallow elastic structure remains challenging. We leverage the diffuse field assumption (DFA) to interpret HVSR in terms of the ratios of the imaginary parts of the Green’s function components, which allows for the inversion of HVSR curves for elastic structure. We measure HVSR at 91 broadband stations of the Southeastern Suture of the Appalachian Margin Experiment (SESAME) and apply the HV-Inv tool to model the Atlantic Coastal Plain sediment structure beneath each station. While we observe clear lateral variability in HVSR across the array due to variations in sediment structure, we generally find little azimuthal variability of HVSR at most stations, allowing local simplification of the structure to horizontal layers. To resolve tradeoffs in the elastic structure due to fitting HVSR alone, we plan to incorporate additional observations of high-frequency Rayleigh wave ellipticity and multimode phase velocity estimated from ambient noise cross-correlations. Together, these complementary datasets will contribute to a robust framework for subsurface characterization in complex geological settings.

Seismic ambient noise has revolutionized regional crust and upper mantle imaging for broadband ocean-bottom seismometer (BBOBS) studies. However, the ocean poses unique challenges including tilt and compliance noise and presence of interface waves with considerable water-column sensitivity. Transfer function (TF) noise removal techniques enable separation of fundamental Scholte waves from overtone Rayleigh waves. However, such noise removal strategies do not always provide the expected signal improvement. Here, we leverage data from recent U.S.-led BBOBS deployments to evaluate variability in ambient-noise surface-wave quality and efficacy of TF corrections across different oceanic environments and instrumentation. We examine the 15-30 s and 3-10 s period bands to represent typical ranges for lithospheric imaging. We find that water depth, instrument design, sediment thickness, and ocean basin are the most important factors controlling ambient-noise data quality and effectiveness of TF corrections. At 3-10 s, enhancement of higher-mode Rayleigh waves is most successful in deep water with thin sediments. At 15-30 s, signal is improved both in shallow water due to infragravity wave removal and in instances of high tilt noise. Our results provide new insights into the environmental conditions and instrument designs that shape ambient noise tomography performance, offering strategies for optimizing future BBOBS studies as well as novel seafloor-sensing technologies.

The interaction between aging oceanic plates and their underlying mantle is a crucial component of the plate tectonic cycle. Sub-lithospheric small-scale convection explains why plates appear not to thicken after a certain age. Yet, many open questions still surround this process. Here, we link grain-scale processes, dynamic models of asthenospheric flow, and seismic observations to gain new insights into the mechanisms of small-scale convection. We present high-resolution 3D geodynamic models of oceanic plate evolution using the community modeling software ASPECT. These simulations use an Earth-like rheology including coupled diffusion and dislocation creep as well as their interplay with evolving olivine grain size. Our models quantify how the balance between diffusion and dislocation creep affects the morphology and temporal stability of small-scale sub-lithospheric convection, including its onset age, the average depth and wavelength of the small-scale convection rolls, and the amplitude of the temperature and grain size anomalies within the rolls. We directly relate these quantities predicted by the dynamic models to geophysical observables through laboratory-derived constitutive relations, converting variations in temperature, pressure, grain size, water content and stable melt fraction to seismic velocity and attenuation. By creating synthetic seismic tomography models of different dynamic scenarios and comparing them to observations from the Pacific OBS Research into Convecting Asthenosphere (ORCA) experiment, we determine the parameter range in which geodynamic models fit these seismic observations. This provides new constraints on oceanic asthenosphere rheology beneath this part of the Pacific Plate, with potentially broad implications for Earth dynamics.