Abstract Ice shelves are assumed to flow steadily from their grounding lines to the ice front. We report the detection of ice‐propagating extensional Lamb (plate) waves accompanied by pulses of permanent ice shelf displacement observed by co‐located Global Navigation Satellite System receivers and seismographs on the Ross Ice Shelf. The extensional waves and associated ice shelf displacement are produced by tidally triggered basal slip events of the Whillans Ice Stream, which flows into the ice shelf. The propagation velocity of 2,800 m/s is intermediate between shear and compressional ice velocities, with velocity and particle motions consistent with predictions for extensional Lamb waves. During the passage of the Lamb waves the entire ice shelf is displaced about 60 mm with a velocity more than an order of magnitude above its long‐term flow rate. Observed displacements indicate a peak dynamic strain of 10−7, comparable to that of earthquake surface waves that trigger ice quakes.

Ambient seismic recordings taken at broad locations across Ross Ice Shelf and a dense array near West Antarctic Ice Sheet (WAIS) Divide, Antarctica, show pervasive temporally variable resonance peaks associated with trapped seismic waves in near-surface firn layers. These resonance peaks feature splitting on the horizontal components, here interpreted as frequency-dependent anisotropy in the firn and underlying ice due to several overlapping mechanisms driven by ice flow. Frequency peak splitting magnitudes and fast/slow axes were systematically estimated at single stations using a novel algorithm and compared with good agreement with active source anisotropy measurements at WAIS Divide determined via active sources recorded on a 1 km circular array. The approach was further applied to the broad Ross Ice Shelf (RIS) array, where anisotropy axes were directly compared with visible surface features and ice shelf flow lines. The near-surface firn, depicted by anisotropy above 30 Hz, was shown to exhibit a novel plastic stretching mechanism of anisotropy, whereby the fast direction in snow aligns with accelerating ice shelf flow.

Abstract Observations of teleseismic earthquakes using broadband seismometers on the Ross Ice Shelf (RIS) must contend with environmental and structural processes that do not exist for land-sited seismometers. Important considerations are: (1) a broadband, multi-mode ambient wavefield excited by ocean gravity wave interactions with the ice shelf; (2) body wave reverberations produced by seismic impedance contrasts at the ice/water and water/seafloor interfaces and (3) decoupling of the solid Earth horizontal wavefield by the sub-shelf water column. We analyze seasonal and geographic variations in signal-to-noise ratios for teleseismic P-wave (0.5–2.0 s), S-wave (10–15 s) and surface wave (13–25 s) arrivals relative to the RIS noise field. We use ice and water layer reverberations generated by teleseismic P-waves to accurately estimate the sub-station thicknesses of these layers. We present observations consistent with the theoretically predicted transition of the water column from compressible to incompressible mechanics, relevant for vertically incident solid Earth waves with periods longer than 3 s. Finally, we observe symmetric-mode Lamb waves generated by teleseismic S-waves incident on the grounding zones. Despite their complexity, we conclude that teleseismic coda can be utilized for passive imaging of sub-shelf Earth structure, although longer deployments relative to conventional land-sited seismometers will be necessary to acquire adequate data.

Ocean swell interacting with Antarctic ice shelves produces sustained (approximately, 2 x 106 cycles per year) gravity-elastic perturbations with deformation amplitudes near the ice front as large as tens to hundreds of nanostrain. This process is the most energetically excited during the austral summer, when sea ice-induced swell attenuation is at a minimum. A 2014-2017 deployment of broadband seismographs on the Ross Ice shelf, which included three stations sited, approximately, 2 km from the ice front, reveals prolific swell-associated triggering of discrete near-ice-front (magnitude greater than or similar to 0) seismic subevents, for which we identify three generic types. During some strong swell episodes, subevent timing becomes sufficiently phase-locked with swell excitation, to create prominent harmonic features in spectra calculated across sufficiently lengthy time windows via a Dirac comb effect, for which we articulate a theoretical development for randomized interevent times. These events are observable at near-front stations, have dominant frequency content between 0.5 and 20 Hz, and, in many cases, show highly repetitive waveforms. Matched filtering detection and analysis shows that events occur at a low-background rate during all swell states, but become particularly strongly excited during large amplitude swell at rates of up to many thousands per day. The superimposed elastic energy from swell-triggered sources illuminates the shelf interior as extensional (elastic plate) Lamb waves that are observable more than 100 km from the ice edge. Seismic swarms show threshold excitation and hysteresis with respect to rising and falling swell excitation. This behavior is consistent with repeated seismogenic fracture excitation and growth within a near-ice-front damage zone, encompassing fracture features seen in satellite imagery. A much smaller population of distinctly larger near-front seismic events, previously noted to be weakly associated with extended periods of swell perturbation, likely indicate calving or other larger-scale ice failures near the shelf front.

Abstract Gravity waves impacting ice shelves illicit a suite of responses that can affect ice shelf integrity. Broadband seismometers deployed on the Ross Ice Shelf, complemented by a near‐icefront seafloor hydrophone, establish the association of strong icequake activity with ocean gravity wave amplitudes (AG) below 0.04 Hz. The Ross Ice Shelf‐front seismic vertical displacement amplitudes (ASV) are well correlated with AG, allowing estimating the frequency‐dependent transfer function from gravity wave amplitude to icefront vertical displacement amplitude (TGSV(f)). TGSV(f) is 0.6–0.7 at 0.001–0.01 Hz but decreases rapidly at higher frequencies. Seismicity of strong icequakes exhibits spatial and seasonal associations with different gravity wave frequency bands, with the strongest icequakes observed at the icefront primarily during the austral summer when sea ice is minimal and swell impacts are strongest.

A significant aspect of bottom-interaction in deep water acoustic propagation, from point sources to point receivers, is the diffraction (or scattering) of energy from discrete seafloor locations along repeatable, deterministic paths in three-dimensions. These bottom-diffracted surface-reflected (BDSR) paths were first identified on the North Pacific acoustic laboratory experiment in 2004 (NPAL04) for a diffractor located on the side of a small seamount. On the adjacent deep seafloor, ambient noise and propagation in the ocean sound channel were sufficiently quiet that the BDSRs were the dominant arrival. The ocean bottom seismometer augmentation in the North Pacific (OBSANP) experiment in June-July 2013 studied BDSRs at the NPAL04 site in more detail. BDSRs are most readily identified by the arrival time of pulses as a function of range to the receiver for a line of transmissions. The diffraction points for BDSRs occur on the relatively featureless deep seafloor as well as on the sides of small seamounts. Although the NPAL04 and OBSANP experiments had very different geometries the same diffractor location is consistent with observed arrivals in both experiments within the resolution of the analysis. On OBSANP the same location excites BDSRs for 77.5, 155, and 310 Hz transmissions.

All underwater acoustic sensors require accurate on-board clocks for subsequent data analysis and interpretation. Unfortunately, most clock oscillators change their frequency over time, which occurs due to environmental changes, aging, and other factors. Typically, clock errors are accounted for by referencing the instrument clock to a known accurate clock (such as GPS) before and after the deployment and applying a correction during data post-processing. This method, however, does not accurately estimate clock errors during a specific experimental event. A method to estimate clock performance during the specific event of interest has been developed. It is shown that advanced signal processing techniques can be used to accurately reconstruct the motion of the acoustic source, which, in turn, can improve the accuracy of the acoustic receivers deployed on the seafloor in the deep ocean. A small subset of data collected on the last day of the Ocean Bottom Seismometer Augmentation in the North Pacific (OBSANP) Experiment in June–July 2013 is analyzed for the demonstration of these techniques. [Work supported by the MITRE Corporation’s R&D Program.]

AbstractThe Ross Ice Shelf (RIS) is host to a broadband, multimode seismic wavefield that is excited in response to atmospheric, oceanic and solid Earth source processes. A 34-station broadband seismographic network installed on the RIS from late 2014 through early 2017 produced continuous vibrational observations of Earth's largest ice shelf at both floating and grounded locations. We characterize temporal and spatial variations in broadband ambient wavefield power, with a focus on period bands associated with primary (10–20 s) and secondary (5–10 s) microseism signals, and an oceanic source process near the ice front (0.4–4.0 s). Horizontal component signals on floating stations overwhelmingly reflect oceanic excitations year-round due to near-complete isolation from solid Earth shear waves. The spectrum at all periods is shown to be strongly modulated by the concentration of sea ice near the ice shelf front. Contiguous and extensive sea ice damps ocean wave coupling sufficiently so that wintertime background levels can approach or surpass those of land-sited stations in Antarctica.

Understanding deformation in ice shelves is necessary to evaluate the response of ice shelves to thinning. We study microseismicity associated with ice shelf deformation using nine broadband seismographs deployed near a rift on the Ross Ice Shelf. From December 2014 to November 2016, we detect 5,948 icequakes generated by rift deformation. Locations were determined for 2,515 events using a least squares grid‐search and double‐difference algorithms. Ocean swell, infragravity waves, and a significant tsunami arrival do not affect seismicity. Instead, seismicity correlates with tidal phase on diurnal time scales and inversely correlates with air temperature on multiday and seasonal time scales. Spatial variability in tidal elevation tilts the ice shelf, and seismicity is concentrated while the shelf slopes downward toward the ice front. During especially cold periods, thermal stress and embrittlement enhance fracture along the rift. We propose that thermal stress and tidally driven gravitational stress produce rift seismicity with peak activity in the winter.

In bottom-interacting ocean acoustics discrete, deterministic arrivals with significant energy are returned from the seafloor at angles that are not predicted by Snell’s law. This observation has led to some confusion between the terms “diffraction” and “scattering” as used in ocean acoustics. The common dictionary definitions of diffraction, as the process by which a beam is spread out or bent after passing the edge of an obstacle, and of scattering, as deflecting waves in a more or less random fashion, do not describe the observed seafloor interaction where a pulse of energy is returned deterministically, not randomly, from the seafloor in a non-Snell’s law direction. In ocean acoustics and marine seismology, it is rare to consider “bending diffraction” but it is quite common to see “scattering diffraction,” for example, as diffraction hyperbolas on bottom profiling and seismic records. For impulsive sources, in addition to specular reflections and random scattering from the seafloor, a significant aspect of bottom-interacting acoustics in deep water is diffraction of energy from discrete seafloor locations along repeatable paths in three-dimensions. [Work supported by ONR.]

We present an upper mantle P-wave velocity model for the Ross Sea Embayment (RSE) region of West Antarctica, constructed by inverting relative P-wave travel-times from 1881 teleseismic earthquakes recorded by two temporary broadband seismograph deployments on the Ross Ice Shelf, as well as by regional ice- and rock-sited seismic stations surrounding the RSE. Faster upper mantle P-wave velocities (similar to +1%) characterize the eastern part of the RSE, indicating that the lithosphere in this part of the RSE may not have been reheated by mid-to-late Cenozoic rifting that affected other parts of the Late Cretaceous West Antarctic Rift System. Slower upper mantle velocities (similar to -1%) characterize the western part of the RSE over a similar to 500 km-wide region, extending from the central RSE to the Transantarctic Mountains (TAM). Within this region, the model shows two areas of even slower velocities (similar to -1.5%) centered beneath Mt. Erebus and Mt. Melbourne along the TAM front. We attribute the broader region of slow velocities mainly to reheating of the lithospheric mantle by Paleogene rifting, while the slower velocities beneath the areas of recent volcanism may reflect a Neogene-present phase of rifting and/or plume activity associated with the formation of the Terror Rift. Beneath the Ford Ranges and King Edward VII Peninsula in western Marie Byrd Land, the P-wave model shows lateral variability in upper mantle velocities of +/- 0.5% over distances of a few hundred km. The heterogeneity in upper mantle velocities imaged beneath the RSE and western Marie Byrd Land, assuming no significant variation in mantle composition, indicates variations in upper mantle temperatures of at least 100 degrees C. These temperature variations could lead to differences in surface heat flow of similar to +/- 10 mW/m(2) and mantle viscosity of 10(2) Pa s regionally across the study area, possibly influencing the stability of the West Antarctic Ice Sheet by affecting basal ice conditions and glacial isostatic adjustment. (C) 2019 Elsevier B.V. All rights reserved.

AbstractIce shelves play an important role in buttressing land ice from reaching the sea, thus restraining the rate of grounded ice loss. Long-period gravity-wave impacts excite vibrations in ice shelves that can expand pre-existing fractures and trigger iceberg calving. To investigate the spatial amplitude variability and propagation characteristics of these vibrations, a 34-station broadband seismic array was deployed on the Ross Ice Shelf (RIS) from November 2014 to November 2016. Two types of ice-shelf plate waves were identified with beamforming: flexural-gravity waves and extensional Lamb waves. Below 20 mHz, flexural-gravity waves dominate coherent signals across the array and propagate landward from the ice front at close to shallow-water gravity-wave speeds (~70 m s−1). In the 20–100 mHz band, extensional Lamb waves dominate and propagate at phase speeds ~3 km s−1. Flexural-gravity and extensional Lamb waves were also observed by a 5-station broadband seismic array deployed on the Pine Island Glacier (PIG) ice shelf from January 2012 to December 2013, with flexural wave energy, also detected at the PIG in the 20–100 mHz band. Considering the ubiquitous presence of storm activity in the Southern Ocean and the similar observations at both the RIS and the PIG ice shelves, it is likely that most, if not all, West Antarctic ice shelves are subjected to similar gravity-wave excitation.

We construct a new seismic model for central and West Antarctica by jointly inverting Rayleigh wave phase and group velocities along with P wave receiver functions. Ambient noise tomography exploiting data from more than 200 seismic stations deployed over the past 18 years is used to construct Rayleigh wave phase and group velocity dispersion maps. Comparison between the ambient noise phase velocity maps with those constructed using teleseismic earthquakes confirms the accuracy of both results. These maps, together with P receiver function waveforms, are used to construct a new 3‐D shear velocity (Vs) model for the crust and uppermost mantle using a Bayesian Monte Carlo algorithm. The new 3‐D seismic model shows the dichotomy of the tectonically active West Antarctica (WANT) and the stable and ancient East Antarctica (EANT). In WANT, the model exhibits a slow uppermost mantle along the Transantarctic Mountains (TAMs) front, interpreted as the thermal effect from Cenozoic rifting. Beneath the southern TAMs, the slow uppermost mantle extends horizontally beneath the traditionally recognized EANT, hypothesized to be associated with lithospheric delamination. Thin crust and lithosphere observed along the Amundsen Sea coast and extending into the interior suggest involvement of these areas in Cenozoic rifting. EANT, with its relatively thick and cold crust and lithosphere marked by high Vs, displays a slower Vs anomaly beneath the Gamburtsev Subglacial Mountains in the uppermost mantle, which we hypothesize may be the signature of a compositionally anomalous body, perhaps remnant from a continental collision.

Continuous seismic observations across the Ross Ice Shelf reveal ubiquitous ambient resonances at frequencies >5 Hz. These firn‐trapped surface wave signals arise through wind and snow bedform interactions coupled with very low velocity structures. Progressive and long‐term spectral changes are associated with surface snow redistribution by wind and with a January 2016 regional melt event. Modeling demonstrates high spectral sensitivity to near‐surface (top several meters) elastic parameters. We propose that spectral peak changes arise from surface snow redistribution in wind events and to velocity drops reflecting snow lattice weakening near 0°C for the melt event. Percolation‐related refrozen layers and layer thinning may also contribute to long‐term spectral changes after the melt event. Single‐station observations are inverted for elastic structure for multiple stations across the ice shelf. High‐frequency ambient noise seismology presents opportunities for continuous assessment of near‐surface ice shelf or other firn environments.

The ocean acoustic noise floor (observed when the overhead wind is low, ships are distant, and marine life silent) has been measured on an array extending up 987 m from 5048 m depth in the eastern North Pacific, in what is one of only a few recent measurements of the vertical noise distribution near the seafloor in the deep ocean. The floor is roughly independent of depth for 1-6 Hz, and the slope (∼ f-7) is consistent with Longuet-Higgins radiation from oppositely-directed surface waves. Above 6 Hz, the acoustic floor increases with frequency due to distant shipping before falling as ∼ f-2 from 40 to 800 Hz. The noise floor just above the seafloor is only about 5 dB greater than during the 1975 CHURCH OPAL experiment (50-200 Hz), even though these measurements are not subject to the same bathymetric blockage. The floor increases up the array by roughly 15 dB for 40-500 Hz. Immediately above the seafloor, the acoustic energy is concentrated in a narrow, horizontal beam that narrows as f-1 and has a beam width at 75 Hz that is less than the array resolution. The power in the beam falls more steeply with frequency than the omnidirectional spectrum.

Bottom-diffracted surface-reflected (BDSR) arrivals are a ubiquitous feature in long-range ocean acoustic propagation and are not predicted by existing forward models based on available bathymetric and bottom properties data. They were first identified in the LOAPEX Experiment in the North Pacific in 2004 where a BDSR from the side of “Seamount B” was observed from 500 to 3200 km range for an M-sequence centered at 75 Hz. In a follow-up research cruise in 2013, a BDSR was also observed from the side of “Seamount B” for ranges from 15 to 35 km and the same azimuth. Are the seafloor diffractors observed in 2004 and 2013 the same? Various measures of the diffractor location and their resolution are reviewed. Both diffractors fall within a 1km radius region. Elsewhere in the 2013 survey pairs of BDSRs were observed within a kilometer of each other. So it is possible that the two BDSR arrivals arise from two distinct and unresolvable diffractors. It is quite likely however that the two diffractor points are the same. The location of the 2004 diffractor is sufficient to explain the arrival times of the BDSR from the 2013 diffractor within the scatter of the data. [Work supported by ONR.]