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 GNSS 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 2800 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.

Mapping absolute P-wavespeeds in the Canadian and Alaskan mantle will further our understanding of its present-day state and evolution. S-wavespeeds are relatively well constrained, especially across Canada, but are primarily sensitive to temperature while complimentary P-wavespeed constraints provide better sensitivity to compositional variations. One technical issue concerns the difficulties in extracting absolute arrival-time measurements from often-noisy data recorded by temporary seismograph networks. Such processing is required to ensure that regional Canadian datasets are compatible with supplementary continental and global datasets provided by global pick databases. To address this, we utilize the Absolute Arrival-time Recovery Method (Boyce et al., 2017). We extract over 180,000 new absolute arrival-time residuals from seismograph stations across Canada and Alaska that include both land and ocean bottom seismometers. We combine these data with the latest USArray P-wave arrival-time data from the contiguous US and Alaska. Using an adaptively parameterised least-squares tomographic inversion we develop a new absolute P-wavespeed model, with focus on Canada and Alaska (CAP21). Initial results suggest fast wavespeeds characterise the upper mantle beneath eastern and northern Canada. A sharp transition between the slow wavespeeds below the North American Cordillera and the fast wavespeeds of the stable continental interior appears to follow the Cordilleran Deformation Front (CDF) in southwest Canada. Slow wavespeeds below the Mackenzie Mountains may extend further inland of the CDF in northwest Canada. In Alaska, CAP21 illuminates both lithospheric structure and the along strike morphology of the subducting slab. The newly compiled data may also improve resolution of subducted slab remnants in the mid-mantle below the North American continent, crucial to help constrain the formation of the Alaskan peninsular at ≥50Ma.

Inverse methods form the basis of many investigations of the structure of the lithosphere-asthenosphere system as they provide the basis for physics-based subsurface imaging from surface and/or near-surface measurements. Steady increases in computational capabilities and methodological improvements have resulted in increasingly detailed three-dimensional models of the Earth based on inverse methods. While these models can show an impressive array of features, it may be difficult for non-specialists to assess which aspects can be considered reliable and which are tenuous, or are artefacts of the mathematical formulation or data collection. In this paper we address the fundamental issues of feature reliability due to limited resolution and model sensitivity to data noise for researchers who do not work with intimately with inverse methods. We include and introductory overview of the mathematical formulation of inversion methods and define commonly used terms and concepts. We then present two case studies based on data from USArray in the western United States. The first case study utilizes magnetotelluric array data to construct a three-dimensional model of electrical resistivity to a depth of approximately 300 km. We use this example to demonstrate fundamental issues regarding data fit, data coverage, and model parameterization. The second case study discusses how we can incorporate petrological and mineral physics information directly into the inversion approach to create models that are compatible with constraints on the temperature and composition of the lithosphere. We will discuss the implications for practical use of these models in interpretations and provide guidelines on how to evaluate such models.