Seismic Structure of Central Canada at Multiple Scales

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The physical principles of seismology are scale independent, governing waves propagating in a few metres of soil or over distances of thousands of kilometres. However, in practice these different scales are typically examined using different approaches by different people. I propose to address this through a program that exploits scale independence, examining the large-scale structure of North America and adjacent oceans, regional-scale crust and upper-mantle structure in the midcontinent, and small-scale bedrock topography in the Red River valley. The largest-scale project I propose is a new tomographic model of the Canadian lithosphere from two-station surface-wave analysis, combining our recently published two-station data for the Cordillera with eastern-Canada data from the UQAM group. The model will be easily updated as new data are available. I also propose a new source of surface-wave data: ocean-bottom seismographs. Using data from past experiments, I will develop dispersion measurement techniques that permit use of ocean-to-shore and ocean-to-ocean paths. This will allow the Canadian model to extend offshore, examining lithosphere beneath the continental shelf and slope. The approach will be more valuable once instruments of the National Facility for Seismic Imaging begin to produce data in the next few years, and will bridge the currently separate domains of continental and marine seismology. I propose to continue work in central North America using data from the Superior Province Rifting Earthscope Experiment. Tomography has not located a major Mid-Continent Rift (MCR) related lithospheric feature, while receiver functions have found a pervasive underplated layer along the MCR axis. The lack of a mantle anomaly beneath the MCR is in contradiction with the large amount of underplated material, unless the underplate was extracted from a deeper source. I will address this gap by applying shear-wave splitting to mantle fabric along the MCR axis, to assess resetting due to rifting. I will constrain the underplate using a transfer-function technique that I developed. Better knowledge of how rifting interacted with lithosphere will aid in understanding North American tectonics and modern rifts. On a small scale, I propose to improve techniques for measuring depth to bedrock using portable seismic equipment, and test their viability in the Red River valley. Existing ambient-noise MASW methods require directional noise; I propose to adapt interferometry used at the crustal scale to recover waves from diffuse noise, which is present everywhere. Better depth-to-bedrock measurement is essential for engineering/geotechnical studies and overburden characterization. The common goal is bridging domains of seismology through scale independence, from continent to ocean and from lithospheric to crustal to near-surface, and so extending the applicability of seismic techniques while better characterizing the tectonic history of North America.