ABSTRACT

Often called the "San Andreas of the North", the Queen Charlotte fault (QCF) system is a strike-slip plate boundary that separates the Pacific and North American tectonic plates offshore western Canada and Southeast Alaska. The QCF is arguably the most active fault of its type in the world: the entire ~900 km offshore length has ruptured in seven >M7 earthquakes during the last century and it sustains the highest known deformation rates (>50 mm/yr). The fault system represents the largest seismic hazard to southeastern Alaska and Canada outside of Cascadia, and caused Canada?s largest recorded earthquake (M8.1) in 1949. Despite rapid response efforts following >M7 earthquakes in 2012 and 2013, first-order questions regarding how the fault system deforms and the processes controlling fault failure during earthquakes remain unanswered due to the lack of modern geophysical imaging. This experiment will be the first comprehensive attempt to characterize this plate boundary at depth on a regional scale. Using seismic energy from marine acoustic and earthquake sources, the project will measure the depth and extent of seismicity, image the fault zone at depth, and determine velocity and thermal structure across the fault. All these data will lead to an improved understanding of this, and other major strike-slip fault systems, for better hazard assessment and earthquake forecasting. The science team is a collaborative, international group of US and Canadian researchers, led by three early-career women. Outreach to local communities will be conducted through a residency at the Sitka Science Center in Alaska and lectures at local high schools and community centers.

Compared to convergent continental-oceanic plate boundaries, the time-space evolution of continental-oceanic transform margins is understudied, despite their important role in the planet?s plate tectonic system. Continental-oceanic transform faults are potentially one of the most favorable tectonic settings for subduction initiation due to the juxtaposition of lithospheres of contrasting density and thermal structure -- small degrees of convergence can lead to failure. The QCF system provides an ideal location to investigate how a continental-oceanic transform fault responds to systematically increasing degrees of convergence at the lithospheric scale. The study area includes two potential fault segment boundaries that mark abrupt changes in transpressive deformation mechanisms as suggested by changes in seafloor morphology and shallow seismic reflection structure: strain partitioning and underthrusting in the south transition to highly localized strike-slip deformation in the north. Lack of information on microseismic depths and locations, the deformation history and geometries of faults at depth, and lithospheric velocity structure leave multiple fundamental questions unanswered: Why has the QCF formed where it is, and what is its deformation history? What is the history of PAC underthrusting along the margin and the fate of underthrust material north of the area of maximum convergence? What are the primary physical conditions controlling seismogenesis along oceanic-continental transforms? How are strike-slip and compressive strain accommodated and partitioned over geologic and seismogenic timescales? Using a combined active- and passive-source marine seismic imaging strategy, this research will characterize crustal and uppermost mantle velocity structure, fault zone architecture and rheology, and seismicity. Data will be acquired using long-offset 2D seismic reflection and wide-angle reflection-refraction capabilities of the R/V Marcus G. Langseth and a combined US-Canadian broadband ocean bottom seismometer array of 64 instruments deployed for ~1 year.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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