A Seismically Active Shear Zone in the Uppermost Mantle Beneath the Canadian Rockies

Abstract Continental mantle earthquakes are uncommon but hold important clues for understanding lithospheric rheology. Few of these earthquakes (<10) have been documented in western North America, though it is likely more exist owing to difficulties in resolving focal depth for small earthquakes. Mapping their distribution in western North America is key to better characterizing cratonic evolution in this area. Here, we evaluate 25 nominally lower crustal and upper mantle earthquakes in the Milk River region of southwestern Alberta. We absolutely relocate each earthquake and compare depths to published estimates of Moho depth and find 17 earthquakes locate ∼8–16 km below the Moho. High‐precision relative relocations and first‐motions define a steeply west‐dipping, Moho‐crossing normal fault. We hypothesize these earthquakes are related to regional mantle flow patterns interacting with lithospheric edges and are part of a larger trend of upper mantle seismicity along the western edge of North American cratonic lithosphere.