The crust and upper mantle beneath the New England Appalachians exhibit a large offset of the Moho across the boundary between Laurentia and accreted terranes and several dipping discontinuities, which reflect Paleozoic or younger tectonic movements. We apply scattered wavefield migration to the SEISConn array deployed across northern Connecticut and obtain insights not previously available from receiver function studies. We resolve a doubled Moho at a previously imaged Moho offset, which may reflect westward thrusting of rifted Grenville crust. The migration image suggests laterally variable velocity contrasts across the Moho, perhaps reflecting mafic underplating during continental rifting. A west‐dipping feature in the lithospheric mantle is further constrained to have a slab‐like geometry, representing a relict slab subducted during an Appalachian orogenic event. Localized low seismic velocities in the upper mantle beneath the eastern portion of the array may indicate that the Northern Appalachian Anomaly extends relatively far to the south.

Subduction, terrane accretion, and continental rifting are fundamental plate tectonic processes. Geologic features such as igneous rocks produced during arc magmatism, terrane boundaries separating regions with different origins, and rift basins filled with sedimentary units reflect such tectonic processes. It is likely

Observations of remote dynamic triggering of intermediate and deep (I&D) earthquakes can potentially clarify the faulting process but have not been systematically compiled. We relocate earthquakes following the two largest recent deep earthquakes with a multiple-event location algorithm and compile a comprehensive catalog of dynamically triggered I&D earthquakes from 1964 to 2015. We find 119 large I&D earthquakes that likely dynamically triggered aftershocks at distances between two mainshock fault lengths and 400 km during the first 4 days following these mainshocks. Dynamic triggering is more common for deep than for intermediate depth earthquakes and is overrepresented in areas with low seismicity that are adjacent to slabs, which suggest that subducting slabs in the transition zone are surrounded by material that is critically stressed but warm enough to inhibit rupture initiation. The passage of seismic waves may initiate runaway ductile shear failure, which nucleates over a period of minutes to days.Plain Language Summary Seismic waves from earthquakes consist of dynamic stress and strain, which can trigger other earthquakes at close distances as well as at locations far away from the original earthquakes. We compile a comprehensive catalog of dynamically triggered intermediate (100-300 km depth) and deep (300-700 km depth) earthquakes from 1964 to 2015 on a global scale. We also focus on particularly significant dynamic triggering that occurred after two large deep earthquakes, and we obtain earthquake locations in both cases with higher resolution than standard global catalogs. We find that dynamic triggering is relatively common within 400 km distance and within the first 5 hr after the original earthquakes occurred. Our data set shows that dynamic triggering is more common for deep earthquakes than for intermediate earthquakes. Furthermore, at the greatest depths, the dynamically triggered earthquakes tend to be located in regions of little or no other seismicity. This observation implies that deep subducting slabs are surrounded by "thermal halos" where the temperature is high enough to prohibit the onset of earthquakes generally but are prone to hosting triggered earthquakes after the passage of high amplitude seismic waves.