PN VELOCITY BENEATH EASTERN NORTH AMERICA USING DT/DΔ MEASUREMENTS FROM THE EARTHSCOPE TRANSPORTABLE ARRAY

Recent surface and body wave tomography studies have imaged laterally varying seismic velocities in the uppermost mantle of the northeastern US and southeastern Canada. Higher velocities are found beneath the Proterozoic craton and lower velocities are detected along the Phanerozoic Appalachians. A prominent feature of these models is a zone of low S-wave velocities in southern New England extending offshore in the Atlantic. Seismic refraction studies of the crust and uppermost mantle in northern New York and western New England found a pronounced increase in P-wave velocity in the 50-60 km depth range, but no such increase was found for eastern New England. Regional earthquakes recorded by permanent stations augmented by stations of the EarthScope Transportable Array (TA) provide an opportunity to test the continental-scale tomography models and to fine tune the spatial variations of P-wave structure in the uppermost mantle using dT/dΔ measurements across different subsets of the stations. The broad coverage of TA stations allows us to map sub-mantle velocity on a regional basis. Our current analysis focuses on the distance range 200 to 1000 km, which gives information on the velocity structure from the Moho down to about 100 km depth. As an example, P-wave arrivals at epicentral distances from 500 km to 800 km from the 20 November 2013 M3.5 southern Ohio earthquake clearly show lateral variations in the upper mantle P-wave velocities across eastern North America. Stations to the northeast indicate a sub-Moho velocity of 8.08 km/s, typical of the Phanerzoic platform. To the north, dT/dΔ analysis shows a velocity of 8.68 km/s. High velocities are commonly found in the upper mantle of shield regions, although 8.68 km/s at shallow mantle depths is unusual. For stations to the south, a sub-Moho velocity of 8.05 km/s is observed, which suggests that the slow upper mantle seen beneath the northern Appalachians also occurs beneath the southern Appalachians. These observations can be further augmented and refined as more earthquakes occur during the time that the TA is active along the North American East Coast. Our goal is to map apparent velocities of the uppermost mantle with a focus on delineating in detail the transition between higher velocity cratonic upper mantle and the lower velocity upper mantle of the Appalachian region.