Seismic wave arrivals from LONGSHOT, 0° to 27°

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Abstract
Seismic wave arrivals from the LONGSHOT underground nuclear explosion were recorded at 27 locations in Alaska and two in Yukon Territory at distances between 2.6° and 26.6° from ground zero on Amchitka Island. Five of these stations were on the island chain, three on the Alaska Peninsula, two on other islands, and the remainder on the mainland from Anchorage to Point Barrow. The upper mantle travel times to distances of about 11° may be represented by t = Δ(km)/7.85 + 4.9 ± 0.5. The estimated depth to the M discontinity beneath the island chain is from 22 to 26 km depending on the choice of data. In the distance range from 6° to 16°, recognition of the initial P wave may be a function of the signal-to-noise ratio. From 13° to 27°, travel times average from 1.5 to 4.0 seconds earlier than the JB surface-focus travel times with a steep gradient from “early” in central and northern Alaska to an average of 2 seconds “more early” in eastern Alaska including Circle Hot Springs, Anchorage, and Kodiak. PcP arrivals are recognized at 15.5° and beyond, and their travel times are parallel to the JB PcP times but are from 2 to 4 seconds earlier. PcP was not anomalously early at Kodiak as was P. P wave amplitude maxima to the Bering Sea stations of St. Paul Island and Cape Romanzoff were an order less than at like distances on the Peninsula.

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