Abstract

Arrival times of Pn and Sn phases from southeastern United States earthquakes listed in the Earthquake Data Reports from July 1970 to June 1977 show a scatter of 2 seconds (one standard deviation). In contrast, the Pg phase arrivals show a scatter of only 0.5 seconds. The difference in scatter was interpreted as indicating variation in the depth to the Moho. In order to evaluate Moho depths, a detailed refraction line and crustal model were developed along the axis of the Southern Piedmont Province from Central Georgia across South Carolina. Accurately located recent earthquakes and explosions occurring and recorded along the axis of the Southern Piedmont Province were used to define the travel time curves. The resulting travel times for the Southern Piedmont are described by; P = x/(5.5 km/sec) Pg = 0.4 + x/(6.05 km/sec) and Pn = 7.6 + x/(8.2 km/sec) with a scatter of ±0.5 seconds. The Sg and Sn velocities were 3.5 and 4.5 km/sec respectively. The implied crustal structure consists of a 2.6 km thick surface layer over a 30.4 km thick crust. No evidence for an intermediate layer in the crust was observed in the traveltimes but a 6.7 km/sec layer less than 6 km thick could exist at the base of the crust. Crustal thicknesses in the southeastern United States were computed from Pn-Pg times by comparison to the Southern Piedmont crustal model. In the Coastal Plain Moho depths of 40 km shallow toward the southeast to 29 km near the coast. Moho depths of 36 to 54 km were measured for the mountains of north Georgia, eastern Tennessee and western North Carolina.

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