Arrival-time data from a southeastern United States earthquake are analyzed to formulate an average regional crustal velocity model. The event studied is the magnitude 3.7 (mb), shallow earthquake (2-km depth) that occurred on 26 August 1979, near the Lake Jocassee Reservoir in northwestern South Carolina. Over 90 seismograms were collected and read by at least two independent interpreters. Some 250 phases have been timed, including 60 Pn, 80 Pg, 50 Sn, and 70 Sg-Lg arrival times.
The arrival-time data can be interpreted in terms of a single-layer crust with average velocities of 6.25 ± 0.04 km/sec for compressional waves and 3.65 ± 0.11 km/sec for shear waves. The average upper mantle velocities are 8.11 ± 0.19 km/sec (P waves) and 4.59 ± 0.13 km/sec (S waves). The Mohorovic˘ić discontinuity is found to lie at varying “average” depths throughout the region: 31 km (Coastal Plain), 33 km (Piedmont), 40 km (Valley and Ridge, Central United States).
A regional velocity model derived from the above results has the aforementioned average velocities, a crustal thickness of 40 km, and a set of time corrections for mantle refraction phases: Coastal Plain (Pn + 1.68 sec, Sn + 2.74 sec), Piedmont (Pn + 1.40 sec, Sn + 2.28 sec), Valley and Ridge and Central United States (no corrections). This new velocity model is tested for precision by relocating several regional earthquakes and comparing the resulting travel-time residuals with those from the earlier locations. The tests demonstrate that the new model can be used to develop good results.