Seismic-refraction measurements of crustal structure between Nevada Test Site and Ludlow, California

Seismic-refraction measurements from nuclear and chemical explosions were made along a line from the Nevada Test Site (NTS) to Ludlow, California, and additional recordings from nuclear explosions were made southward toward Calexico, California. The time of first arrivals from the Ludlow shotpoint is expressed as T₀ = 0.00 + Δ/2.50 (assumed), T₁ = 1.00 + Δ6.10, T₂ = 2.81 + Δ/6.80, and T₃ = 5.48 + ~7.76, where T is in seconds and distance Δ is in km. First arrival times from NTS fit the lines T₁ = 0.74 + Δ/6.10, T₂ = 2.81 + Δ/6.80 (assumed), T₃ = 6.70 + Δ/8.04 to a distance of 265 km, beyond 265 km T₃ = 5.83 + Δ/7.75. The difference in the apparent velocities of the P_n (T₃) arrival is caused by variations in the dip of the Mohorovicic discontinuity. The thickness of the successive layers at NTS are H₀ s 1.0 km (V₀ ~ 2.5 km/sec), H₁ = 13 km (v₁ = 6.1 km/sec), and H₂ = 20 km (v₂ = 6.8 km/ sec); the total crustal thickness is 34 km. The successive crustal layers at Ludlow have a thickness of H₀ = 1.4 km, H₁ = 13 km, and H₂ = 13 km; the total crustal thickness is 27 km.