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The time of the first arrival of seismic waves generated by explosions at the Nevada Test Site and recorded along a 300‐km line southeastward through Kingman, Arizona, is expressed as T0 = Δ/5.2, T1 = 0.34 + Δ/6.15, and T2 = 5.82 + Δ/7.81, where time is in seconds and the shot‐detector distance (Δ) is in kilometers. Assuming constant velocities for the layers, the thicknesses are H0 = 1.7 km, H1 = 26.7 km, and H0 + H1 ≐ 28 km (below a 1‐km datum). The average Bouguer anomaly is about −120 milligals, and the average elevation is about 1.1 km. Seismograms were examined for P waves indicating the presence of other discontinuities within and below the H2 layer, but the separation between seismic stations was too great to establish the presence of such discontinuities. A questionable alignment of weak arrivals following the T2 refraction time by less than 1 second may indicate the presence of a discontinuity below 28 km. Fair alignments of strong second arrivals in the range 200 to 400 km might be interpreted as direct P waves or channel waves in the H1 layer.

A profile of measurements of shear stress perpendicular to the San Andreas fault near Palmdale, California, shows a marked increase in stress with distance from the fault. The pattern suggests that shear stress on the fault increases slowly with depth and reaches a value on the order of the average stress released during earthquakes. This result has important implications for both long- and shortterm prediction of large earthquakes.

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A reversed seismic‐refraction profile was recorded between Santa Monica Bay, California, and Lake Mead, Nevada, during November 1961. Depth to the M discontinuity was determined to be approximately 29 km at Santa Monica Bay, 36 km under the Trans‐verse Ranges, 26 km under the Mojave Desert, and 30 km at Lake Mead. Prominent events on the seismograms in the distance range 30 to 150 km are interpreted as reflections from the M discontinuity and from a crustal layer of intermediate velocity. These reflected events are used to make a detailed interpretation of crustal structure. The velocity of compressional waves in the mantle immediately below the M discontinuity was determined to be 7.8 km/sec. The velocity of compressional waves in the intermediate layer is near 7.0 km/sec. The apparent velocity of the direct arrival in the crustal rocks near the surface is 6.3 km/sec northeast of Santa Monica Bay and 6.1 km/sec southwest of Lake Mead. The higher apparent velocity for the direct arrival from Santa Monica Bay seems to be the result of thinning toward the east of low‐velocity rocks near the surface. These low‐velocity near‐surface rocks are Cenozoic sedimentary rocks and fractured and weathered granitic and metamorphic rocks. The velocity of Sg was determined to be 3.4 km/sec near Lake Mead. A prominent phase of apparent velocity 6.3 to 6.4 km/sec was recorded at distances beyond 200 km. This phase is identified as P and is interpreted as a reflection from the intermediate layer. Amplitude measurements support the conclusion that the P phase is a reflected arrival.