Deep reflections from Southern California blasts

Two records showing reflections from the Mohorovicic discontinuity at normal incidence have been obtained from large quarry blasts in Southern California. Each record shows a strong reflection with corrected travel time near 10.6 sec. An earlier reflection appears near 9.0 sec Strong reflections were also obtained at distances slightly beyond critical. Computations of the thickness of the crust near the quarries have been made using velocity data obtained from blast and earthquake refraction studies. The results using any crustal model that fits the refraction data or by assuming a single‐layer crust agree closely. In this area the Mohorovicic discontinuity is 32 km below sea level, and the mean velocity in the crust is 6.2 km/sec The data give additional evidence for the existence of a zone in which velocity decreases with depth.