Monolith Quarry, Monolith California - blast site chart (part 1): Supplement 1 from "Crustal structure and reflections from the Mohorovičić discontinuity in southern California" (Thesis)

Description

The thickness and structure of the crust of the earth have been studied in a limited area of southern California by means of reflected and refracted seismic waves. Two usable records of reflections from the Mohorovičić discontinuity at nearly vertical incidence were obtained from large quarry blasts. Each record shows a strong reflection with travel time near 10.6 sec. Strong reflections from the Mohorovičić discontinuity were also obtained at distances slightly beyond critical from blasts at one quarry. Many other records of blasts and of earthquakes show phases that may be interpreted as reflections from the Mohorovičić discontinuity. Studies of refracted waves from blasts and earthquakes have given additional information about crustal velocities. When combined with the reflection data, they indicate that there is at least a slight decrease of velocity with depth somewhere in the crust and that there are significant variations in the arrival time of P_n at distant stations, indicating lateral variation either in crustal velocities or in the thickness of the crust. Computations of the thickness of the crust near Monolith and Corona, California have been made from various assumptions of velocity structure. The results obtained by using any model that agrees with the refraction data or by assuming a single-layer crust agree closely. In this area the Mohorovičić discontinuity is 32 km below sea level and the mean velocity in the crust is 6.2 km/sec.

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