Seismograms recorded from a blast of about 70 tons of Du Pont “Nitramon” in tunnels at a quarry near Corona, southern California, are discussed. Arrival times of P waves indicate a velocity of between 5.7 and 6.0 km/sec. in the upper 6 km. of the region, and a velocity of about 6 1/2 km/sec. at a depth of 10 km. The Mohorovičić discontinuity is at a depth of the order of 40 km. The velocity below it is 8.1 to 8.2 km/sec. The amplitudes of S waves are only slightly more than one-tenth of those in an earthquake having P waves of equal amplitudes. The ratio of the velocity of P to that of the first recognizable S is found between 1.6 and 1.7. The first S waves at distances up to about 140 km. indicate a velocity of transverse waves of about 3¾ km/sec. at a depth of the order of 10 km. A phase with an apparent velocity of about 3 1/2 km/sec. can be traced to more than 400 km. It is followed by several slower phases. On the assumption that the amplitudes of Pn do not differ appreciably from those in an earthquake of the same magnitude, the blast would have had a magnitude of about 4.