Summary
Investigation of several blasts subsequent to those considered in our previous study, and in particular a major blast set off near Victorville, California, on September 12, 1931, has yielded results as follows.
1. In two cases the time of detonation was determined (a) very accurately in the case of the blast near Victorville by a precise method, and (b) probably with substantially equal accuracy in the case of the blast in the Arroyo Seco, though the method employed was inherently less precise.
2. Phases, or definite changes in registered motion, have been recognized and measured which appear to correspond to successive arrivals of waves traveling, apparently, along direct paths with different apparent velocities, as follows: 6.0− 5.9+, 5.55, 5.4−, 5.0+, 4.1, 3.5−, 3.4−, 3.25, 3.15, 3.0−, 2.7± kilometers per second.
3. In the present study no positive evidence has been found to indicate reflected waves, or waves refracted along deeper layers. Phases considered in the previous study to indicate such waves are now found to be susceptible of alternative explanations.
Difficulties have been encountered, as discussed in the body of the paper, in the interpretation and satisfactory explanation of certain of the very numerous apparent phases, particularly those which appear to indicate waves with apparent velocities of 6.0− to 5.9+, and 4.1 kilometers per second.
From this it is again clear that further opportunities for similar investigations are of great importance. For this reason, and also in order that blasts of moderate, or larger, magnitude may not be included in lists of small local earthquakes, it is suggested that all such blasts set off in the region of Southern California should be reported to the Seismological Laboratory at Pasadena—and, whenever practicable, sufficiently in advance to permit preparations for thorough study.
P- and S-phase data from local earthquakes in Southern California for 1966 to 1975