Ten seisms with epicentral distances between 3° and 23° are studied.
Gutenberg's method to find the variation of Pn amplitudes with distance is applied. It results in observations too scattered to permit any conclusion.
The second method uses a combination of seisms, each observed in a limited range. A minimum is found around 7° and a maximum around .
The essence of the proposed theoretical explanation is the coexistence of “Muskat's rays”† (refracted rays) and of “direct rays” and the existence of “Muskat's rays” along the boundary of two media, in one of which the velocity increases with depth. The results are well explained by comparing the amplitudes due to these two rays.
The following structure of the earth's crust best explains the results: speed above the Mohorovičić discontinuity, 6.3 km/sec.; speed immediately below it, 8.1 km/sec.; speed at 80 km. depth, between 8.10 km/sec. and 8.128 km/sec.; around 80 km., decrease in the rate of increase of speed with depth, or decrease of speed with depth < 0.001293 km/sec/km.; depth of the discontinuity, 35 km.; σ1 = σ2 = 0.25; density ratio = 1.103.
Note added in proof: Since the time of this writing, various papers have been published by Macelwane's students using the term “head-wave” for what is here called “Muskat's rays.” As the former term is not ambiguous and is now in current use, it should be preferred.