Otto Nuttli, John D. Whitmore; An observational determination of the variation of the angle of incidence of P waves with epicentral distance. Bulletin of the Seismological Society of America ; 51 (2): 269–276. doi:
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Apparent angles of incidence of the P wave were obtained from the seismograms of the Galitzin-Wilip instruments at Florissant. The “half-periods” of these waves varied from to seconds, and the epicentral distances from 16.5° to 103.2°. The data indicate that the velocity of P waves at the earth's surface is approximately 8 km/sec. This suggests that these P waves do not see or are not affected by the earth's crust, although the crustal thickness is known to be approximately 35 km. P wave data from five other stations for the Alaska earthquake of July, 1958 support the conclusions obtained from the Florissant data.