On the travel times of P as determined from nuclear explosions

Abstract

A study has been made of the travel times of P waves as recorded from 14 shots on the Nevada Test Site (NTS) and from the Gnome (GN) shot. The travel times on different lines extending from the test sites were considered separately. Of special interest were the travel times obtained on the line joining the NTS and GN site. The two sets of travel times were in good agreement, but from epicentral distance 700 km onwards they were delayed and scattered. This was an indication of the presence of a low velocity layer at small depth on the line. Travel times on lines extending in other directions from the NTS differed somewhat but not very greatly from those on the NTS-GN line. On the line from the Gnome site to the northeast travel times are considerably smaller owing to a strong increase of velocity with depth close to the Gnome site. Travel times in agreement with those observed were calculated. Some California stations at distances around 1700 km observed P of the Gnome shot with an exceptional delay of $412$ seconds. The new Madrid earthquake of February 2, 1962, had its epicenter on the Gnome northeast line, but the surface velocity of P_n of the earthquake is smaller on the line than that of the Gnome shot. P_n of the earthquake was well recorded in all directions from the epicenter; there is no low velocity layer at small depth in these regions. In an earlier study of some earthquakes in northeastern America P_n and also S_n were found to be well recorded phases. It may seem as if shallow, low-velocity layers are confined to the western mountain regions.