Abstract

It has been assumed heretofore that the relatively large phase called forumla in records of near-by earthquakes in the continents is the direct longitudinal wave, and that the corresponding velocity of about 5.6 km/sec is characteristic of the “granitic layer.” This leads to contradiction with the calculated origin-time of transverse waves in earthquakes and cannot be reconciled with observations of the longitudinal waves from blasts. It seems more likely that the velocity of longitudinal waves below the sediments is about 6 km/sec, increases to forumla km/sec or more at a depth of roughly 10 km, and possibly decreases below a depth of between 10 and 15 km. Such a decrease in velocity is to be expected in rocks with an appreciable content of quartz, since in laboratory experiments a decrease of elastic constants of quartz with increasing temperature has been found approaching the temperature at which transformation from alpha- to beta-quartz occurs (corresponding to a depth of 25 km or more). At the bottom of a deeper layer with higher velocity (usually forumla km/sec) the Mohorovičić discontinuity at a depth of between 30 and 40 km, but deeper under some mountain chains, forms the boundary between the simatic crustal layers and the ultra-basic material below (wave velocity 8.2 km/sec). Most earthquake foci seem to be in the hypothetical low-velocity layer.

Geophysical and geological evidence indicates a greater difference between the structure of the Pacific basin and the surrounding continental areas than between the bottom of the Atlantic or Indian oceans and the surrounding shelves or continents. In the Pacific, the surface layers seem to consist of sediments, erupted and perhaps some simatic material. Below them is probably ultra-basic material with a boundary (Mohorovičić discontinuity) at a depth of only a few kilometers. In the Atlantic Ocean (and probably similarly the Indian Ocean) the transition from the continents to the basins seems to be more gradual, and the Mohorovičić discontinuity seems to be at greater depth than in the Pacific but much shallower than in the continents. While at present there is no indication of extensive sialic material in the bottom of the Pacific, there may be limited areas with sialic material at least in the eastern part of the Atlantic Ocean while relatively basic simatic (but not ultra-basic) material seems to be close to the surface at least in parts of the western Atlantic basin.

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