Three major low-velocity layers seem to exist in the earth's crust, of which two, the lithosphere channels, are found respectively in the 'granitic' and 'basaltic' ('gabbro') layers of the continents; a third channel extends from the Mohorovicic discontinuity downward into the asthenosphere. Several types of waves are guided by these channels, especially Pa and Sa by the asthenosphere channel, Lg1, Lg2 and Rg by the lithosphere channels; waves guided by low-velocity layers in sediments must also be expected. Many records of the Southern California earthquake of July 21, 1952 show channel waves with periods and velocities as reported for other paths. The regular microseisms with periods of 4 to 10 sec have properties similar to those of the Lg--Rg group in earthquake records and are probably propagated by the same mechanism.Most of these interpretations and conclusions are tentative; pertinent observations are scanty; and complications have thus far prevented development of adequate equations to calculate the amplitudes of waves guided by a given channel.

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