A note on the use of microseisms in determining the shallow structure of the Earth's crust

Abstract In the VELA UNIFORM Special Issue I of Geophysics, M. N. Toksoez presented the result of his attempt at determining the shallow structures of the earth's crust from the phase velocity of microseisms. He stated that his attempt resulted only in partial success on account of the fact that the microseisms arrived from more than one direction at the same time with comparable strength. He also concluded that there was no way of improving the results by the use of special arrays because, according to him, there were two unknown parameters, direction and phase velocity, and without the knowledge of one the other cannot be found. I thought this problem was already solved in my paper (Aki, 1957), in which a statistical theory of determining the phase velocity of random waves was given with a successful application to microseisms in Tokyo in the frequency range of 5 to 15 cps. Since Toksoez's conclusion might have given a pessimistic view on the use of microseisms, I feel it is necessary to report a brief summary of my old paper published in a Japanese journal which might not be well circulated in the United States.