Investigation of Microearthquakes - On the Nature of the Crust -

Abstract

1. The ambiguities in the interpretation of explosion seismic data can be compensated by studying the seismic data of microearthquakes. It was found that the upper crust and the lower crust were defined by a fairly sharp contrast in their compressional waves. 2. The Poisson's ratio of the upper crust is about 0.24, which is a rather lower value than that to be expected from rock experiments. There is also a sharp contrast in the Poisson's ratio between the upper and lower crust. This suggests that there must exist a change of chemical and/or mineral composition between these two layers. 3. By comparing the phase velocities of a Rayleigh wave in the central U.S. and one in the western part of Japan, areas which are infered to have nearly the same crustal structures according to the results of explosion seismology, it was found that the Poisson's ratio of the upper mantle and/or lower crust in the latter district was larger than that of the former district by 0.03-0.05 in average. The lower crust and the upper mantle of the island arc may be characterized as having a high Poisson's ratio. It is interesting relating to the formation of the crust and its development.