水成岩層の地震波傅播に及ぼす影響

概要

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Since k in Omori's empirical formula d=kt may be made to assume the physical meaning k=v1.v2/(v1-v2), where d is the distance of the seismic focus from a station, t the duration of the preliminary tremors at the said station, and v1 and v2 the respective transit velocities of the dilatational and distortional waves, it will be seen that the factor is by no means a constant, but must vary with v1 and v2, and must therefore be some function of the focal depth f, say, and the epicentral distance △. For near distances, Omori deduced for it the value 7.42, but evidence is not wanting that in the younger geo-logical formations it may take as low a value as 2, or even 1.5. For the local shocks so frequent in the Kii district, one of us obtained the value 3.0. Consequently, in determining the seismic focus by t obtained at four or more stations with the assumption of wave transmission along a single straight line with k as constant, inaccuracies in the results will be inevitable. Similarly, the usual method of estimating focal depths by the time-distance curve of P-, P or S--P- must also be subject to several errors when the presence of a superficial sedimentary layer has been ignored.
東京帝国大学地震研究所,Earthquake Research Institute, Tokyo Imperial University,地震研究所の論文
1930-04-08