On the Determination of Crustal Structure by the Dispersion Curves of Surface Waves (III)

Abstract

In the previous paper, the author investigated which method will be the best analyses for determining the crustal structure from the seismic surface waves propagating through it, which has no vertical discontinuities between an epicenter and a point of observation. In many cases, however, seismic surface waves do cross several vertical discontinuities before they reach observing points.
In the present paper, the author investigated dispersive waves crossing discontinuities taking a very simple model (two elastic plates are in contact in a plane perpendicular to them).
The observed group-velocity U is given by
Δ/U=Δ₁/U₁+Δ₂/U₂+…+Δ_n/U_n
for a given period, where
Δ=Δ₁+Δ₂+…+Δ_n=total epicental distance,
Δ₁=distance between a source and the first discontinuity,
Δ_m=distance between the (m-1)-th and the m-th discontinuities,
Δ_n=distance between the n-th discontinuity and a point of observation,
U_m=group-velocity of surface waves under consideration in the m-th medium.
It must be noted that this relation holds only when waves are incident almost perpendicularly to the discontinuity planes.
The observed phase-velocity gives informations of the local structure in the vicinity of observing points.
In the latter part of this paper, the author deduced the position of discontinuity line between the Pacific Ocean and the Continent of North America, using the dispersion of Rayleigh waves observed by R. M. Brilliant and M. Ewing.