The effect of surficial sedimentary layers on continental surface waves

Abstract

Surface waves in the 1/2-second to 12-second period range, recorded at several stations in eastern North America from the eastern Tennessee shock of June 23, 1957, are the bases for several deductions concerning the effect of sedimentary layers on continental surface wave propagation. These are: (1) The velocities of surface waves of the fundamental Love and Rayleigh modes having periods less than about 10 seconds may be strongly affected by sedimentary layers of average thickness. The decrease in velocity accounts, at least in part, for the prolongation of surface-wave trains in this period range when sedimentary layers of appreciable thickness have been traversed. (2) Higher-mode propagation for both types of surface waves is a possible explanation for the velocities, frequencies, and amplitudes of the phase Sg at moderate epicentral distances, and of its long-distance counterpart the high-frequency component of Lg. The lower-frequency components of Lg have been explained previously by other aspects of normal-mode propagation in the crust. (3) Study of dispersion of short-period surface waves can result in fairly detailed knowledge of velocity-depth relation within the sedimentary column and may also reveal information on anisotropy. (4) The results of this study must bear heavily on studies of microseism propagation. As an example, the increase of microseismic activity along the entire east coast of the United States when a storm moves onto the continental shelf may be attributed to channeling of the waves in the deep sedimentary trough beneath the shelf.