“Surfseisms”—Elastic Waves Generated in the Bottom by Pounding Surf

Abstract

The following experiment was performed on the beach at Fire Island in an area whose geophysical structure is well known: vertical component geophones were planted in the sand near the surf, and recordings were made of particle velocities in the surface sand layer. At the same time motion pictures were taken of the breaking water waves. The pattern of signal bursts and relative silences found in these surfsesims correlates well with surf behavior. Spectrum analyses of surfseisms have been obtained over a frequency range from 10 to 50 cps. An analysis of the dispersive wave train produced by a simulated shot source shows very low group velocities of the order of several hundred ft/sec. A qualitative theory of surfseism phenomena will be presented. (This work was supported by the Office of Naval Research.)