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LAMONT GEOLOGICAL OBSERVATORY COLUMBIA UNIVERSITY, PALISADES, NEW YORK
Abstract
The propagation of short-period oceanic surface waves with predominant periods between 5 and 20 seconds was studied for a large number of paths in the Atlantic, Indian and Pacific oceans. In the ocean basins these waves are controlled largely by the sedimentary layer. The amplitudes of the short-period waves are greatly dependent upon the nature of the propagation path and on the properties of the source. Surface waves with periods between 5 and 10 seconds are often observed for paths that cross the continental margins of the Pacific, but are recorded only rarely for similar transmission paths in the Atlantic. Probably this effect is due to the differences in the structural configuration of the margins, particularly to such differences in the sedimentary strata.
Seismic refraction and reflection measurements indicate that most of the oceanic paths for which the 5 to 10 second waves are observed are characterized by sedimentary thicknesses that do not average more than a few tenths of a kilometer. The predominant periods of the short-period wave train are increased to as much as 12 to 20 seconds for paths that traverse the thick sediments of the Argentine Basin. This increase in period, which was predicted theoretically for regions of relatively thick sediments, indicates that the low-rigidity sediments play a prominent role in determining the character of the short-period wave train. The increase in the predominant periods of the waves associated with the first Love and first shear mode also accounts for the absence of the 5 to 10 second waves in areas of thick sediments such as the continental margins of the Atlantic.
Dispersion data for the Rayleigh, first Love and first shear modes were used in conjunction with reflection and refraction results to estimate the average shear velocity in the sediments of the Argentine Basin. The average shear velocity in the upper 0.5 km of sediments in about 0.2 to 0.4 km/sec, and the velocity in the kilometer of sediments below this is about 0.5 to 0.7 km/sec. Sedimentary shear velocities of a few tenths of a kilometer per second were also obtained for paths along which the average sedimentary thicknesses are a few tenths of a kilometer.
For group velocities between 4.2 and 3.4 km/sec the particle motion in the first shear mode is retrograde; ratios of horizontal-to-vertical motion as large as 3.5 are observed at island and coastal stations.
Seismic waves with periods of 10 to 30 seconds and with group velocities of 4.0 to 4.4 km/sec are sometimes recorded on the vertical components of long-period seismographs. These waves may be explained as higher modes of the Rayleigh type. Oscillatory waves with periods of 5 to 10 seconds are observed to follow the P wave in many oceanic areas. These arrivals are attributed to a type of leaking mode that results from the multiple reflection of SV waves in a low-rigidity layer.
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