Group and phase velocities of Rayleigh waves along 57 paths between Central America and the central United States were determined. The waves examined were of period between 15 and 60 seconds and crossed the Gulf of Mexico and Caribbean Sea in many directions. The following conclusions about the structure of the region are drawn:

Along the paths crossing the western and central Gulf and the regions near the coast of Texas and the eastern coast of Mexico the velocities were found to be very low. This is attributed to the great thickness of the sediments in these regions.

The velocities along paths crossing the eastern Gulf and westernmost Caribbean were found to be higher than the typical continental velocities. This is considered as an indication of a difference in structure between the eastern and western Gulf of Mexico. Along one path in the western Caribbean Sea between Colombia and Cuba the velocities were almost continental. High velocities found for part of the eastern Caribbean indicate an almost oceanic character for this region.

For each earthquake examined the initial phases were determined with an error less than ± π/4. For a given earthquake in all cases except two the initial phases were found to be the same at all stations.

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