Three-component, short-period data on Lg in the Eastern United States show unusually large regional variations in the average value of the ratio of the largest horizontal (H) to the largest vertical (Z) component of ground motion. This ratio is found to have a value of about 3 or larger for most sites in the central platform and the coastal plains regions and less than 2 for the hard-rock sites in the Appalachian Mountains region. Large values of H/Z in the region of coastal plains sediments can be explained as being due to the presence of low-velocity overburden but such an explanation is probably not valid for sites in the central platform region. Large variations in H/Z from one site to another are of special importance in earthquake engineering since most damage to structures is generally caused by the horizontal components of ground motion.

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