As the number of long-period large-scale structures increases world-wide, estimation and realistic modeling of long-period earthquake ground motions become an important consideration in seismic design. Structures that maybe characterized as large-scale are high-rise buildings, large fluid storage tanks and long-span suspension bridges. Recent events have allowed the observation of the effects of long-period motions on large-scale structures: amplification and elongation of structural response on high-rise buildings , sloshing damage to oil storage tanks, and increase in displacement demands on members of long-span suspension bridges.
Intense long-period ground motions are usually induced at large distances from the source by large subduction-zone earthquakes (e.g. 2003 Tokachi-Oki, Japan, Mw 8.3) and moderate-to-large crustal earthquakes (e.g. 1999 Chi-Chi, Taiwan, Mw 7.6). Usually, long-period ground motions consist primarily of surface waves, which are generated by conversion of incident body wave energy on sedimentary deposits. Distinctive examples of well documented basin-induced surface waves in the recent past were the motions recorded on the Mexico Valley during the 1985 Michoacán earthquake and the motions recorded on the Po Plain during the 2012 Emilia earthquake in Northern Italy.