Effects of sedimentary basins on ground motions

Many urban areas around the world are situated on sediment-filled basins, with sediment thickness ranging from tens of metres up to 10 or more kilometres. Depending on the angle of incidence, seismic waves entering the basin can become trapped in it, causing resonances resulting in very strong motions that are very damaging. This effect explains much of the damage to buildings and the collapse of the elevated I-10 freeway structure in the Los Angeles basin in the 1994 Northridge earthquake (Graves et al., 1998).

The most dramatic example of this effect occurred in the 1985 Mexico earthquake. In Mexico City, 400 km from the source, while the earthquake was generally not felt at sites away from the basin, in the Lake Zone of the basin resonances were extremely large, causing catastrophic damage to longer-period structures that were resonant with the predominant period of the resonating ground motion (Sections 8.3.6 and 5.2.2).

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