Ground Response and Damage Intensity Correlations in Damage Zones on Basin Fringes

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9416378 Rollins A preliminary review of damage distribution in the San Fernando and Los Angeles basins following the 1994 Northridge earthquake indicates that damage was concentrated in low-rise structures at the fringes of the basins where soil profiles are relatively stiff and shallow. Accelerograms located on alluvium at the fringes of the valleys also exhibited unusually high accelerations, particularly in the vicinity of San Fernando. In contrast to the 1985 Mexico Earthquake, it appears that incoming rock motions were rich in short period energy due to the proximity to the source, causing significant amplification in the stiffer shallow deposits (with shorter natural periods of vibration) located at the fringes of the basins. Amplification of short period energy would be expected to result in a concentration of damage to short period structures (such as single story structures) located on these deposits. It is also possible that 2- or 3-D basin- shape effects played a role in the amplification at these locations. The objectives of this project are: (1) To evaluate the ability of widely used computer codes such as SHAKE and DESRA to match recorded motions, particularly in areas where damage intensity was relatively high. These ground response techniques have proven useful at deep soft soil sites such as those in Mexico City and San Francisco, but have not been verified against field data for stiff shallow sites. Other siesmically active areas, such as Salt Lake valley, have stiff shallow soils adjacent to mountain fronts and it is important to calibrate the methods for these site conditions. (2) To calibrate correlations between the damage potential index (DPI) defined by Seed and Sun (1989),and observed damage to structures. Useful correlations between DPI and damage intensity were observed following the 1967 Caracas, Venezuela earthquake and the 1985 Mexico Earthquake. However, the Northridge earthquake provides the best opportunity to d ate to calibrate the correlation for conditions in the U.S. ***