Northridge, California, earthquake of 1994: density of red-tagged buildings versus peak horizontal velocity and intensity of shaking

The 1994 Northridge earthquake occurred underneath a densely populated metropolitan area, and was recorded by over 200 strong motion stations in the metropolitan area and vicinity. This rare coincidence made it an ideal case to study, in statistical sense, the correlation of damage to structures with the level of strong shaking, in particular with respect to (1) instrumental characteristics of shaking and (2) the reported site intensity scale. In this paper, statistics for the incidence of red-tagged building in 1 × 1 km² blocks in San Fernando Valley and Los Angeles is presented and analyzed, as function of the observed peak ground velocity or the local intensity of shaking. The ‘observed’ peak velocity is estimated from contour maps based on the recorded strong motion. The intensity of shaking is estimated from the published intensity map and from our modification of this map to make it more consistent with observed high damage to buildings in some localized areas. Finally, empirical scaling equations are derived which predict the average density of red-tagged buildings (per km²) as a function of peak ground velocity or site intensity of shaking. These scaling equations are specific to the region studied, and apply to Wooden Frame Construction, typical of post World War II period, which is the prevailing building type in the sample studied. These can be used to predict the density of red-tagged buildings per km² in San Fernando Valley and in Los Angeles for a scenario earthquake or for an ensemble of earthquakes during specified exposure, within the framework of probabilistic seismic hazard analysis. Such predictions will be useful to government officials for emergency planning, to the insurance industry for realistic assessment of insured losses, and to structural engineers for assessment of the overall performance of this type of buildings.