Structural Damage Claims Attributed to Aftershocks

The January 17, 1994 Northridge earthquake (Magnitude 6.7) was the most costly natural disaster in the United States. The Northridge main shock was followed by thousands of aftershocks ranging up to a magnitude 5.9. For a variety of reasons, earthquake-related insurance claims often report damage from not only the main shock, but from aftershocks that can occur many months later. The focus of this paper is the structural damage claimed to be caused by aftershocks of the Northridge earthquake, and the analytical basis for differentiating between aftershock and main shock damage. Methodology is suggested for estimating the ground shaking intensity at a site due to aftershocks, particularly for those cases where nearby ground motion records are not available. Mechanisms of aftershock damage estimation, including site-specific effects and cumulative damage from main and aftershocks, are also reviewed.