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Within the dense urban core of our cities in seismic areas, older tall buildings with poor earthquake systems present high risks in large earthquakes (e.g., in design basis or maximum considered earthquake events). Towers that topple in side-sway can impact neighboring buildings, with increased economic and life-safety consequences. Current approaches in seismic design for new buildings, and proposed earthquake ratings systems for existing buildings, consider each building individually, in isolation from its neighbors. Yet impacts to surrounding structures are inevitable with the collapse of closely-spaced tall buildings, and a different, more complete picture emerges when the urban core is viewed in its complex totality. 

Zones of concern can be visualized using two indices: an 'exposure index' to indicate the built area at risk from any building, and a 'threat count’ to indicate the number of buildings that could potentially strike a building in question. Below are examples for the City of San Francisco.

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