Equivalent constant rates for post-quake seismic decision making

The objective of this paper is to develop a methodology to quantify the time-varying post-quake aftershock threat in a manner consistent with the development of conventional design ground motion levels, which are typically based on a constant probability of exceedance of a given ground motion intensity. We use the frequency of collapse of a mainshock-damaged building in the aftershock environment as a proxy for the life-safety risk faced by an arbitrary building occupant. We demonstrate that the time-varying aftershock threat can be transformed to an equivalent constant collapse rate for a mainshock-damaged building by considering the total expected number of aftershock collapse events in the time interval of interest and a ‘social discount factor’, typically taken to be from 3% to 5%. The ability to quantify the time-varying life-safety risk in the aftershock environment in a manner compatible with existing building code provisions will allow informed building evacuation and re-occupancy decisions to be made. While this concept has been introduced for the aftershock environment, it may also be a potential solution to a wider class of safety problems involving transient life threats.