EFFECTS OF RUPTURE DIRECTIVITY ON EARTHQUAKE LOSS ESTIMATION

A seismic loss estimation analysis, whether for a single structure or a portfolio of properties, requires a relationship between a chosen ground motion intensity measure, IM, and economic loss. This relationship, called a Loss Function, LF, is developed here by a two-step procedure that combines analytical and empirical data. First an analytical relationship is built via engineering analyses performed on a mathematical model of a structure, and then the resulting function is calibrated using loss data from past earthquakes. One key aspect of developing the analytical function is to create a “link” between one (or more) IM(s) and one (or more) measure(s) of the demand that the ground motion imposes on the structure. This link is often established using regression analysis techniques. Typically, the spectral acceleration, Sa, at the structure’s fundamental period of vibration, T1, is used as the single IM of choice. Studies have shown, however, that near-source, forward-directivity ground motions are, on average, more damaging than other accelerograms with the same Sa(T1). As intuition suggests, this is true for moderate-to-long period structures that are dynamically driven by the velocity pulses often observed in fault-normal components of forwarddirectivity records. This study illustrates that this can also be the case for some stiffer structures whose T1 values are significantly shorter than the period of those velocity pulses. We show the impact of this finding in a loss estimation analysis for residential woodframe structures located at single sites and in a portfolio in the San Francisco Bay Area.