An Outlook Into Time-Dependent Aftershock Vulnerability Assessment

Abstract eng:
Operative seismic aftershock risk forecasting is particularly useful as support for rapid decision-making in the presence of an ongoing aftershock sequence. Arguably, an operative forecasting framework can be built on the basis of adaptive time-dependent seismic aftershock risk assessment. In such a context, the fragility curves represent the progressive state of damage in a structure. Focusing on adaptive time- and event-dependent fragility assessment, this work explores various issues related to probability-based performance assessment in the immediate post main-shock environment. A fragility curve is expressed herein as the daily probability of (first-excursion) exceeding of a prescribed limit states given the firstmode spectral acceleration. A time-dependent structural performance variable, defined as the ratio of residual demand to residual capacity, is adopted in order to measure the cumulative damage in the structure. The sequence of event-dependent fragility curves are calculated by evolutionary linear logarithmic regression of the structural performance variable versus spectral acceleration, called herein as the sequential cloud method. It is demonstrated that the aftershock recurrence model used to estimate the daily number events of interest significantly affects the resulting fragility curves. Two alternative aftershock occurrence models based on the ETAS model and the modified Omori are studied herein. The sequence of daily fragility curves for a given limit state are finally integrated together with daily aftershock hazard curves (based on above-mentioned aftershock models) in order to calculate the daily aftershock risk. As a numerical example, the daily aftershock risk is calculated for the L’Aquila 2009 aftershock sequence (central Italy). An equivalent single-degree-of-freedom structure with cyclic strength and stiffness degradation is used in order to evaluate the progressive damage caused by the sequence of aftershock events.

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