Conditional pulse probability for near-source hazard analysis

Near-source ground motion records affected by “directivity” may show unusual features resulting in low frequency pulses in the velocity time-history, especially in the fault-normal component. Although not all near-source recordings show pulses, such an effect is of particular interest for practitioners as it may cause the seismic demand for structures to deviate from that of, so-called, “ordinary” records. Consequently many seismology and earthquake engineering researchers have tried to parameterize the causes and the effects of directivity pulses. In the framework of the probabilistic seismic assessment of structures in near-source conditions, a quantification of the pulse threat is required. In fact, the recently developed probabilistic seismic hazard analysis for near-source sites requires a probabilistic model for the occurrence of pulses in ground motion. Herein this issue is investigated and models are obtained via logistic regression of a set of pulse-like records from the NGA database. Analyses are limited to ground motions recorded within 30km from the source and to strike-slip events. Occurrence probability of velocity pulses is computed as conditional in respect to those factors considered by seismologists to affect the amplitude of directivity effects.