Selection of Near-Fault Pulse Motions

Abstract

Earthquake ground motions in the near-fault region frequently have intense, double-sided pulses in the velocity-time series that can be very damaging to structures. Many of these velocity pulses are attributed to the effects of forward directivity, which occurs when a fault ruptures toward a site. However, pulses are not always observed in the forward directivity region, and some pulses cannot be explained by forward directivity. The relative contribution of pulse-type motions to the overall seismic hazard should be considered when selecting records in a suite of design ground motions for a site in the near-fault region. This study uses a new scheme to classify records from an enhanced database of records from shallow crustal earthquakes that have the closest site-to-source distances less than 30 km with moment magnitudes greater than 6.0 as either pulse or nonpulse motions. The resulting database of 673 records from 52 earthquakes contains 141 pulses, including 74 explained well by forward directivity. Logistic regression is used to develop a simple model to estimate the proportion of pulse motions as a function of closest site-to-source distance and epsilon of the seismic hazard. The resulting relationship can be used to estimate the number of pulse-type motions that should be included within a suite of ground motions to represent the proper contribution of pulse motions to the seismic hazard. Guidance is also provided for selecting pulse records.