Near-fault ground motions are different from ordinary ground motions in that they often contain strong coherent dynamic long period pulses and permanent ground displacements, as expected from seismological theory. The dynamic motions are dominated by a large long period pulse of motion that occurs on the horizontal component perpendicular to the strike of the fault, caused by rupture directivity effects. Forward rupture directivity causes the horizontal strike-normal component of ground motion to be systematically larger than the strike-parallel component at periods longer than about 0.5 seconds. To accurately characterize near fault ground motions, it is therefore necessary to specify separate response spectra and time histories for the strike-normal and strike-parallel components of ground motion. An empirical model for dynamic near-fault ground motions that assumes monotonically increasing spectral amplitude at all periods with increasing magnitude, representing directivity as a broadband effect at long periods, was developed by Somerville et al. (1997). However, near fault recordings from recent earthquakes indicate that the pulse is a narrow band pulse whose period increases with magnitude, causing the response spectrum to have a peak whose period increases with magnitude, such that the near-fault ground motions from moderate magnitude earthquakes may exceed those of larger earthquakes at intermediate periods (around 1 second). A preliminary response spectral model has been developed to incorporate the magnitude dependent shape of the response spectrum of the forward rupture directivity pulse.
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