Magnitude scaling of the near fault rupture directivity pulse

Abstract

Current ground motion models all assume monotonically increasing spectral amplitude at all periods with increasing magnitude. However, near fault recordings from recent earthquakes confirm that the near fault fault-normal forward rupture directivity velocity pulse is a narrow band pulse whose period increases with magnitude. This magnitude dependence of the period of the near fault pulse is expected from theory, because the period of the pulse is related to source parameters such as the rise time (duration of slip at a point on the fault) and the fault dimensions, which generally increase with magnitude. This magnitude dependence of the pulse period causes the response spectrum to have a peak whose period increases with magnitude, such that the near fault ground motions from smaller earthquakes may exceed those of larger earthquakes at intermediate periods (around 1 s). This paper presents preliminary equations relating the period of the fault-normal component of the forward rupture directivity velocity pulse to the earthquake magnitude, and a preliminary model for the acceleration response spectra of near fault fault-normal ground motions that includes the magnitude dependence of the period of the response spectral peak.