Spatial Variability of the Directivity Pulse Periods Observed during an Earthquake

Abstract

The ground velocity pulses generated by rupture directivity effects in the near‐fault region can cause a large amount of damage to structures. Proper estimation of the period of such velocity pulses is of particular importance in characterizing near‐fault seismic hazard and mitigating potential damage. We propose a simple equation to determine the pulse period as a function of the site location with respect to the fault rupture (defined by the hypocentral distance hypD , the closest distance to the rupture area clsD , and the length of the rupture area that breaks toward the site D ) and some basic rupture properties (average rupture speed and average rise time). Our equation is first validated from a dataset of synthetic velocity time histories, deploying simulations of various strike‐slip extended ruptures in a homogeneous medium. The analysis of the synthetic dataset confirms that the pulse period does not depend on the whole rupture area, but only on the parameter D . It also reveals that the pulse period is not sensitive to the level of slip heterogeneity on the fault plane. Our model is tested next on a real dataset build from the Next Generation Attenuation‐West2 Project database, compiling 110 observations of velocity pulse periods from 10 strike‐slip events and 6 non‐strike‐slip events. The standard deviation of the natural logarithm residuals between observations and predictions is ∼0.5. Furthermore, the correlation coefficient between observations and predictions equals ∼0.8, indicating that despite its simplicity, our model explains fairly well the spatial variability of the pulse periods.