A procedure for modelling near-field earthquake intensities

Close to the rupture surfaces of large earthquakes, the pattern of intensity is expected, on physical grounds, to reflect the fault geometry. But there are usually not enough observational data to constrain isoseismals at such short distances. In order to obtain a plausible model to sustain hazard estimation exercises, a simple procedure for evaluating intensities is presented. It involves using a point-source attenuation function to calculate the contribution to the ground motion due to an element of the source, then integrating along the fault trace or, for dipping faults, over the entire rupture surface. In the far field, the intensity so derived is exactly equivalent to that obtained if the entire rupture is represented by a point source.