Reply to Comment by Hough et al. (1991) on “A Comparison of Eastern North America Ground Motions with Theoretical Predictions”

Abstract

INTRODUCTION I thank Susan Hough, Leonardo Seeber and Klaus Jacob for the opportunity to discuss the implications of the Saguenay ground motion data for ENA ground motion relations. Perhaps the most contentious issue of the ground motion relations developed by Dave Boore and myself has been our assumption that the seismological source can be modeled by a simple ‘constant stress drop’ model, with a mean stress parameter of 100 bars for eastern events (Atkinson, 1984; Boore and Atkinson, 1987; Atkinson and Boore, 1990). (This compares with a stress parameter value of 50 bars for western events (Boore, personal communication; Somerville, personal communication).) The adoption of any simple model to characterize the ‘average’ seismological source parameters will inevitably lead to significant discrepancies in the case of individual events, Saguenay being an excellent example. Because earthquakes are subject to great inter-event variability, it is not necessarily the particular model or value that is selected which causes disagreements between theory and observation. A single earthquake can neither validate nor disprove a median prediction. Hough et al. have two specific comments regarding the analyses and conclusions presented in my paper: 1. they believe that the NCEER strong motion data were excluded on the basis of an incorrect assumption, and that these excluded data might change the conclusions reached; and 2. they believe that Saguenay is more ‘typical’ of large ENA events than Nahanni; including Nahanni in the ground motion data set has the effect of downweighting the Saguenay data, which they feel is unjustified. . . .