Comment on 'A model of earthquake triggering probabilities and application to dynamic deformations constrained by ground motion observations' by Joan Gomberg and Karen Felzer

摘要

Felzer and Brodsky [2006] argued that a uniform power law distance-decay of aftershock density to 50 km from small main shocks provides evidence that aftershocks are triggered by dynamic stress, since static stress would be negligible more than a few kilometers from the main shocks. This study was succeeded by Gomberg and Felzer [2008], which extended the analysis. Figure 1 of Gomberg and Felzer [2008], reproduced here as Figure la, is presented to show that the decay of seismicity density from main shocks "is a constant over distances spanning a fraction of a main shock fault length to hundreds of main shock fault lengths, at least out to absolute distances of 50-100 km" (paragraph 6). Gomberg and Felzer's [2008] Figure 1 caption states that "All aftershocks are M > 2 and occur in the first 5 min after their main shock; the short time window separates aftershocks from unrelated background earthquakes." But inspection of the Felzer and Brodsky [2006] source panels for Figure 1 indicates that the circles are actually the first 2 days of M > 3 aftershocks. Apart from the mislabeling, resealing the aftershock density from 2 days to 5 min is too uncertain to make the compatibility argument advanced, and the restricted ranges of the two data sets give an appearance of continuity that is inconsistent with the full data.