Spatial distribution of the difference between the magnitudes of the main shock and the largest aftershock in the circum-Pacific belt

The distribution of the magnitude difference D₁ between the main shock and the largest aftershock for large circum-Pacific earthquakes is observed to be peaked at 1.2 magnitude units, in accord with Bath's law, but also at 1.8 magnitude units. The peak at 1.8 has been not noted before and is shown here with the T-test to be statistically valid. The observed spatial distribution of the sequences is the basis to suggest that, along circum-Pacific convergent plate boundaries, values of D₁, tend to be larger for earthquake sequences within back-arc areas as compared to sequences closer to the plate interface, although admittedly a good number of exceptions to this generalization are also observed to exist.