The most likely explanation for the trend is that there are several centers of seismic activity, like Tonga or a major aftershock zone, that have these apparently special distances as their separation. To discard this possibility, one should compare time-scrambled catalogues with the real catalog, or examine which event-pairs are occurring for the special distances.

I'm particularly skeptical because these distances do not correspond to much, if any, amplification of the seismic waves.

I don't have time to pursue this myself.

John