Beth, maybe it is real. Probably would be good if Roger and John V chimed in on whether the pre-1960 magnitudes are correctly estimated. For example, Richter magnitude is no good for great quakes: it underestimates magnitude. For great quakes they use Moment magnitude. That scale may not have been invented until a few decades ago. So, I don't know whether your list is corrected for that. I don't know how accurate those magnitudes are that are based on historic damage reports.

If real, might be just statistics of small numbers: might not mean anything.
Chris