If you ask a geologist a question, expect a long ass answer.

There's a relationship known as the Gutenberg-richter relationship. It says if you plot cumulative # of earthquakes vs. magnitude, you get a straight line.

If you average out all the subduction zones in the world, the slope of this line is =1 and is referred to as a b-value.

This means for every magnitude 7.0 earthquake, we expect ten 6.0 earthquakes. For every 6.0, we expect ten 5.0 earthquakes and so on.

The relationship is related to underlying plate age. Younger plates are thermally hot and more buoyant, which lowers the b-value. Meaning we expect a higher frequency of large magnitude earthquakes compared to small ones (for every 8.0, maybe we see only nine 7.0 earthquakes). Cold subduction zones have a much larger b-value, because the underlying plate is much more dense and trying to subduct more quickly. Less friction and locking against the overlying plate.

The b-value of Japan is ~1. Which makes NO sense. The Pacific plate under Japan is some of the oldest oceanic lithosphere anywhere on earth. It should be shooting down towards the mantle, yet doesn't. It acts buoyant, causing massive frictional forces and large magnitude earthquakes. It also "stalls" around 660 km instead of subducting further into the planet.

Tldr: cold, old plate under Japan should want to subduct, but doesn't. It wants to go up. Wtf

Google for inquiring minds: "Gutenberg-richter relationship, b-values, wadati-benioff zone, mantle megalith".