Saw that one on the NEIC website.

Quake appears to be in the descending plate and is a plate bend rift event with the bent plate forming tension cracks with a sudden roof keystone failure resulting in a drop of the keystone over the bend.

There have been a lot of these quakes in the Tohoku region following the 11/03/2011 quake which decoupled the megathrust and relaxed the crust of both plates.

This event was a consequence of descending plate lurch which has caused the outboard part of the descending plate to bend in response to the lurch to even out the stress of the descent.

This is a rather smaller event than the Tonga tsunami but resembles the quake & tsunami in Sanriku of the 1930s, which was a plate bend rift response to the "silent but deadly" quake & tsunami of 1896 which relaxed the crust just N. of Tohoku.

Japanese may have to wait about 30 years or more for the BG plate bend rift & tsunami event of Tonga type to strike in the Tohoku region. Would be interesting to watch but smaller than the original megathrust tsunami.

have a nice day: Ag