This is right at the southern end of the big rupture. These smooth trenches have to maintain themselves by a big M9 every few hundred years, otherwise they would just get filled in. Only an M9 has enough fault displacement to 'clean' things up.
This M7.3 is now at the cusp of a sharp bend, which stopped the last rupture. The next segment goes to Tokyo, and is probably under some stress because of the M9.