They occur somewhere in the country on more than a daily basis. (12 (3M+) have been mapped in the past 7 days alone). Here's a very interesting site that maps Japanese quakes/tremors (3M+) in a time lapse. You can see the time lapse for the past day, 7 days, or (this is the best part) since Mar-11-2011. Go to:
- click the "since Mar 11" option at the top
- click the "faster" button" next to the "Play" button - click it a few times
- watch the 4323 quakes/tremors occur on-screen. It's like the final scene in "War Games"!
Thanks for asking this question - it led me to this site!