I don't know the answer to that off the top of my head, but I just checked the USGS web site at http://gldss7.cr.usgs.gov/neis/eqlists/significant.html, and in the last four years ther have been anywhere from 1 to 4 earthquakes in Japan (each year) of a magnitude 6 or greater. You can expect maybe ten times as many magnitude fives as a rough guesstimate.