Initial reports say a 5.5 quake hit Nagano Prefecture.

Also, the Japanese Meteorological Agency warned that more aftershocks of 7.0 or greater could hit Japan.  Keiji Doi, earthquake prediction division chief, said the chance of aftershocks of 7.0 could continue for at least a year.

So far, more than 400 aftershocks of magnitude 5.0 or larger have occurred since the catastrophic March 11 quake.