At least 19 earthquakes with magnitudes of 6 or higher struck the coastal parts of northeastern Japan as well as inland areas after the record 8.8-magnitude temblor wrought damage and death to eastern Japan on Friday.
According to the Japan Meteorological Agency, an estimated 6.7-magnitude quake struck the Shin-Etsu region in central Japan early Saturday morning. The quake registered an intensity of upper 6 in Sakae, in northern Nagano Prefecture. The village was later struck by two additional tremors that registered lower 6.
Meteorological agency officials said they were unsure whether those quakes were correlated with Friday's massive quake, but said there was a possibility that related tectonic movements may have triggered those tremors.
Officials said there is a possibility that aftershocks with intensities around lower 6 could continue for 30 days or so.
More than 100 aftershocks with intensities of 1 or higher have been confirmed in wide areas of eastern Japan, focusing on the Pacific coast. The largest aftershock measured 7.4 in magnitude.
Officials acknowledged that due to the frequency of tremors that hit a wide area of Japan, they were unable to issue emergency quake alerts appropriately.
They said because the system locates the epicenter after picking up seismic action, it was unable to accurately identify the areas that would be affected as multiple quakes ended up shifting the assumed epicenter.
Meteorological agency officials also warned about the continued threat of tsunami triggered by Friday's giant quake, and issued advisories for even the Sea of Japan coast.
As of 11 a.m. Saturday, 18 areas, mainly between Hokkaido and Kochi Prefecture, were issued large tsunami warnings, while 21 locations from the Kanto region to Kyushu were issued tsunami alerts.
Twenty-seven locations, mainly on the Sea of Japan side, were issued tsunami advisories.
The total of advisories and alerts covered all 66 designated districts for tsunami forecasting, marking the first time that all districts were issued with some form of caution.