The weather agency is warning of possible strong temblors in Nagano Prefecture as shallow-focus earthquakes have occurred frequently in the area over the past several days.

Noting that more than 50 human-felt quakes have hit the region since last Wednesday, the Meteorological Agency says all of the temblors had a focus around 10 kilometers or less below the surface, and that such shallow quakes can produce disproportionately intense shaking.

On Thursday, an earthquake with a magnitude of 5.5 struck central Nagano, registering 4 on the Japanese seismic intensity scale of 7 in Matsumoto.

While the cause of the series of quakes remains unknown, several active faults have been confirmed near the area. Frequent quakes can also be triggered by volcanic activity, but there are no signs of any active movement in the region, the agency said.

Following the magnitude 9.0 earthquake that devastated the Tohoku region on March 11, 2011, another quake with magnitude 6.7 rattled northern Nagano in the early hours of the next day, killing three people and injuring more than 10.

KEYWORDS Nagano, earthquakes