The government on Friday implemented a new alert system in seven coastal prefectures from Chiba to Hokkaido to warn against subsequent earthquakes should a quake of magnitude 7.0 or larger strike in either of two deep-sea trenches in the Pacific Ocean where massive temblors are feared.

The warnings are designed to encourage evacuation preparedness against additional earthquakes arising in the Japan and Chishima trenches off the country's northern Pacific coast.

The government estimates an M9.0 quake in either trench could trigger huge tsunami, producing waves of up to 30 meters in parts of Hokkaido and Iwate prefectures in the country's north.