Japan's government will start a new alert system for potential mega-earthquakes following magnitude-7 or over quakes along two deep-sea trenches in the Pacific off northern Japan.

The system will cover 182 municipalities in Hokkaido and the prefectures of Aomori, Iwate, Miyagi, Fukushima, Ibaraki and Chiba. Authorities estimate the region could see tsunami more than 3 meters high with intensities of lower six or over in the Japanese scale of zero to 7.

The system is set to be in place at noon on Friday.

Under the system, the Japan Meteorological Agency would issue an alert about two hours after a magnitude-7 tremor along the Chishima Trench or the northern part of the Japan Trench.

Following the alert about the risk of subsequent megaquakes, residents in the region would be required to prepare for immediate evacuation for about 7 days while continuing their daily lives.

Businesses would be urged to refrain from operating in places that could suffer tsunami or landslides.

Communities would be asked to help elderly people evacuate, and confirm methods of communication among residents.

Earthquakes of magnitude 7 or higher have occurred repeatedly along the Chishima Trench and the northern part of the Japan Trench. Some have been followed by megaquakes.

In the 2011 earthquake and tsunami disaster, a magnitude-9.0 quake hit northeast Japan on March 11, two days after a tremor of magnitude 7.3.

In 1963, a quake of magnitude 7.0 occurred along the Chishima Trench near Etorofu Island. Eighteen hours later, a megaquake with a magnitude of 8.5 struck.

The Cabinet Office and the Meteorological Agency say areas off the Pacific coast of northern Japan have seen frequent seismic activity.

They also say the new system does not mean megaquakes will definitely happen. They add that it is designed to save as many lives as possible, and pledged thorough efforts to inform people about the system.