Sri Lanka, April 20 -- The Japan Meteorological Agency has issued an alert for a potential mega-quake, following the magnitude 7.7 quake on Monday. The agency says residents in seven prefectures should prepare for disaster and listen for any updates.

Officials say there is an increased probability of a mega-quake along two deep-sea trenches in the Pacific Ocean. The alert covers 182 cities and towns from Hokkaido to Chiba.

The agency raised the scale of the quake from magnitude 7.5 to 7.7 that occurred off the coast of Sanriku, Iwate Prefecture at 4:52 p.m. local time, and had an intensity of upper 5 on the Japanese scale of 0 to 7 in the hardest-hit areas. The depth of the quake is estimated to be 19 kilometers.

Class 3 long tremors i...