A strong earthquake has shaken the coast of northeastern Japan, prompting authorities to issue tsunami advisories for four coastal regions.The Japan Meteorological Agency (JMA) measured Wednesday evening’s earthquake at 6.8 preliminary magnitude and said the epicentre was about 235km south of Kushiro on Hokkaido, Japan’s main northern island.A tsunami of about a half-metre was expected for Aomori and Iwate prefectures. Iwate was in the region heavily damaged by last year’s earthquake and tsunami.Warnings were also issued for central and eastern areas of Hokkaido’s coastline.The earthquake was relatively shallow at 26km from the ocean floor, the JMA said.The Pacific Tsunami Warning Center in Hawaii said there was no threat of a Pacific-wide tsunami.