An earthquake measuring 6.4 on the Richter scale today rocked the southwest island of Japan's Okinawa Prefecture. The Japan Meteorological Agency, a tsunami alert was issued shortly after the quake, which occured 10:28 p.m. local time. The focus of the quake was very shallow in the sea off Miyakojima island of Okinawa. There were no immediate reports of casualties or damage to properties, but the agency said tsunami with a height expected to be about 0.5 meter will reach Miyakojima and Yaeyama areas soon.