TOKYO, Aug. 17 (Xinhua) -- A second earthquake with a preliminary magnitude of 6.5 rocked Ishigaki Island in Japan's southernmost Okinawa Prefecture on Monday, said the Japan Meteorological Agency, which issued a tsunami warning of 50 cm high waves to nearby islands.

    The focus of the quake, which occurred at 7:11 a.m. (1011 GMT), was located around 140 km southwest of Ishigaki at a depth of some10 km under the sea, said the agency.

    There have been no immediate reports of casualties or damage to properties.

    Earlier in the day, a similar quake measuring 6.8 on the Richter scale hits Ishigaki Island at 9:06 a.m. (0006 GMT). And the Japan Meteorological Agency issued a tsunami alert for Miyako Island and neighboring islands in Okinawa Prefecture and lifted the warning at 10 a.m. (0100 GMT).