SYDNEY : A shallow 6.4-magnitude earthquake hit the southern Indian Ocean on Saturday, US seismologists said, prompting Australia to issue a tsunami warning. The US Geological Survey said the quake struck at a depth of seven kilometres (4.4 miles) at 1110 GMT, almost 700 miles northwest of Australia's Cocos Islands, also known as the Keeling Islands. There were no immediate reports of damage or casualties, but the Joint Australian Tsunami Warning Centre (JATWC) issued an alert and advised people not to swim in the sea, and to move away from beaches, harbours, marinas and coastal estuaries. Evacuations from the area were unnecessary, it said.