SYDNEY, AUSTRALIA — A magnitude 6.1 earthquake rattled buildings on Vanuatu’s main island early Sunday but did not appear to have caused major damage, just days after a massive, deadly quake wreaked havoc on the Pacific archipelago. The nation’s most populous island, Efate, is still reeling from the 7.3-magnitude quake that struck Tuesday. It killed 12 people as it toppled concrete buildings in the capital, Port Vila, and set off landslides. The quake on Sunday occurred at a depth of 40 kilometers and was located about 30 km west of the capital. Unlike the earlier quake, no tsunami alerts were