Geological Survey said, but there was no tsunami warning or immediate reports of damage.
The offshore tremor struck at a shallow depth of eight kilometres (five miles), about 100 kilometres southeast of Kupang on the Indonesian portion of Timor island, the USGS said.
It was followed by a shallow 5.6 magnitude quake nearby.
Witnesses described a powerful jolt from the first tremor which lasted a few seconds.
“I was on the second floor of my office and suddenly everybody ran outside because of the earthquake,” said an AFP reporter in Kupang.
“All the chairs were shaking… we were traumatised by all the earthquakes in Lombok.”
A string of deadly earthquakes that rocked Lombok island this summer killed some 555 people.
Indonesia´s disaster agency said things had quickly returned to normal.
“Local disaster mitigation agency officials are still monitoring the impact of the earthquake,” agency spokesman Sutopo Purwo Nugroho said in a statement.
“There are no victims or damage.”
Indonesia, one of the most disaster-prone nations on earth, straddles the so-called Pacific “Ring of Fire”, where tectonic plates collide and many of the world´s volcanic eruptions and earthquakes occur.