The United States Geological Survey recorded a Magnitude 6.7 Earthquake at a depth of 151.5 km beneath the Banda Sea, roughly 278 km to the northeast of Dili in East Timor, slightly after 8.15 am local time (slightly after 0.15 am GMT) on Thursday 21 December 2016. There are no reports of any damage or casualties arising from this event, though people have reported feeling it as far away as Darwin in Australia and Indonesian West Papua.
The approximate location of the 21 December 2016 Banda Sea Earthquake. USGS.
The Timor and the Lesser Sunda Islands are located on the Timor Microplate. This is trapped between the converging Eurasian and Australian Plates, both of which are being subducted beneath it. This is not a smooth process; the two subducting plates continuously stick to the overlying plate, then break away as the pressure builds up, causing Earthquakes in the process. In the south the Australian Plate is passing under the island of Timor, with material from the subducted plate melted by the friction and the heat of the Earth's interior rising through the Timor Plate to feed the volcanoes of the island. In the north the Eurasian Plate is being subducted in the same way, feeding the volcanoes there.
The subduction zones beneath the Timor Microplate. Hamson (2004).
Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organization Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.