The United States Geological Survey recorded a Magnitude 5.6 Earthquake at a depth of 81 km, roughly 8 km to the west of the village of Nereju in Vrancea County, in the southeast Carpathian Mountains of Romania, at about 1.20 am local time on Wednesday 28 December 2016 (about 11.20 pm on Tuesday 27 December GMT). There are no reports of any damage or injuries arising from this event, but it was felt over much of Romania as well as in neighbouring Bulgaria, Moldova, Serbia and Ukraine. This is roughly what would be expected from an Earthquake of this size at this depth; the quake is big enough to be felt over a wide area, but most of its energy has dissipated before the shock-waves reach the surface.
The approximate location of the 28 December 2016 Romanian Earthquake. USGS.
The Carpathian Mountains form part of the suture formed when the Tethys Ocean closed during the Mesozoic, joining the continents of Laurasia (to the north) and Gondwana (to the south). The area is now internal to the Eurasian continent, but the area to the south, known as the Moesian Platform, has a separate origin to the rest of Europe. This system is once again being stressed by the impact of Africa into Eurasia from the south, with the Anatolian Plate (which underlies Anatolian Turkey), Aegean Plate (which underlies southern Greece) and Adriatic Plate (which underlies eastern Italy and the western Balkan Peninsula) caught between the two larger units, leading to a more complex interplay of stresses across southeastern Europe. The Antatolian and Aegean Plates are located to the south of the Moesian Platform, and are being pushed to the west, while the Adriatic Plate lies to the west of the Aegean Plate and Moesian Platform, and is being pushed to the northeast.
Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation 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.