The United States Geological Survey recorded a Magnitude 6.4 Earthquake at a depth of 619.4 km, in western Ucayali Province in Peru, close to the border with Brazil, at about 8.30 am local time (about 1.30 pm GMT) on Sunday 18 December 2016. This event was felt across much of Peru, as well as parts of Brazil, though there are no reports of any damage or casualties; which 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 18 December 2016 Ucayali Earthquake. USGS.
Peru is on the west coast of South America and the western margin of the South American Plate, close to where the Nazca Plate, which underlies part of the east Pacific, is being subducted along the Peru-Chile Trench. The Nazca Plate passes under the South American Plate as it sinks into the Earth, this is not a smooth process and the plates repeatedly stick together then break apart as the pressure builds up, causing Earthquakes. As the Nazca Plate sinks further it is partially melted by the friction and the heat of the Earth's interior. Some of this melted material then rises through the overlying South American Plate, fuelling the volcanoes of Peru and neighbouring countries.
The subduction of the Nazca Plate beneath the South American Plate, and how it causes Earthquakes and volcanoes. Pacific Earthquake Engineering Research Center.
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.