The United States Geological Survey recorded a Magnitude 7.0 Earthquake at a depth of 60.1 km about 83 km off the southeast coast of Mindanao Island, Philippines, slightly before 3.40 am local time on Saturday 29 December 2018 (slightly after 7.40 pm on Friday 28 December GMT). There are no reports of any damage or casualties associated with the event, but it was felt across Mindanao, as well as on the Indonesian islands of Sangihe and Talaud, and a tsunami alarm was briefly put in place, as the region is prone to such events.
The approximate location of the 29 December 2018 Mindanao Earthquake. USGS.
The geology of the central Philippines is Complex. The west of Mindanao Island is located on the Banda (or Sunda) Microplate, and the east on the Philippine Plate, which is being subducted beneath the Sunda (or Banda) Microplate along the central part of the island. Immediately to the east of the Island the Pacific Plate is being subducted along the Philippine Trench, and passes beneath eastern Mindanao as it sinks into the Earth. This is not a smooth process, an the plates constantly stick together then break apart again as the pressure builds up, resulting in Earthquakes.
Earthquakes along subductive margins are particularly prone to causing tsunamis, since these often occur when the overlying plate has stuck to the underlying plate, being pulled out of shape by its movement.. Eventually the pressure builds up to far and the overlying plate snaps back, causing an Earthquake and a tsunami.
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.
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