The United States Geological Survey detected a Magnitude 6.2 Earthquake at a depth of 10 km, about 70 km to the northeast of the island of Tongatapu (the largest of the 177 islands that make up the Kingdom of Tonga) slightly before 1.25 am local time on Tuesday 7 July 2015 (slightly before 12.25 pm on Monday 6 July, GMT). While this was a large quake, it was unlikely to present any danger due to its distance from shore, though it was felt on the island.
The approximate location of the 7 July 2015 Tonga Earthquake. Google Maps.
The islands of Tonga lie along the boundary between the Pacific and Australian Tectonic Plates. The Pacific Plate is being subducted beneath the Australian Plate along the Tonga Trench, which forms the boundary between these two plates, with the volcanic islands that make up the archipelago being formed as the subducting plate is melted by the heat of the planet's interior, so that lighter minerals rise up through the overlying plate as liquid magma, which fuels the volcanoes that build the islands. This subduction is not a smooth process, with the two plates constantly sticking together until the pressure generated by their movement builds up sufficiently to break them apart, causing a sudden forward jerk we experience as an Earthquake.
Diagram showing subduction along the Tonga Trench, and how this feeds the volcanoes of the Tonga Volcanic Arc. York University.
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.
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