The Indonesian Meteorological, Climatological, and Geophysical Agency recorded a Magnitude 7.3 Earthquake at a depth of about 84 km, between the islands of Nias and Hibala, off the south coast of Sumatra, Indonesia at about 3.00 am Western Indonesian Time on Tuesday 25 April 2023 (about 8.00 pm on Monday 24 April GMT). There have been no reports of any damage or casualties following this event, but people have reported feeling the event across the Nias Archipelago, Sumatra and southern Peninsula Malaysia, with reports of shaking going on for about 30 seconds. A tsunami warning was issued following the quake, but in the event the wave associated with the quake was only about 11 cm high when it reached Sumatra.
The Indo-Australian Plate, which underlies the Indian Ocean to the west of Sumatra, is being subducted beneath the Sunda Plate, a breakaway part of the Eurasian Plate which underlies Sumatra and neighbouring Java, along the Sunda Trench, passing under Sumatra, where friction between the two plates can cause Earthquakes. As the Indo-Australian Plate sinks further into the Earth it is partially melted and some of the melted material rises through the overlying Sunda Plate as magma, fueling the volcanoes of Sumatra.
This does not happen at a 90° angle, as occurs in the subduction zones along the western margins of North and South America, but at a steeply oblique angle. This means that as well as the subduction of the Indo-Australian plate beneath the Sunda, the two plates are also moving past one-another. This causes rifting within the plates, as parts of each plate become stuck to the other, and are dragged along in the opposing plate's direction. The most obvious example of this is the Sumatran Fault, which runs the length of Sumatra, with the two halves of the island moving independently of one-another. This fault is the cause of most of the quakes on the island, and most of the island's volcanoes lie on it.
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