On Tuesday 4 September 2012, slightly after 2.20 am local time (slightly after 6.20 pm on Monday 3 August, GMT), a large Earthquake occurred roughly 220 km south of East Java, according to the United States Geological Survey, who measured the quake as 6.4 on the Richter Scale, at a depth of 8.8 km. This is a large, shallow Earthquake which would potentially be very dangerous closer to land, but is unlikely to present any threat this far from land, and will probably only have been felt at the very southern tip of East Java and possibly Bali. It is not thought likly that the quake will have caused a tsunami.
Map showing the location of the 4 September 2012 Earthquake and the areas that will have suffered the worst shaking. The outermost circle, which just touches the south of East Java and Bali, will only have suffered light shaking. USGS.
Java and Bali are located on the southen edge of the Sunda Plate, close to its margin with the Australian Plate, which is being subducted beneath the Sunda Plate along the Sunda Trench, to the south of the islands. This is not a smooth process, as the rocks of the two plates constantly adhere together then break apart as the pressure builds up, causing Earthquakes. As the Australian Plate sinks further into the Earth it is partially melted by a combination of the friction and the heat from the planet's interior. Some of this melted material then rises through the overlying Sunda Plate, fueling the volcanoes of Java and Bali.
See also Seismic activity beneath Mount Tangkubanparahu, Indian Ocean shaken by a series of major Earthquakes, Massive Earthquake off Northern Sumatra, Large Earthquake in Northern Sumatra and and Earthquakes on Sciency Thoughts YouTube.
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