At about 9.35 pm local time on Wednesday the 3rd of August an earthquake with a magnitude of 5.2 on the Richter Scale occurred 156 km off the coast of Sumatra, due west of the city of Bengkulu at a depth of about 12 km. Just after 7.15 the next morning a second earthquake reported as having a magnitude of 5.8 by the United States Geological Survey and 6.0 by the Badan Meteorologi, Klimatologi, Dan Geofisica (Indonesian Agency for Meteorology, Climatology and Geophysics) occurred just off the coast, 37 km southwest of Mukomuko, at a depth of 37 km. Then slightly after 11.30 pm a third earthquake, with a magnitude of 4.8 occurred 226 km west of Bengkulu, at a depth of about 37 km.
USGS Map of the area showing the quake on the 3rd in Yellow and 4th in Blue. The red line is the Sunda Megathrust.
There were no casualties reported from any of the quakes, nor significant damage, nor was a tsunami warning issued, though the second quake did cause considerable panic in Padang, the capital of West Sumatra.
Sumatra is one of the most earthquake prone places on Earth, with many quakes having caused considerable loss of life. In October 2010 an earthquake off the west coast of Sumatra caused a 3 m tsunami which swept 600 m inland, killing at least 435 people and displacing 20 000 more. In September 2009 an earthquake just offshore of Pedang killed at least 1115 and injured about 3000 more, largely due to collapsing buildings. In September 2007 a series of earthquakes of the west Sumatran killed 23 people. In March the same year a quake to the north of Pedang killed 68 people, and damaged over 40 000 buildings. In March 2005 an earthquake off the northwest coast killed at least 1300 people, mainly on the small island of Nias. On Boxing Day 2004 an earthquake off the Sumatran coast caused a tsunami which swept across the Indian Ocean killing over 230 000 people.
A reconstruction of the 2004 Tsunami from the
This is all caused by Sumatra's peculiar seismic setting. Sumatra is located on the boundary between the Eurasian and Australian Plates, which would be hazardous enough in itself, but the situation is made worse by fragmentation of the Eurasian Plate under Indonesia and Southeast Asia as it is caught between the Australian and Pacific Plates. Sumatra actually lies on the boundary between two such fragments; the Sunda Plate on the east side of the island and the Burma Plate on the West. The two plates are sliding past one-another, the Burma plate moving northward and the Sunda Plate to the south, forming a transform fault. To the west of the island lies the Sunda Megathrust, where the Australian plate is being forced under the Burma and Sunda Plates.
The Burma/Sunda transform margin and the Sunda Megathrust under Sumatra.
See also Mount Soputan, Sulawesi, erupts, Magnitude 6.8 Earthquake hits Bam Island, Papua New Guinea and Earthquakes on Sciency Thoughts YouTube.