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M 6.5 earthquake strikes southwestern coast of Sumatra
On 2 June 2016, a magnitude-6.5 earthquake struck the southwestern coast of Sumatra, which lies within the Mentawai seismic gap where a megathrust earthquake (M 8.5 – M 9.0) is expected to occur in the next couple of decades.
According to Professor Kerry Sieh, Director of the Earth Observatory of Singapore (EOS), “The M 6.5 earthquake occurred on the edge of a large, long dormant patch of the Sunda megathrust that EOS researchers have shown is likely to produce a M 9.0 earthquake in the next couple of decades. Although it is unusual, earthquakes like this one sometimes end up being foreshocks to larger earthquakes.”
Unlike megathrust events (earthquakes that take place on the boundary of two tectonic plates), the recent Sumatra quake had ruptured within the subducting plate (the plate that is sliding underneath the other towards the mantle). Its location, which is situated at the lower edge of where the expected giant earthquake may occur (see figure 1), indicates that stress perturbations were produced by the M 6.5 quake within the megathrust zone. These stress perturbations could play an important role in triggering megathrust earthquakes in the future.
UPDATE, 3 June 2016:
In an interview with The Straits Times on why tremors from the Sumatra quake can be felt in Singapore, Assistant Professor Wei Shengji explained that it was an intra-plate quake, which occurs in the interior of a tectonic plate, and which produces stronger high-frequency shaking.