Yesterday’s earthquake off the Indonesian island of Sumatra was completely different to the earthquake in 2004 in the same region, which caused the Boxing Day tsunami.
The Sumatra region is the site of the meeting of two of the earth’s tectonic plates. One is the Indian plate, an oceanic plate, which is mostly sea. As it moves roughly north-eastwards, it collides with the second plate - the Sunda plate - which is what Sumatra is sitting on.
When the two plates collide, the oceanic plate is pushed down (because it is denser) underneath Sumatra. This means the plate boundary between these two is gently dipping at an angle of about 15 degrees across thousands of kilometres, giving the huge area of fault slip necessary to produce an earthquake more powerful than magnitude 9, such as the 2004 earthquake.
When the plates are locked, the underlying plate continues to move, dragging the overlying plate with it to the north-east – but it also drags it down.
When an earthquake happens, that connection between the two plates is released and the overlying plate snaps back into its proper position. That process results in the plate moving upwards and the sea bed is suddenly flipped up, displacing an enormous amount of water, which forms an enormous wave moving in multiple directions. That is exactly what happened in 2004.
Yesterday, instead of the earthquake occurring on the boundary between the Indian plate and the Sunda plate, it actually occurred within the Indian plate. And instead of moving down on a dipping fault, it occurred in a vertical fault that moved sideways – what is technically called a strike-slip fault.
This means that one part of the sea bed has gone left, the other has gone right, and neither has significantly gone up or down, which does not displace a significant amount of water. There might be a bit of motion, but it won’t produce a huge wave. I would speculate that tension has been building up in the Indian Plate ever since 2004, and yesterday the strain finally overcame the strength of the rocks.
It took around 30 minutes after the first earthquake yesterday for enough data to be collected from seismometers to establish the style of earthquake.
Tsunami warnings went out almost immediately after the first earthquake, and that was absolutely the right this to do. Detecting an earthquake that size, in an area known for producing tsunamis, means the warning must get out as quickly as possible. Just the magnitude alone of an earthquake such as that is potentially dangerous. When you get a situation like the one yesterday, you essentially you have a choice - heed a tsunami warning and waste a couple of hours if it’s a false alarm, or ignore it and possibly die. It’s not a huge disruption to leave your house and go to high ground until its all clear – it’s far better to be safe than sorry.