The earthquake that struck roughly 400km off the coast of Sumatra on Wednesday reminded us all that our planet hasn’t gone to sleep. Big earthquakes can happen at any time and often have severe consequences.
The magnitude 8.6 quake occurred roughly 300km from the location of the magnitude 9.1 earthquake that led to the devastating 2004 Boxing Day tsunami. Indeed, this latest earthquake should be considered a late aftershock of that 2004 event.
(Wednesday’s quake also generated its own magnitude 8.2 aftershock – a huge quake in its own right.)
No-one will have forgotten the devastating consequences of the 2004 quake: more than 230,000 people killed in 14 countries following a series of devastating tsunamis that inundated coastal communities with waves as high as 30 metres.
Indonesia suffered the brunt of the destruction, followed by Sri Lanka, India and Thailand.
But there are some key differences between the earthquake of 2004 and the one that happened earlier this week.
Quakes and shakes
The upward motion of the top plate lifted the water column above it, setting off a huge tsunami.
In contrast, Wednesday’s earthquake occurred further away from land (see map above) and was a so-called “outer rise” event. It started inside the Indo-Australian plate and was caused by the forces generated as the plate bent prior to subduction.
It was a “strike-slip earthquake”, which results in the ground moving with a more side-to-side motion, rather than an up-and-down motion.
The side-to-side motion means there isn’t much up-and-down movement of the sea floor, so the water isn’t disturbed much. As a result, large tsunamis usually don’t form from slip-strike earthquakes.
Aftershocks from Wednesday’s earthquake should be smaller than the magnitude 8.6 mainshock. So even if they are thrust faults, they are unlikely to cause as big a tsunami as the 2004 event.
Unlikely, as I say, but not impossible – very occasionally, aftershocks can be bigger than the mainshock, in which case the name is changed and the first event is redesignated as a “foreshock” of the bigger one.
It’s worth mentioning that there are some similarities (but also many differences) between the Sumatran earthquake observed this week and the Christchurch earthquakes that started with a magnitude 7.2 earthquake in September 2010.
Last year’s devastating magnitude 6.3 February 22 Christchurch earthquake was a late aftershock of that initial quake.
But because it was very shallow and centred right under a major city (and had some unusual propagation effects), it caused much more damage than usual for an earthquake of that size. Indeed the second Christchurch earthquake caused much more damage than the earlier, larger quake.
In contrast, Wednesday’s Sumatran aftershock was far away from land and although the shaking was widespread (it was felt as far away as India), it was slower and less damaging than the Christchurch earthquake and claimed significantly fewer lives than the Boxing Day event.
Wednesday’s quake wasn’t the first we’ve seen in the Sumatra region, nor will it be the last. Our planet is very much alive and, if we hadn’t already, it’s time we all took notice.