The Sumatran earthquake didn’t cause a tsunami because …

A prison official examines damage to the prison wall that collapsed after this week’s earthquake in Banda Aceh, Indonesia. Hotli Simanjuntak/EPA

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.

Wednesday’s quake (yellow star) was not the first to occur near the interface between the Sunda and Indo-Australian plates (Click for larger view). US Geological Survey

But there are some key differences between the earthquake of 2004 and the one that happened earlier this week.

Quakes and shakes

The Boxing Day quake was a so-called “subduction thrust” event, which occurred where the Indo-Australian tectonic plate “subducts” (shoves itself) under the Sunda Plate.

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.

Prathap Ravisnankar/The Hindu

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.

Christchurch comparisons

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.

The 2004 Boxing Day tsunami killed more than 250,000 people. madaboutasia

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.

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