Mid-plate earthquake a product of tension between India, Australia

Back in April, we reported on a remarkable earthquake off the Indonesian island of Sumatra, near the location of the magnitude 9.1 quake that caused the devastatingly deadly tsunami in 2004. The more recent event caused little damage despite the fact that it was reported to be a magnitude 8.6 earthquake (with a magnitude 8.2 aftershock two hours later).

This was due to the nature of the fault. Rather than a thrust fault, where one block of rock slides up and over another (displacing water and creating a tsunami), this occurred on strike-slip faults, where one block simply slides laterally past another. No earthquake of this magnitude had ever been recorded on a strike-slip fault before. Additionally, the earthquake occurred not at the boundary where two plates meet (as the 2004 earthquake did), but within one of the plates. It was also the largest such “intraplate” quake ever recorded.

Several new papers in the journal Nature this week present detailed analyses of the event. For starters, one of the groups revises the magnitude of the earthquake, bumping it up 8.7—a not insignificant increase on a logarithmic scale, and one that would further secure records for quakes of this type.

Another finds that it triggered an unusual number of earthquakes around the globe. Earthquakes of this magnitude can affect faults that are near the slipping point as the seismic energy literally circles around the planet (several times, in fact). After the 2012 strike-slip earthquake, the number of quakes of magnitude 5.5 or greater quintupled for nearly a week. Even though we’ve seen several larger earthquakes recently—the 9.1 in Indonesia, the 2010 8.8 in Chile, the 2011 9.0 off Japan—none have them had an effect nearly this large.

Either the conditions were primed (many more faults ready to be triggered than usual), or something about this earthquake was different. The researchers suspect a combination of the two, but do a fair bit of head-scratching about exactly why the increase was so large.

One of the papers focuses on working out the details of precisely where the faults slipped during the earthquake. That’s “faults”—plural—as it appears that four separate segments were involved. About this configuration the researchers write, “Our results show that the great events of 11 April 2012 involve rupture of a very complex network of faults, for which we have no documented precedent in recorded seismic history.” Three faults were parallel but staggered, and the fourth was oriented 90 degrees from the rest.

The stretches where the faults moved ranged from roughly 30 to 100 miles in length. Over a little less than three minutes, the faults slipped in sequence, with blocks sliding by as much as 37 meters. The researchers also find (as others had previously) that the faults were exceptionally deep, extending into the upper mantle—again, very surprising and interesting behavior.

So why all the weirdness? And what’s such a large earthquake doing over 100 kilometers from a plate boundary? This comes down to the tortured nature of the tectonic plate once known as the IndoAustralian plate. It’s now known that this plate is actually composed of two or three distinct subplates that aren’t getting along well.

While the oceanic plate south of Indonesia is being pulled strongly northward into the Sunda Trench (a subduction zone where oceanic crust descends into the mantle), the part of the plate that includes India has hit a roadblock- the Eurasian continental plate. In order for both of those things to be occurring, the plate must be crinkling and cracking in between. Since, there’s no clean boundary there where one plate could slide merrily by, there are distributed zones of deformation that relieve the stress, typically where pre-existing weaknesses exist (such as old faults that formed for other reasons).

The April earthquake occurred in one of those zones. Perhaps the most interesting thing about the Indonesian earthquakes of 2004 and 2012 is the insight they provide into the long-term processes going on between the various sections of this massive tectonic plate. One group writes, “The mega-earthquakes of [2004 and 2005] acted as instantaneous boosters, suddenly illuminating where and how intraplate deformation was at work.” It’s a little like pressing the fast-forward button on the northward movement of the Australian section of the plate to see clearly where the deformation between there and the Indian section of the plate occurs.

Two of the papers believe the results of those “boosters” could tell us something about the future of these jostling subplates. In the words of one, “The long-term scenario is that a nascent plate tectonic boundary is forming: the Australian plate is becoming detached from the Indian plate.” It would take many millions of years for such a boundary to form, and it will take more information to determine that this is definitely the future of that distributed zone of deformation.

Regardless, while this was the first time that such large earthquakes were recorded in that area, it almost certainly won’t be the last. As the saying goes, time stops for no one. Neither does plate tectonics.

Nature, 2012. DOI: 10.1038/nature11520, 10.1038/nature11492, 10.1038/nature11504 (About DOIs).