Saturday, November 10, 2012

Tectonic Breakup. Earthquakes. Will There Be A New Earth Geography?

Unusual Indian Ocean earthquakes hint at tectonic breakup

April 2012 quakes occurred away from plate edges, suggesting formation of a new boundary.

A pair of massive earthquakes that rocked the Indian Ocean on 11 April 2012 may signal the latest step in the formation of a new plate boundary within Earth’s surface.
Geological stresses rending the Indo-Australian plate apart are likely to have caused the magnitude-8.6 and magnitude-8.2 quakes, which broke along numerous faults and unleashed aftershocks for 6 days afterwards, according to three papers published online today in Nature1–3.
Seismologists have suspected since the 1980s4that the Indo-Australian plate may be breaking up. But the 11 April earthquakes represent “the most spectacular example” of that process in action, says Matthias Delescluse, a geophysicist at the Ecole Normale Supérieure in Paris and lead author of the first paper1. Worldwide, “it’s the clearest example of newly formed plate boundaries,” he says.
According to prevailing theories of plate tectonics, the Indo-Australian plate began to deform internally about 10 million years ago. As the plate moved northwards, the region near India crunched against the Eurasian plate, thrusting the Himalayas up and slowing India down. Most scientists think that the Australian portion forged ahead, creating twisting tensions that are splitting the plate apart in the Indian Ocean.
Delescluse and his team inferred the presence of these seismic stresses by modelling stress changes from shortly before the 2012 earthquakes. They found that two earlier earthquakes along the eastern plate boundary — the magnitude-9.1 tremor in 2004 that unleashed a massive tsunami across the Indian Ocean, and another quake in 2005 — probably triggered the 2012 event by adding to pent-up stresses in the plate’s middle region.
Gregory Beroza, a seismologist at Stanford University in Palo Alto, California, says that the model is a likely explanation. “The 2004 and 2005 earthquakes by themselves would not have caused this other earthquake. There had to be other stresses,” he says.

At least four faults within the Indo-Australian plate ruptured simultaneously in April 2012, resulting in two magnitude-8 earthquakes within two hours. (Red stars indicate the epicentres.)

Earth begins to break

Indo-Australian Tectonic Plate Is Breaking Up
You may not have felt it, but the whole world shuddered on 11 April, as Earth’s crust began the difficult process of breaking a tectonic plate. When two huge earthquakes ripped through the floor of the Indian Ocean, they triggered large aftershocks on faults the world over, and provided the best evidence yet that the vast Indo-Australian plate is being torn in two.
Geologists have spent five months puzzling over the twin quakes - of magnitude 8.6 and 8.2 – which took place off the coast of North Sumatra. Events that large normally occur at the boundary between tectonic plates, where one chunk of Earth’s crust slides beneath another, but these were more than 100 kilometres from such a subduction zone. What’s more, both involved rocks grinding past each other sideways with very little vertical movement – what geologists call strike-slip earthquakes. Yet strike-slip quakes this large had never been reported before.
Matthias Delescluse at the École Normale Supérieure in Paris, France, and his colleagues have an explanation. They analysed quakes in the area since December 2004, when a magnitude-9.1 quake in a subduction zone near Sumatra triggered a devastating tsunami. They found earthquakes during this period were nearly 10 times more frequent compared with the previous eight years. What’s more, 26 of the quakes that happened between December 2004 and April 2011 were similar to the 11 April quakes in that they involved rocks being pushed and pulled in the same directions.
Taken together, the events suggest that the Indo-Australian plate is breaking up along a new plate boundary, say the researchers, and that may account for both the location and the size of April’s quakes . Although both are currently on the same plate, Australia is moving faster than India. This is causing a broad area in the centre of the Indo-Australian plate to buckle. As a result, the plate may be splitting (see map).
John McCloskey at the University of Ulster in Coleraine, UK, is not yet convinced, saying the evidence from the April events is still too weak to support such a bold claim. But Lingsen Meng at the University of California, Berkeley, who studied the rupture pattern of the larger 11 April quake, is more confident. “I think it’s a fair argument that the 11 April earthquakes may mark the birth of a plate boundary,” he says. Things should become clearer as more earthquakes shake the region.
If they are anything like the 11 April events, the rest of the world may shake too. In another new study, Fred Pollitz at the US Geological Survey in Menlo Park, California, and his colleagues found that the global rate of quakes with a magnitude of 5.5 or greater increased almost fivefold in the six days after 11 April – something that has never been seen before, even after very large earthquakes .
“This was the most powerful event (ever recorded) in terms of putting stress on other fault zones around the world,” Pollitz says.

Mid-Ocean Ridge Earthquakes and Complex Plate Tectonics in a Quiet Week


Earthquakes of at least M5.0 for the week ending 9 October 2012 – Image courtesy of USGS
The 240 recorded earthquakes of at least magnitude 2.5 shown on the USGS real time earthquake map for the week 3-9 October showed the expected pattern with the majority of tremors (including all greater than or equal to M5.0) occurring along the boundaries of the earth’s tectonic plates, and a scattering of smaller events occurring within plates in more stable areas (including tremors of M3.0 in Oklahoma and M2.5 in eastern Canada).

Mid Ocean Ridge Earthquakes

The map shows a trio of tremors (two of M5.5 and one M5.7) occurring in the central Atlantic Ocean, along the ridge which forms the axis of the ocean where the African and American plates are moving apart and new ocean crust is being created. Such events, though by no means uncommon, are fewer in number and smaller in magnitude than those at transform or convergent plate boundaries.
At ocean ridges, plate movement is largely extensional, although “all types of faulting styles are observed,” according to Bergman and Solomon’s 1984 article,Source mechanisms of earthquakes near mid-ocean ridges from body waveform inversion: Implications for the early evolution of oceanic lithosphere.

Transform faults offset mid ocean ridges -Image courtesy of Pimvantend-
Because the structure of ocean ridges includes fault segmentation, with each length of the ridge being offset by lateral (transform) faults, earthquakes are an expected feature of the process. Without detailed knowledge of the local tectonics of the three earthquakes it’s impossible to say exactly which type of faulting was responsible, although their location (a few kilometres from the ridge itself) suggests that they may have been generated by movement along these transform faults.

No absolutes: How shifting plates completely remake the Earth

Tracking geologic hotspots shows a wobbly Earth in constant flux.

Plate tectonics is one of the most successful theories in the history of science. Beyond its scientific successes, it's widely accepted by the public, since it explains a lot about the world that we see around us.
But like other successful theories, it has its share of awkward inconsistencies. A recent paper in theJournal of Geophysical Research attempted to tackle one of these inconsistencies—finding an absolute reference frame for the movement of the plates—but failed so badly that its authors advise other scientists not to even bother trying. But as part of their failure, they came up with a new measure of one of the more unexpected consequences of plate tectonics.
All of plate tectonics is driven by density differences in the material beneath our planet's solid surface. These drive the shifting plates, power hot-spot volcanoes, and recycle material to the planet's surface. They also make sure that the mass of the Earth is never evenly distributed. As that mass shifts internally, it actually causes the Earth's spin to wobble around a bit. As a result, even the Earth's axis of rotation doesn't provide an absolute reference frame. In the process of failing to find an absolute reference frame, though, the authors have provided a detailed map of how the Earth's true pole has wandered over millions of years.

Making the numbers add up

One of the larger successes of plate tectonics is that it offers an explanation for island chains. Groups of volcanic islands, like the Hawaiian islands, can trace straight lines for thousands of miles if one considers the largely submerged remains of former islands. Plate tectonics offers an explanation: there are stationary hot spots in the mantle that drive volcanic eruptions. The plates simply slide over a hot spot, which builds volcanoes that later become inactive and erode as they slide past.
This process is so regular that it's one of the ways that scientists have tracked past plate motion. For the Hawaiian chain, it's even possible to see a sudden left turn, as the Pacific plate changed its direction of motion. These measurements tend to line up well with others based on measurements made at the boundary of plates.
So, stationary hot spots, moving plates. That would make hot spots a great reference frame for plate movement. Just pick absolute locations for the hot spots, then you could track the entire planet's plates as they slid across them. Just one small problem: it doesn't work. "It was soon realized," the authors write, "that a reference frame defined by fixed hot spots from the Pacific Ocean could not adequately reproduce hot spot tracks in the Indian and Atlantic Ocean."
In other words, although a hot spot appears to be a fixed reference frame for a given plate and its neighbors, our best data indicates that different hot spots appear to be moving relative to each other.
But our best data is an ever-changing thing, and the authors decided it was time for another try. They went through the literature and pulled out any information they could find about rates and directions of plate motion, and integrated it all into a single model. Despite several iterations that made for a progressively better fit to the data from individual hotspots, there was no way to get things to work out globally. "Our attempts to define a global fixed hot spot reference frame have failed to produce acceptable fits to the segments of hot spot tracks formed from Late Cretaceous to Paleogene time (80–50 Ma [million years])," the authors concede.
Their conclusion? It's time to give up on the idea of hot spots being fixed. If we're ever going to have an absolute reference frame, it's not going to come from hot spots.

Shifting references on an unstable Earth

Based on the paper, though, it's hard to tell what else might provide an absolute reference. The authors' work provides further evidence that the entire surface of the Earth (the lithosphere) is moving relative to its interior, the mantle. In other words, the surface of the Earth is not completely coupled to the core, something that had previously been suggested to be the case for Saturn's moon Titan. In the case of Earth, the so-called "lithosphere rotation" involves a slow drift westward, shifting about a tenth of a degree every million years on average. However, the rate isn't even, and has been nearly three times that at some point in the past, apparently at the time when the Indian plate was accelerating toward Asia.
It isn't just that we lack a fixed reference frame to track the plates. It's that plate tectonics itself shifts such enormous masses around that these skew the reference frame.
As it turns out, this also wipes out another potential reference frame, the axis of the Earth's rotation. If the Earth were a uniform, solid sphere, its axis of rotation would remain stable. But the whole idea behind plate tectonics is that the mass isn't distributed evenly. The hot spots at issue here push to the surface of the crust precisely because they're hotter and thus less dense than the surrounding material. On larger scales, it's this density-driven convection that powers the shifting of the continents themselves. As crust is driven into the Earth's interior at subduction zones, it places sheets of solid material deep under the crust that take millions of years to come to equilibrium with their new surroundings.
So, not only is the Earth not uniform, but its internal differences are constantly shifting around. All of which feeds back into the dynamics of its rotation, leading to a phenomenon called "true polar wander"—its axis of rotation hasn't always run through the sites of the current North and South Poles. In fact, during the period from 90 million to 40 million years ago, the poles drifted nearly 10 degrees and then snapped back.

Technicalities and the big picture

The paper itself is long, dry, and very technical; it's not the sort of thing that I'd recommend anyone read unless they're actively working in this field. But the ideas within it are important and compelling.
One important idea is that even our most successful scientific theories are filled with enough discrepancies and inconsistencies to keep scientists gainfully employed for generations. But it's important to keep these in perspective. Not knowing something, or even getting it wrong, doesn't mean that we don't know anything, or that every little inconsistency means we should throw the entire structure out.
The story of plate tectonics itself highlights how oddities on their own aren't enough to overthrow a dominant idea. Instead, you have to come up with something that explains not only the discrepancies, but everything else that the dominant idea gets right. Even then, it's not easy, something that is very clear from the reception that plate tectonics got when it was first suggested almost precisely 100 years ago.
One of the reasons that many people have a hard time accepting some aspects of science is that these ideas make them feel uncomfortable. Plate tectonics doesn't have the same emotional impact as the Copernican revolution, which told us that our place in the Universe wasn't special. But it does tell us that our place wasn't even a place for most of its history: continents shift, islands grow and vanish, and there are apparently no fixed frames of reference.
Without the energy brought to the Earth's surface by plate tectonics, however, it's not even clear that life itself would have been able to flourish, and it certainly wouldn't have evolved the way it has without the changing climates and landscapes.
Giving up a fixed frame of reference to have all that seems like a worthwhile tradeoff.
Intraplate quakes signal tectonic breakup
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Two April earthquakes (red stars) off the coast of Sumatra are indicators of the ongoing breakup of the Indo-Australian tectonic plate. Over millions of years, the split will put India and Australia on different courses.
Keith Koper
Two giant earthquakes in the eastern Indian Ocean have shown geologists that breaking up is easy to do — for tectonic plates, that is.
The pair of quakes hit on April 11, startling seismologists with their size (magnitudes 8.6 and 8.2) and location (hundreds of kilometers from the active zone that spawned the deadly 2004 magnitude 9.1 earthquake and tsunami). Now, three studies reveal that the April quakes were an indication that one great slab of Earth’s crust is slowly fracturing into two.
The work, reported online September 26 inNature, confirms that seismic risk remains high in the area.
“You’d be nuts to think it was all over in offshore Sumatra,” says Kerry Sieh, a seismologist at the Earth Observatory of Singapore who was not involved in the new research.
The bigger April quake leapt straight into the record books. It was the largest earthquake ever recorded in the middle of a tectonic plate, rather than at a plate’s edges where most quakes happen. It was also the largest earthquake recorded along a strike-slip fault, in which two chunks of Earth’s crust slide past each other horizontally, like along California’s San Andreas. And it was the most complex strike-slip rupture ever seen, breaking along at least four separate faults interlaced like a geological lattice. 
Add together the 2004 killer Sumatra quake, two nearby great quakes in 2005 and 2007, and these April Indian Ocean quakes, says Sieh, and “you get the greatest release of seismic energy anywhere on Earth in the past half-century.”
Blame it on the massive Indo-Australian crustal plate, which stretches from the Himalayas in the north to well below Australia in the south. You can think of the plate like a motorcycle with a sidecar, says Matthias Delescluse, a marine geophysicist at the École Normale Supérieure in Paris. The motorcycle — the part of the plate carrying Australia — is driving quickly northeast beneath Indonesia. But the sidecar — the part carrying India — is slamming into a geological wall of the Himalayas. The motorbike and sidecar are thus shearing apart. Millions of years from now, the Indo-Australian plate will split into an Indian and an Australian plate.
April’s quakes reminded scientists that this is happening, maybe even faster than once thought. The 2004 quake, to the east, sped up the rate of earthquakes across the region and probably hastened the April quakes, Delescluse and his colleagues report in Nature. They calculated how the monster 2004 and 2005 quakes changed stress patterns in the Earth’s crust, and found that releasing stress on the faults diving under Sumatra to the northeast actually raised stress in the strike-slip faults to the southwest, in the Indian Ocean.
In a second Nature paper, seismic records illuminate the complex way the seafloor ruptured in April.
The first April 11 quake unzipped four perpendicular faults one after another in less than two minutes, the scientists found. Each fault ruptured with the equivalent energy of at least a magnitude 8.0 quake in that event. Two hours later, the magnitude 8.2 aftershock struck just south of the main rupture.  “This was a gee-whiz event for us,” says team member Thorne Lay, a seismologist at the University of California, Santa Cruz.
But the story wasn’t over once the two quakes were done. They continued to resonate around the globe, triggering big aftershocks as far away as Mexico, a third study finds.
Fred Pollitz, a seismologist at the U.S. Geological Survey in Menlo Park, Calif., became interested in the quakes when his colleagues’ pagers kept going off for days afterward with alerts of other big quakes. “That struck me as rather suspicious,” Pollitz says. So he and his colleagues went through catalogs of global earthquakes, looking for changes in patterns of seismicity.
They found that the number of quakes of magnitude 5.5 or greater, located more than 1,500 kilometers from the April 11 quakes, went up nearly fivefold for six days afterward. The biggest such quake was a magnitude 7 in Baja California, about 22 hours afterward.
Most giant quakes don’t trigger temblors so far away — or if they do, the triggered quakes are well below magnitude 5. The difference, Pollitz says, lay in the strike-slip nature of the April 11 quakes. This type of fault geometry allows the stress of a crustal movement to propagate much farther across the planet’s surface, compared with deep-diving plates that transmit their energy into the bowels of the Earth. The quick rupture also allowed seismic wave energy to travel out in pulses that “we believe shook up the faults more efficiently,” Pollitz says.
Though the strike-slip geometry may have triggered other quakes, it also meant that only a few people died in the April 11 events. Horizontal ground movements don’t push the ocean water around in ways that generate a deadly tsunami, like the one that killed a quarter of a million people in 2004.
Still, the seismic risk around Sumatra remains high, because other parts of the plate diving beneath Indonesia have not broken for some time. And the fact that big earthquakes can pop off where they’re not expected, like along strike-slip faults in the east Indian Ocean, suggests to researchers that other surprises lie in store.

Back Story | How the plates break up
Geoatlas/Graphi-Ogre, adapted by E. Feliciano
As geologists’ defining theory of the processes that shape the face of the planet, plate tectonics is rock solid. Yet there’s no clear definition on exactly what constitutes a tectonic plate or how many plates there are. At its most basic, a tectonic plate is a chunk of Earth’s outer surface (its crust and a portion of the upper mantle) that moves as a single entity. Where plates meet, they grind against or dive beneath one another.
Seven huge plates dominate the plate tectonic scene. At least eight smaller plates show up on most maps, mostly on the margins of oceans where Earth’s crust is thinner and more prone to fracture. But some of these plates, big and small, are in the process of breaking up (see main story), and in a few cases may have already done so. Several scientists argue that the Indo-Australian plate, which is the third largest plate and is in the midst of a drawn-out breakup, has already shed one fragment, a smaller plate dubbed Capricorn, on its southwest edge. Global-positioning satellite data show that the southwest part of the larger plate is not moving in exactly the same direction as the northwest and east. That, some geologists argue, suggests that Capricorn has already broken away.

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