September 29, 2012 - You may not have felt it, but the whole world shuddered on 11 April, as Earthâ€™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 â€“ which took place off the coast of North Sumatra. Events that large normally occur at the boundary between tectonic plates, where one chunk of Earthâ€™s crust slides beneath another, but these were more than 100 kilometres from such a subduction zone. Whatâ€™s more, both involved rocks grinding past each other sideways with very little vertical movement â€“ what geologists call strike-slip earthquakes. Yet strike-slip quakes this large had never been reported before.
Matthias Delescluse at the Ă‰cole Normale SupĂ©rieure 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. Whatâ€™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 Aprilâ€™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 above).
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. â€śI think itâ€™s a fair argument that the 11 April earthquakes may mark the birth of a plate boundary,â€ť 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 â€“ something that has never been seen before, even after very large earthquakes .
â€śThis was the most powerful event (ever recorded) in terms of putting stress on other fault zones around the world,â€ť Pollitz says.