Indo-Australian Tectonic Plate Is Breaking Up
Notes/ Comments/ Questions
http://www.earth-issues.com/2012/10/01/earth-begins-to-break/
26 September 2012 by Colin Barras
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).
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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.
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