Beneath the surface broken by the quakes, the researchers identified a broad region that appeared to be dramatically weaker after the quakes. This suggests there was widespread cracking of greywacke 3 miles (5 km) around the fault. In contrast, earthquakes of similar magnitude in the crust elsewhere typically only "produce zones of cracked rock around the fault which are a few hundred meters wide," said study lead author Martin Reyners, a seismologist at research institute GNS Science in Lower Hutt, New Zealand.
Until now, scientists had assumed that the strength of Earth's crust remains constant during aftershocks. But these new findings, detailed online Nov. 24 in the journal Nature Geoscience, suggest energetic quakes can lead to widespread weakening of the crust.
"Such widespread weakening is not common, and has not been reported previously," Reyners told LiveScience's OurAmazingPlanet.
To explain why weakening was seen in that particular region and not elsewhere after strong quakes, Reyners noted the increasing pressure and temperature seen with increasing depth in the crust that usually means that at depths of more than about 6.8 miles (10.9 km), rocks are no longer brittle. As a result, the rocks often flow, not crack, when force is applied to them.
"This is known as the brittle-plastic transition," Reyners said.
However, "because of the very strong rock unit underlying Canterbury, the brittle-plastic transition is very deep - it lies at about 35 kilometers [22 miles] depth," Reyners said. As such, widespread cracking and weakening of the rock occurred.
The researchers will now focus on figuring out how widespread this strong block of rock is at shallow depths throughout the eastern portion of the South Island of New Zealand. "This is important for defining the seismic hazard for communities in this region," Reyners said.
Original article at LiveScience's OurAmazingPlanet.
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