Deadly New Zealand Earthquakes Weakened Earth's Crust
 |
 An aerial view of Christchurch, New Zealand, where a magnitude 6.3 earthquake struck Feb. 22. Credit: NASA Earth Observatory |
A series of deadly earthquakes that shook New Zealand in 2010 and 2011 may have weakened a portion of Earth's crust, researchers say.
New Zealand lies along the dangerous Ring of Fire — a narrow zone around the Pacific Ocean where about 90 percent of all the world's earthquakes, and 80 percent of the largest ones, strike.
A devastating magnitude- 6.3 quake struck New Zealand's South Island in 2011. Centered very close to Christchurch, the country's second-largest city, it killed 185 people and damaged or destroyed 100,000 buildings. The earthquake was the costliest disaster to ever strike New Zealand, consuming about one-sixth of the country's gross domestic product.
This lethal earthquake was the aftershock of a magnitude-7.1 temblor that struck 172 days earlier (in 2010) in the area, causing millions of dollars in damage to bridges and buildings, and seriously injuring two people. Although the 2010 temblor was stronger than its aftershock, it caused less damage because it occurred farther away from any city. The 2011 earthquake was, in turn, followed by a number of large aftershocks of its own.
Scientists found that most of the earthquakes that struck New Zealand during these two years released abnormally high levels of energy, consistent with those seen from ruptures of very strong faults in the Earth's crust. To learn more about this long series of energetic quakes, researchers analyzed the rocks beneath the area hit, known as the Canterbury Plains.
Widespread weakening
Approximately 6 miles (10 kilometers) below the Canterbury Plains lies a large, extremely strong block of volcanic rock called the Hikurangi Plateau, which was pulled underground about 100 million years ago, when the portion of the Earth's surface it rested on dove under the edge of the ancient supercontinent Gondwana. It remains attached to Earth's crust, welded to chunks of a dark, gray sandstone known as greywacke.
The scientists analyzed seismic waves detected before and after the quakes by GeoNet, a network of seismographs across New Zealand. Based on this data, including seismic waves from more than 11,500 aftershocks of the 2010 quake, they mapped the 3D structure of the rock under the Canterbury Plains, similar to the way ultrasound data can provide an image of a fetus in a womb.
A map of the earthquakes in the Canterbury earthquake sequence around Christchurch, New Zealand. Stars note the largest events, with intervening aftershocks in the same color (red is the Sep. 3, 2010, magnitude-7.1 earthquake; black is the Feb. 22, 2011, magnitude-6.3 quake; blue is the June 13, 2011, magnitude-6.0; green is the Dec. 23, 2011 magnitude 5.8 and 5.9). Yellow rectangles outline fault planes for these largest events inferred from geodetic data.Credit: Reyners, et al., Nature Geoscience
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.
Why there?
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.
Canterbury earthquakes highly unusual
CHARLES ANDERSON
| |
|
The Canterbury earthquakes were even more unusual than first thought and unlikely to occur anywhere else in the world, new research reveals.
The research, led by seismologist Martin Reyners of GNS Science, showed the unusual rock structure of the region meant the Canterbury earthquakes produced some of the strongest vertical ground accelerations ever seen in an earthquake.
The makeup of this unique dense and thick slab of rock could have implications for other regions around the lower South Island.
''There will be few other places in the world where a similar earthquake sequence might occur," Reyners said.
The research, published in Nature Geoscience showed that the strong quakes in Canterbury also could cause widespread cracking and weakening of the earth's crust - challenging the common assumption that the strength of the crust was constant.
Normally rocks become hot and ''plastic'' at depths of about 10km. However, the researchers found that strong, brittle rocks continued to a depth of about 30km under Canterbury.
''Strong rocks store and release strain differently to weak rocks," Reyners said.
This unusually thick and dense slab of rock helps to explain the long and energetic aftershock sequence in Canterbury, he said.
Seismic energy would have dissipated more quickly in softer rock.The researchers were now focussed on determining how widespread this strong rock unit is in the lower half of the South Island.
"This is important for defining the earthquake hazard for people living between mid-Canterbury and Southland," Reyners said.
The researchers had initially set out to determine the three-dimensional structure of the crust under Canterbury by using a technique called seismic tomography - similar to a medical CAT scan or ultrasound.
This helps to get more accurate aftershock locations and better define the many smaller faults that ruptured in the earthquakes.
Instead, researchers found that rock properties had changed significantly over a wide area around the Greendale Fault, which ruptured on 4 September 2010 producing a magnitude 7.1 quake.
"This finding was entirely unexpected, but it explains why the main shock released so much energy," Reyners said.
Most of the quakes in the two-year-long Canterbury sequence released abnormally high levels of energy - this was consistent with the ruptures occurring on very strong faults that store energy slowly and gradually and are hard to break.
The Canterbury quakes had their genesis 100 million years ago when very strong rocks became emplaced under Canterbury, he said.
The delay between the September 2010 and Feburary 2011 quakes also may have been caused by a ''strength recovery'' required for the crust following the cracking following the September quake, the research said.The research involved analysing the seismic waves produced by 11,500 aftershocks in Canterbury.
This enabled the team to build a 3D picture of rock structure to a depth of about 35km below the surface.
Reyners said post-quake analysis such as this research was important as it helps to understand how strain builds up in thecrust and how it is released during earthquakes.
"But to do that accurately, we need to understand the types of rocks that exist at depth.''
http://www.stuff.co.nz/science/9444492/Canterbury-earthquakes-highly-unusual--
__._,_.___
No comments:
Post a Comment