March 7, 2011

Christchurch Earthquake Mapped From Space

The Christchurch, New Zealand earthquake that took place on February 22 has been illustrated in new radar imagery.

The magnitude 6.3 tremor killed over 160 people and shattered a city reeling from a previous seismic event in September.

Data downloaded from the Japanese Alos spacecraft mapped the way the ground deformed during the most recent quake.

It shows clearly that the focus of the tremor was right under the city's south-eastern suburbs.

The type of image is known as a synthetic aperture radar interferogram.

It is made by combining a sequence of radar images acquired by an orbiting satellite "before" and "after" a quake.

The technique allows a precise measurement to be made of any ground motion that takes place between the image acquisitions.

The colored bands represent movement towards or away from the spacecraft.

The peak ground motion is about 19-inches of motion towards the satellite.

"It's like a contour map but it's showing to the south-east of Christchurch that the ground motion is towards Alos. That's uplift," Dr John Elliott from the Centre for the Observation and Modelling of Earthquakes and Tectonics (Comet) at Oxford University, UK told BBC News.

"And then right under Christchurch, we see subsidence. That's partly due to liquefaction but it's mainly due to the way the Earth deforms when you snap it like an elastic band."

Liquefaction is a phenomenon that afflicts loose sediments in an earthquake and is akin to a lateral landslide.

The issue is big for Christchurch because the city is built on an alluvial plain, which is a type of ground that will amplify any shaking during a tremor.

Scientists are using the Alos information to better understand the future seismic hazards in this part of New Zealand.

It has become obvious from recent events that Christchurch sits close to "blind faulting," which is a type of faulting that is at risk of rupture.

"It means much more work needs to be done around Christchurch," Elliot told BBC News.

"People knew they could get earthquakes further into the mountains [in the west of South Island]; that's how they've been built in some ways, through earthquakes and all the faulting.

"But to get an earthquake right under their city will have been a surprise to nearly every single person."

The interferogram is noticeably incomplete and there are several areas where the fringing is missing.

To the east is ocean, and this technique does not work over water.

To the west, the issue is related to the satellite track and the fact that it views the Earth in strips. 

Elliot told BBC: "Here, the patches are the result of de-correlation between the acquisition images, where we just can't match them - they're too different.

"There are a few reasons for that. Usually it's the result of vegetation growth, but here it could be due to more extreme shaking or liquefaction."

Researchers are investigating the relationship between September's Magnitude 7.1 quake and February's 6.3 event.


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