Radar imagery shows how ground deformed during quake

6:22 am on 8 March 2011

The upheaval wrought by the Christchurch earthquake is illustrated in new radar imagery.

Data from the Japanese Alos spacecraft has been used to map the way the ground deformed during the 6.3-magnitude quake on 22 February, the BBC reports.

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

The type of image displayed on this page is known as a synthetic aperture radar inteferogram. It is made by combining a sequence of radar images acquired by an orbiting satellite "before" and "after" a quake.

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

The coloured bands, or fringes, represent movement towards or away from the spacecraft.

In this interferogram, the peak ground motion is almost 50cm 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," explained Dr John Elliott from the Centre for the Observation and Modelling of Earthquakes and Tectonics (Comet) at Oxford University, UK.

"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."

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

It has become obvious from recent events that Christchurch sits close to "blind faulting" - faulting that is at risk of rupture, but which betrays little evidence of its existence at the surface, meaning the potential danger it poses is not properly recognised.

"It means much more work needs to be done around Christchurch," said Dr Elliot.

"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."