Christchurch quake mapped from space

Written by elena on March 7, 2011

Christchurch quake mapped from spaceBy Jonathan AmosScience correspondent, BBC NewsThe coloured bands, or fringes, represent movement towards or away from the spacecraftContinue reading the main storyRelated StoriesNo bodies in NZ quake cathedralNZ quake: The epicentre townDepth and location key in NZ quakeThe upheaval wrought by the 22 February earthquake in Christchurch, NZ, is illustrated in new radar imagery.The Magnitude 6.3 tremor killed more than 160 people and shattered a city already reeling from a previous seismic event in September.Data from the Japanese Alos spacecraft has been used to map 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 displayed on this page is known as a synthetic aperture radar interferogram.Continue reading the main story“Start QuoteTo get an earthquake right under their city will have been a surprise to nearly every single person”John ElliottOxford UniversityIt 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."Where the rainbow fringes become most tightly spaced is where the fault break came closest to the surface, although the data indicates the fault is unlikely to have broken right through to the surface.Blind dangerLiquefaction is a phenomenon that afflicts loose sediments in an earthquake and is akin to a lateral landslide.It is a major issue for Christchurch because the city is built on an alluvial plain, and this type of ground will amplify any shaking during a tremor.BBC News reader Gillian Needham took this image of central Christchurch moments after the quake struck New Zealand's second city on 22 FebruaryScientists 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 fully 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."