Information about earthquakes yielded by satellite imaging techniques is forcing engineers to rethink where they build, discovers Andrew Mylius.
An enormous earthquake, measuring Richter 8 or more, has been expected in Japan for three decades now.
Japan was left reeling by the 1995 Kobe quake, which measured 7.2 - 5,000 people died and 350,000 were made homeless.
Nobody can stop the next giant earthquake, which is likely to fall on the Tokai district west of Tokyo, happening. But there are hopes that close monitoring of movements in the earth's crust may offer some forewarning, enabling residents of the densely built Tokai district to prepare for the approaching calamity. The area has been staked out with a grid of GPS beacons which will help reveal any lateral movement.
Combined with minute measurement of vertical movements, achieved using millimetre accurate advanced satellite mapping techniques, Tokai is probably the most closely monitored patch of earth anywhere in the world.
Satellite mapping techniques are still far from mainstream, but are exciting increasing interest. Two very different approaches are being taken: One involves capturing relatively low resolution photographic images which, when earthquakes strike, can reveal areas of damage. This is hugely useful to engineers working in the disaster relief and mitigation fields. Information gathered by 'spy' satellites is being used to identify collapsed buildings, blocked traffic arteries and collapsed bridges, for example, in areas where the chaos following large earthquakes makes it difficult to survey for damage from the ground. The images can play a crucial role in speeding aid to where it is most needed.
While these pictures will show up major alterations to the landscape they are unable to reveal subtler changes, however. For the kind of pinpoint accuracy needed to spot non-destructive movement of the earth's crust, an altogether different form of satellite imaging is used.
InSAR - a user-friendly acronym for interferometric synthetic aperture radar - involves beaming C-band radiation at the earth from an orbiting satellite. The radiation travels at a frequency of 6.3GHz, with a wavelength of 56mm, and is bounced back by solid surfaces and received by the satellite in much the same way as conventional radar used by ships or aircraft. The satellite's orbit takes it over the same territory once every 35 days, enabling a series of pictures to be taken from an identical viewpoint.
Movement in the earth's surface is detected by comparing the phase of reflected radiation waves. Reflections from a solid, unmoving surface will be in phase each time the satellite passes overhead. But any vertical ground motion will throw the waves out of phase, creating interference, explains says Ren Capes, applications development manager at satellite mapping specialist NPA. By measuring how far out of phase waves are it is possible to judge vertical shifts in the landscape to accuracies of up to 0.1mm.
InSAR mapping has made it possible to identify ground movements that were previously invisible, says Cambridge University professor James Jackson. While main faults are fairly easy to identify, in countries like Iran, Turkey and Greece, InSAR has revealed minor faults fanning out from their ends and unknown secondary faults some distance from known ruptures. It has also made clear that extensional faults, where one tectonic plate slides down the sloping side of another, grow progressively longer over time.
'You wouldn't want to build a liquid natural gas tank on one of those if it's going to move 20-30cm, ' Jackson notes.
Geologists and engineers have been able to observe the infinitessimally slow process by which plateaus are raised and mountains formed. As land masses rise rivers are forced to cut through them, forming deep gorges.
Engineers have in the past siezed on these natural bottlenecks to build dams without stopping to ask what geological processes are at work. 'In the 1960s and 70s people were building things in places you wouldn't with modern understandings, ' he says, ominously.
'Engineers haven't kept up with this kind of advance. They seem to think if you can't see fault it's not there, ' Jackson adds.
'But earthquakes can distort the surface without breaking it.
They can break pipelines or foundations. And earthquakes have effects way beyond the immediate area. If a fault goes 10km down it will disturb ground for 10km around.'
As well as helping to identify new faults and expose how known faults are developing, radar interferograms yield information about what kinds of earthquake have occurred, says Imperial College reader in earthquake engineering, professor Nicholas Ambraseys. To this end, six months ago the Natural Environment Research Council launched a five year project, the Centre for Observation and Modelling of Earthquakes and Tectonics (COMET for short), to monitor movement of the earth's surface. InSAR images can tell about the nature of a fault - rupture type, depth, geometry and strength, says COMET project manager Barry Parsons. It can also detect long-term motion of fault systems in between earthquakes. One of COMET's aims, Parsons says, is to see how strain is relaxed by faulting.
If you can gauge how much strain is locked up in a fault you are some way to being able determine the nature, and maybe even the approximate timing of earthquakes, Parsons suggests. Ambraseys is unwilling to venture into the chancy game of predicting earthquakes, but InSAR is enabling engineers to assess hazard and risk, he says: 'You can say with quite a lot of certainty the consequences of a quake will be here and here, and plan your lifelines accordingly.'
In densely developed Japan engineers are using InSAR to watch and learn about the fault structures beneath their feet.
They may not be able to predict when the big quake will strike, but they can prepare.
For a detailed explanation of how radar interferometry is being used, see James Jackson's 'Living with earthquakes - know your faults', Journal of Earthquake Engineering, Vol 5, 2001, Special Issue 1, at www. worldscinet. com COMET: http: //comet. nerc.ac. uk InSAR: www. npagroup. com