AT LEAST two thirds of the world's megacities - cities with present-day populations of 1.5M or more - are built in zones of high seismic activity. And according to Cambridge University reader in earth sciences Professor James Jackson it is not just megacities which tend to be sited in danger areas. Since recorded history began, people worldwide have chosen to live close to and on top of fault lines.
When earthquakes strike repercussions can be catastrophic, sometimes claiming tens of thousands of lives. Yet there are more advantages in living in an earthquake zone than disadvantages, Jackson claimed, delivering the eighth Society of Earthquake & Civil Engineering Dynamics Mallet-Milne lecture at the ICE this week.
People are attracted to the fertile hillsides and valleys created by deformation of continental plates. They tap into freshwater springs which well up from the ground along fault lines and into rivers flowing from folded catchments. Minerals pushed up to the surface of the earth can be extracted. And features such as natural harbours can be exploited.
In short, said Jackson, people do not want to live with earthquakes but in many regions of the world they cannot live without them.
It was clear 20 years ago that there was a close correlation between an area's population and its seismicity. But advances in mapping and surveying technology in the last decade have made clear that seismic activity is often spread over far larger areas than previously thought - in some cases unrecognised fault lines have been discovered.
It is increasingly possible to translate new findings from studies in India, Iran, Turkey and Greece, where seismic activity is high, to older seismic landscapes such as Scotland and the Alps, to understand better how they were formed.
Most significantly, the detail of information now being gathered enables seismologists to estimate more accurately than ever before the depth and length of fault lines - and the potential effects of future quakes.
Jackson's research has been particularly focused on Iran, where the desert lends itself to relatively straightforward monitoring and where there is little vegetation to screen deformations from view. GPS is now well established as a means of measuring, to within millimetres, vertical, longitudinal and lateral movement in landmasses, said Jackson.
Since the early 1980s it has become possible using GPS to visualise the transformation and plot the course of a landmass.
Engineers are now aware of extrusions as land is squeezed 'like chocolate in a vice' between two larger masses, of rotations, folds, and fractures as never before.
But it is in the application of synthetic aperture radar interferometry that has delivered the most startling improvements in recent years. This imaging technique's great virtues are sensitivity and scope - areas viewed are typically 200km by 300km square. It is enabling seismologists to view hitherto invisible ridges and ripples in the landscape, to pinpoint faults and centres of activity with previously unimagined accuracy, and to locate new faults altogether.
Synthetic aperture radar interferometry involves the comparison of radar phase information gathered before and after a seismic event. Two incredibly detailed radar pictures are taken by a satellite orbiting above the landscape, pre- and post-earthquake. Contours and features in the landscape are represented by different radar phases, translated into a map of coloured pixels. If landscapes are unchanged radar phase information is identical. Ground displacement caused by an earthquake, though, will alter the phases reflected back from the earth's surface.
By transposing the second pixellated map on the first, differences in phase information can be compared. As early as 1993, it was found that earthquakes produce distinctive patterns or fringes - coloured, pixellated contours that are steepest around the fault. The fringes show all surface transformations arising from an event.
Such information is providing engineers with 'a far better idea of what to look for and to expect than we had 30 years ago', said Jackson. To some extent, by correlating similar topographic forms, it is possible to translate findings about the behaviour of earthquakes resulting from observation of rural Iran to Los Angeles or Tokyo, he added.
Geologists and seismologists are closer than ever to answering the 'difficult question of 'how did the landscape get like that?', ' said Jackson. The more progress they make, the more 'it will yield insights which have direct practical application in engineering', he added.
'We are by no means infallible, and it is almost certain that we will be surprised by occasional disasters. But such occurrences should become rarer.'