A DESTRUCTIVE earthquake is a full-scale laboratory from which keen observers may make significant discoveries, Professor Nick Ambraseys told a packed audience for his Rankine Lecture at Imperial College in London on 31 March.
'Be they seismologists, geologists, engineers, sociologists or economists, an earthquake site offers a unique opportunity to develop an understanding of the effects of the phenomenon, which cannot be gained simply from lectures, reading reports or by surfing the internet, ' he said.
Ambraseys, emeritus professor and senior research fellow at Imperial College, used the 44th annual Rankine Lecture to put the point that engineering seismology is the link between earth science and engineering.'It aims primarily at the solution of engineering problems, ' he said. 'It requires a thorough understanding of the fundamentals of both disciplines, and experience which must be acquired with time through the field study of earthquakes.'
Ambraseys drew heavily on his field experiences around the world. 'As our knowledge of the complexity of earthquakes has increased, we become more and more aware of the limitations which nature has imposed in our capacity to assess on purely theoretical bases, ' he said.
In a vote of thanks, Professor James Jackson described Ambraseys as a giant in his field and said in his 50-year career he had 'stuck his neck out' to the benefit not only of engineers but people all over the world who lived in earthquake zones.
The event was attended by many past Rankine lecturers including Ralph Peck from the US.
Ambraseys' key points
The seismicity of many regions over the last few decades is not representative of their long-term seismic hazard; short-term observations may lead to very conservative and expensive engineering solutions.
Extrapolating from instrumentally recorded earthquakes has considerable perils. 'How can one be assured that the short-term record is statistically representative, particularly when we know that its time distributions are clustered?'
In the epicentral region of an earthquake, fault displacements and ground deformations can be destructive, regardless of the level of ground acceleration for which engineering buildings have been designed.
Not all faults are visible on the ground.
In particular, blind faults that had not been recognised before an earthquake are a serious threat to buildings and lives. However, they can be identified from indications of long-term cumulative deformation which are always visible in the geomorphology.
Field observations confirm that yielding of the soil will prevent large bedrock motions from reaching the ground surface, at the expense of large differential settlements and ground spreading. In such cases damage will be post-seismic and due to excessive settlements and deformation of the ground caused by gravity, rather than by large earthquake forces.
Slope failures and ground settlements may occur during and after an earthquake.
Aftershocks can be as damaging as the main one, not because they are stronger, but because of the weakening of a structure and the pore pressure changes following the main shock.
Building codes and regulations - and the efficacy of their enforcement and implementation - can only be tested after an earthquake.
Many problems in engineering seismology can now be solved, at least in theory. However, theoretical and numerical solutions place the onus on the engineer to assess or assume the required design parameters, which are difficult enough to estimate even after an earthquake.
Time rarely allows for thorough feasibility studies and for the acquisition of adequate data; the engineer is on occasion forced to step across that hazy borderline of safety.
In assessing the seismic hazard, conservatism should be applied only once: that is, having picked an exceedingly unlikely earthquake, one should not apply unlikely ground motions or other parameters to accompany an already unlikely event.
Any number in assessing 'hazard' represents a judgement.
A primary conclusion is that the large degree of uncertainty must be kept in mind when deciding the level of conservatism to be applied.
For the engineer to accept what is an acceptable earthquake risk, a certain amount of informed judgement, detailed technical evaluation of the project and experience is needed, rather than results from a probabilistic treatment of short-term seismicity data.
Much statistical ingenuity has been spent devising techniques for tackling the assessment of hazard, but there are doubts about how useful and how well founded some of these statistical techniques really are, and the statistician here should take a background role.
A statistician can point to features in the hazard data that look anomalous because they depart from some standard model, but whether the anomalies are to be ascribed to peculiarities of the statistical model, or to peculiarities of the physical processes, is not a question the statistician should be asked to answer. If an important effect is really present, it should not take a statistician to bring it out.