How much will

it shake where

you live?

In Mexico City in 1985, more than 8000 people died in an earthquake of magnitude 8.1, which was centred more than 400 km away.  In the same earthquake, the damage in Acapulco, 100 km nearer the epicentre, was relatively minor.  This earthquake taught us an important lesson which has been repeated many times since:  the composition of the ground directly beneath a building can be just as important in determining the amount of shaking the building will will experience as the distance from the earthquake.

The question most frequently asked by earthquake engineers is:  "You can tell me something about where earthquakes will occur, and how big they will be, but how much will the ground move under this building?"  We are now beginning to answer that question, but it takes a detailed knowledge of how seismic waves are generated by earthquakes, and how they travel through the earth.  And it takes a computer analysis to put it all together to make predictions for a particular location.

Damage in an earthquake depends on three factors:  how large the ground motion is (the amplitude), how long it lasts (duration), and how rapid (frequency).  The larger the amplitude, the greater the force on the building.  The longer the duration, the more likely the building is to be damaged because continuous flexing may cause columns to crack and then fail.  The frequency determines what size structures are likely to be affected.  Tall buildings tend to be damaged by slow vibrations whereas short buildings are vulnerable to rapid motion.  the buildings damaged in Mexico City in 1985 were mostly between 10 and 14 stories tall.

The main reason for the large amount of damage in Mexico City was the fact that the amplitude of the seismic waves was magnified by the ground conditions.  The city is founded on a dry lake bed of soft sediments.  This caused the seismic waves to slow down as they travelled, and as they did so they built up in amplitude.  Not only that, but the particular size and shape of the lake bed meant that the waves were amplified at a frequency that matched some of the buildings.  So it was like pushing a child on a swing:  if you push at exactly the right frequency the amplitude becomes large.  This had disastrous effects for these buildings.

The geological structure near the surface can have a significant effect on the level of shaking during an earthquake (figure 1).  Thick dry sediments can amplify the shaking.  Shaking can be amplified in steep hilly areas because the waves are focussed by the shape of the land surface.  The study of these effects is known as microzoning.

There have been some clear examples in New Aealand.  In the 1942 Masterton earthquake, damage in Wellington was much more severe in locations on soft sediments than in the hill suburbs.  But being able to predict the level of shaking in any particular place is not as simple as that.  Current research involves measuring ground motions on a variey of ground conditions, identifying the types of waves that are amplified, developing new computer techniques to predict ground motions and testing their results against actual recordings, and comparing ground motions recorded in deep boreholes with those from the surface to understand how seismic waves are amplified as they reach the surface.

The Wellington Regional Council commissioned a study by the institute of Geological and Nuclear Sciences and Victoria University to produce a set of maps of ground-shaking hazard in selected parts of the Wellington region (figure 2).  The set shows which areas can expect the greatest shaking in an earthquake.

If sandy or silty soil is loosely packed and sturated with water, it can behave like a liquid when it is shaken stronglyduring an earthquake.  it loses its strength, so that cars and even buildings can sink into the ground.  the soil changes from solid to liquid abruptly, as one eye-witness observed:  "... the ground boiled, fissures opened, and the earth shook with violence ..."  The most recent occurrence of liquefaction  in New Zealand was during the Edgecumbe earthquake in the Bay of PLenty in 1987.  In Wellington, low-lying areas near the harbour, particularly the reclaimed land, are most at risk.

FUTURE  DIRECTIONS

Our understanding of the factors affecting ground motion is growing rapidly.  Geographic Information Systems (GIS) are being used to piece together a vast amount of information, and to combine geological information, topography, recorded ground motions and other data on to maps of land use.

The Wellington regional Council has an all-hazards map which uses this technology to combine the hazards due to ground shaking, liquefaction potential, fault movement, tsunami, and landslip potential.  The map allows city planners and the Wellington Earthquake Lifelines group to identify critical areas that are particularly vulnerable to earthquakes and to establish the importance of lifeline facilities in those areas.

It may also be possible to use this information about areas subject to strong ground motion, at the time of an earthquake.  When an earthquake strikes, the seismic waves travel about 35 km in 5 seconds, whereas radio waves are almost instantaneous.  It may be possible to develop rapid warning systems so that automated systems shut down gas lines or turn on critical facilities in the first few seconds between the beginning of an earthquake and the arrival of the most damaging earthquake waves.  Even a few seconds' warning may be enough to reduce the likelihood of fire, or stop high speed trains or lifts, and thus save lives.