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Inangahua ground-shaking figures disbelieved - 24/05/1998

Ground-shaking recordings taken on the West Coast during the 1968 magnitude 7 Inangahua earthquake were so strong that some scientists refused to believe them.

The recordings were made by a network of instruments called scratch plate accelerographs which measure the shaking of an earthquake at ground level.

Crude by today's standards, the instruments consist of a needle on two springs that scratches a pattern on a piece of smoked glass which is then analysed to determine the strength of ground-shaking.

The recordings they make are valuable to engineers, developers, scientists and planners in helping to make buildings, structures and community lifelines more resistant to earthquake shaking.

In 1968 physics graduate Bill Stephenson had recently joined Physics and Engineering Laboratory, a division of the former DSIR. It was his job to look after the national network of ground-shaking instruments. On the West Coast they were located at Greymouth, Reefton and Westport.

After the Inangahua earthquake Mr Stephenson flew to the West Coast to retrieve the scratch plates. He found the strongest ground-shaking was recorded at Reefton, 30km south of Inangahua. It was 0.61 of a gravity unit, which would have the ground shaking so much that it would be impossible to stand up.

This figure was almost twice the strength of any ground-shaking recorded anywhere in the world to that date.

'' I accepted what my instruments told me, but some scientists and engineers had great difficulty with this figure -- it was completely outside their comfort zone. Some of them didn't want New Zealand to be out of step with the rest of the world.''

It was suggested to Mr Stephenson that the instrument at Reefton was probably faulty, so he tested it exhaustively and found it was accurate.

However, there was still doubt about his figures and it was not until the 1990s that they were finally verified and published.

The ground-shaking recording of 0.61g has since been exceeded in a number of overseas earthquakes. It was also exceeded during the 1987 Edgecumbe earthquake in a recording made at the Matahina Hydro Dam in eastern Bay of Plenty.

Mr Stephenson now works as an engineering seismologist with the Institute of Geological & Nuclear Sciences Limited which operates a network of 266 accelerographs throughout New Zealand. The digital accelerograph, the 1990's version of the scratch plate instruments of the 1960s, records information during earthquakes that is valuable for improving building design codes.