Dam designed for big earthquakes

Press, 27 November 1982, Page 3

Dam designed for big earthquakes

PA Wellington The Clyde dam is designed to cope with an earthquake of almost the same magnitude as that which destroyed Napier in 1931, according to the Minister of Works and Development, Mr Friedlander. In answer to a Parliamentary question, he said that the dam was designed to cope with ■ a maximum credible earthquake of magnitude 7.6 originating on the Dunstan fault. , The Napier earthquake registered 7.9 and the 1968 Inangahua earthquake 7 on the Richter scale. Filling Lake Dunstan, which will be created behind the dam, was unlikely to generate an earthquake, Mr Friedlander told Dr M. J. Cullen (Lab., St Kilda). “However, examples are known when dams of similar heights have induced a number of shocks in regions

previously considered to have low natural seismicity," Mr Friedlander said. The risk of damaging earthquakes could not be assessed quantitively, he said, but was judged on present day seismological and geological knowledge to be low. The nearest active fault line that could cause a destructive earthquake was a kilometre upstream from the dam. “The periodicity of large earthquakes resulting from fault rupture on the Dunstan fault may be about 25,000 years,” Mr Friedlander said. “It is estimated that a maximum credible earthquake of 7.5 magnitude has an average recurrence interval over the central and east Otago region as a whole of 1000 years.” Asked if he could guarantee that the foundation rocks for the dam would not give way because of the reported

presence of weak and fractured rocks, Mr Friedlander said that any such rocks were being either removed or strengthened to ensure the dam had an adequate foundation. Dr Cullen said that the Minister’s replies indicated that “big question marks” still hung over the safety of the dam site. It would not be sensible to be unduly alarmist about it, he said. “However, it. is clear that seismic activity occurs in the region, and that the seismic activity is possibly sufficiently large to prove a danger to the dam structure. It is also clear that far too little research was undertaken before very substantial work was done on the dam itself, This kind of cart-, before-the-hofse planning must never be repeated.” Dr Cullen asked the Government to publish the full geological report.