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No cause for alarm over freshly discovered fault - 01/01/1998

Residents of Whiteman's Valley, 30km north of Wellington, need not pack up and leave just because an active fault has been discovered running through the valley, a geologist says.

Institute of Geological & Nuclear Sciences Limited geologist John Begg said the recently-discovered ''second-order'' fault in the valley was typical of many other secondary faults in the Wellington region.

'' Many of the Wellington region's known secondary faults are in urban areas. They're difficult to investigate once houses have been built on them and the ground surface has been modified,'' Dr Begg said.

Until a few years ago, geologists had thought that all the significant active faults had been discovered. But extensive revisions of all the likely areas had thrown up a number of new faults.

Dr Begg estimated the Whiteman's Valley fault was active '' about every 15,000 years'' compared with the main Wellington fault which is active ''every 600 to 800 years''.

The Whiteman's Valley fault, which was investigated using funding from the Earthquake Commission, had ruptured within the past 10,000 years. It had produced an earthquake of about magnitude 7 on the Richter scale, and a vertical displacement of about 2m.

Dr Begg said his studies had concluded that the Whiteman's Valley fault had little effect on seismic activity in the Wellington region.

He defined a second-order fault as one that did not trend northeast-southwest (the orientation of the main faults), was less prominent on the landscape than the main faults, and was less active than the main faults.

The Institute was continually upgrading its seismic model of Wellington and an important part of that process was quantifying the potential hazard from all known faults.

The major faults in the Wellington region have been known since early this century. They are the Wellington fault, the Wairarapa fault to the east, the Ohariu fault to the west, and the Gully fault which runs between Cape Terawhiti and Pukerua Bay. All are capable of producing an earthquake of magnitude 7.5, or greater.

There are about 30 major faults in New Zealand. Major faults are classified as ones that have an average slip rate (movement rate) of at least 1m every 1000 years. Slip rates vary significantly. The alpine Fault in the South Island is the biggest mover among the major faults -- it moves at the rate of about 30m every 1000 years.

Dr Begg said there are more than 20 second-order faults in the Wellington region.

'' Four are known to be active and a number of the others that might be active, have not been fully investigated yet.''

Earthquake hazards in Wellington come from three sources -- the interface between the Australian and Pacific tectonic plates 30km under Wellington, the four major faults, and the many second-order faults.