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Boo Boo fault part of a bigger hazards picture - 07/05/2004

The recently revealed Boo Boo Fault in Cook Strait is one of many thousands of bits of geological information that scientists use to calculate earthquake hazard throughout New Zealand.

Boo Boo fault

Boo Boo Fault's vital statistics will be added to the national seismic hazard model - a kind of seismology black box that scientists and engineers can interrogate to find out how vulnerable various parts of New Zealand are to earthquake shaking.

Developed four years ago by Geological and Nuclear Sciences (GNS), the national seismic hazard model contains information on 300 onshore active faults plus a growing number of offshore faults.

New Zealand's offshore fault information comes from NIWA (the National Institute of Water and Atmospheric Research) and the onshore fault information comes from GNS. Boo Boo Fault was mapped recently by using high precision sonar equipment on the NIWA research ship, Tangaroa.

As new geological information comes to hand, it is included in the national seismic hazard model. This can result in earthquake hazard estimates increasing slightly in some parts of New Zealand and decreasing in others.

The 70km-long Boo Boo Fault is expected to increase the annual earthquake and tsunami hazard in Wellington and Marlborough by less than 10 percent. This means there will be a small increase in the expectation that damaging ground shaking will occur in Wellington and Marlborough in any 12 month period.

The lion's share of earthquake hazard in the Wellington region comes from the Wellington Fault. This is unlikely to change with the discovery of more offshore faults.

Boo Boo Fault is more active than first thought, with a slip rate of 10mm to 12mm a year. Slip rate is the horizontal or vertical distance a fault moves when it ruptures, divided by the time taken for the movement to occur.

There are only a handful of active faults in New Zealand with slip rates above 10mm a year.

Initial estimates put the Boo Boo Fault rupturing at intervals of hundreds to thousands years. It is not known when it last ruptured.

GNS seismologist Mark Stirling says in simple terms, the length of a fault and its slip rate combine to give a fair indication of the maximum sized earthquake it can generate, and how often that earthquake can be expected.

" In physical dimensions, Boo Boo Fault is similar to the Wellington Fault which we estimate will produce an earthquake of magnitude 7.2 to 7.6 the next time it ruptures.

" We can expect an earthquake of similar magnitude from Boo Boo Fault, and that's a big earthquake by any measure."

But because of its distance from built up areas, violent ground shaking will have dissipated somewhat by the time seismic energy from a Boo Boo Fault rupture reaches Wellington and Blenheim.

While information about offshore faults is valuable, onshore faults exert more influence on levels of expected ground-shaking throughout New Zealand.

" One of the areas we've been working on which will have a significant impact on the national hazard model is the Taupo Volcanic Zone."

Once new information from the many complex geological faults in this region is added to the national seismic hazard model, it will change estimates of earthquake vulnerability in the central North Island and Bay of Plenty. Ultimately, this may have an effect on building codes in those areas.

Traditionally geologists believed that the faults in the Taupo Volcanic Zone ruptured independently. However, recent findings indicate that fault ruptures in a number of places are linked.

Geologists have also found a number of faults they thought were active are inactive.
The reverse has also been true for several faults.

Most developed countries have a national seismic hazard model, but New Zealand, Canada, the United States and Japan are leaders in this area.

Information on New Zealand's national seismic hazard model has been published in scientific journals and is available to the public, in summary form, on the GNS website: http://www.gns.cri.nz/what/earthact/modelling

The model's public expression is a document called General Structural Design and Design Loadings For Buildings, published by Standards New Zealand. It was last published in 1992 and a revised and updated edition is due to be published later this year. The new edition incorporates many changes brought about by increased knowledge of active faults in New Zealand.

The 1992 document can be viewed at www.standards.co.nz

  • Boo Boo Fault gets its name from Boo Boo Stream at Cape Campbell in Marlborough.