Natural Hazards Update - No.14 May 2007

Finding faults in the Wanganui Basin

Recent research shows that there are many more active offshore faults in the Wanganui Basin than originally believed. An area of faulting along the eastern boundary of the Basin, known as the Kapiti-Manawatu Fault System (KMFS), is characterised by shallow seas and thick sedimentary deposits. NIWA marine geologists Geoffroy Lamarche and Scott Nodder, who have mapped the faults, have found evidence of sediment displacement, indicating that there have been a number of earthquakes within the last 20 000 years.

The techniques used mean that not only can the location of active faults be confidently mapped, so too can their geometry and slip, or movement, rate. ‘We have a good idea of the lateral and vertical extent of faults in the KMFS now,’ says Geoffroy. ‘This means we can better estimate the potential maximum magnitude of earthquakes generated by the faults.’

Residents of coastal Kapiti and Manawatu can be reassured that the earthquake and tsunami hazard in the area is still considered low. While it is estimated that earthquakes of magnitude 5.8–7.5 could be generated from the offshore faults, the evidence suggests that recurrence intervals are relatively long (>5000 years). However, the holy grail of researchers – the question of when a ground-rupturing earthquake is actually going to happen – remains out of reach for now.

The research, funded by FRST, has generated several journal papers and will be presented to relevant local authorities. The data will be incorporated into the National Seismic Hazard model by GNS Science so that the faultrelated earthquake risk of the region can be better assessed.

Natural Hazards 2006 – A year in review

A new Natural Hazards Centre publication, ‘Natural Hazards 2006’, is due for release in June. This 28–page review, produced jointly by NIWA and GNS Science, is the first in an annual series. It provides an informative summary of natural hazard events and the activities of major organisations involved in natural hazards work.

Contents include:

Maps and summaries of the main hazard events of 2006
Features on key hazards, including snow, landslides, and coastal erosion
An update on NIWA and GNS Science natural hazards research
Items from the Insurance Council, EQC, and MCDEM
Selected hazards publications lists.

Natural Hazards 2006 will be distributed via our regular mailing list. Additional copies will be available from either: Harriet Palmer at NIWA h.palmer@niwa.co.nz, or John Callan at GNS j.callan@gns.cri.nz.

Reporting on the Northland floods

On 28–29 March, over 400 mm of rain fell in the hill catchments of eastern Northland. Severe weather warnings had been issued but the total amount of rain greatly exceeded expectations and 1-in-150 year events occurred in Kaeo, the hills west of Kerikeri, and the hills north of Whangarei. The resulting floods covered more than 5000 hectares of farmland as low-level flood protection schemes were overwhelmed. A number of rural communities were badly affected, and some homes were condemned as a result of flood damage. Numerous landslides also occurred, the most high-profile of which seriously damaged main access routes and tourist attractions.

The estimated cost of landslide and flooding damage greatly exceeded $20 million. NIWA hydrologist Graeme Smart was part of a team which visited some of the worst-affected areas. The team concluded that much of the flooding and landslide damage had occurred in places where dwellings or assets had been situated in precarious locations. Many of the damaged sites had experienced similar events before, but increasing population density means that more people are now exposed to the hazards. Recommendations made include more research into better warning systems, better hazard maps, and the incorporation of hazard knowledge into regional development plans

The April Solomon Islands tsunami

On 2 April at 08.40 NZST, a magnitude 8.1 earthquake at a depth of around 10 km occurred south of Gizo, Solomon Islands. It caused a tsunami that predominantly beamed out to the north-east, hitting Gizo and surrounding islands, and to the south, into the Coral and Tasman Seas. For New Zealand, the tsunami wave train was focused towards the west coast between Cape Farewell and Greymouth.

Data from NIWA’s open-coast sea-level gauges on the west coast showed that the highest wave height above the tide level was 0.5 m high, or 1.1 m peak-to-trough wave height. These waves arrived at Charleston, near Westport, almost 20 hours after the original earthquake. No damage was caused, but if the waves’ arrival had coincided with a high tide and higher wave activity, some minor damage in low-lying areas could have occurred, particularly from the surging effect of waves coming up and down beaches with periods of 6–12 minutes.

NIWA scientists had recognised that the Solomon Islands could be a possible source for substantial tsunamis along the west coast of New Zealand, based on evidence of prehistoric tsunamis. As part of a consultancy project for several regional councils, NIWA, using their RiCOM tsunami and storm surge computer model, looked at a variety of tsunami scenarios. The predictions made by the model of wave height, areas most likely to record the largest waves, and arrival times of a tsunami originating from the Solomons area, matched well with the actual sea-level records analysed by NIWA.

Natural Hazards Centre