Natural Hazards Update - No.8 2004

Are the recent Wellington earthquakes related to ground deformation on the Kapiti coast?

A swarm of earthquakes has been felt in the Wellington and Hutt Valley region over the past year or so. The two largest, of magnitude 4.4 and 4.6, occurred on 3 April this year, and several others were felt later in April and May. The earthquakes were all in a tight cluster about 4 km north of Upper Hutt at a depth of 24–26 km.

A continuously operating GPS receiver has been located at a trig station on Paekakariki Hill since 2000. The motion of the site was steadily westward at 25 mm/yr from 2000 to about May 2003, when it suddenly slowed to 15 mm/yr westward. Since May 2003, there has also been a change in vertical motion, with a relative uplift of about 10–15 mm through to the present. A similar receiver at Wellington Airport has continued to show fairly steady westward motion at 30 mm/yr and no clear change in vertical motion.

The earthquake swarm at Upper Hutt and the change in motion at Paekakariki Hill both began at about the same time. These places are less than 15 km apart. Could the earthquakes and the change in motion be related?

The reason for the steady westward motion at Paekakariki and Wellington is that the Pacific tectonic plate is steadily subducting beneath the North Island. In the southern North Island, the subducting Pacific plate and the overriding North Island are coupled along their interface, so as the Pacific plate moves west it drags the North Island with it. If part of this subduction interface were to slowly slip, rather than remaining tightly coupled, it could produce both the effects we have observed – the Upper Hutt earthquakes and the (presumably temporary) change in motion at Paekakariki Hill.

To explain the two effects requires a slip of up to about 50 cm occurring slowly over the past year on a 30–40 km deep part of the subduction interface. To produce the observed 10 mm/yr change in horizontal motion at Paekakariki while producing very little change in motion at Wellington Airport, the slip direction needs to be roughly parallel to the plate motion direction rather than being approximately down dip. For oblique slip to occur on the deeper part of the subduction interface is reasonable because the proposed slip event is occurring to the west of most of the upper-plate strike-slip faults.

A slip event of this type is similar to the slip that occurs during an earthquake, except that it happens so slowly (over months) that it doesn’t cause the ground shaking felt in a normal ‘fast’ earthquake (which happens in seconds). However, it does have other effects. The slip event causes changes in stress in the rock in the surrounding region which may make particular types of earthquake more or less likely.

The types of earthquake that are most common in the Upper Hutt swarm are normal-faulting earthquakes near the top of the subducting plate. The stress changes from the supposed slow slip event have been estimated by Dr Russell Robinson of GNS, and are exactly what are required to favour this type of earthquake.

We emphasise that our idea – that a slip event has been taking place slowly over the past year on a deep part of the subduction interface below the Kapiti coast – is based on extremely limited data. Better knowledge of the existence and effects of such slip events, and their implications for understanding subduction mechanics and the likelihood of future earthquakes, depends on gathering improved data and on undertaking geological and geophysical research on the data.

There is good news for the future in that EQC is funding the installation of additional continuous GPS stations and seismic stations in the southern and eastern North Island over the next few years as part of the GeoNet project. This will provide important data for addressing these issues, not only in the Wellington region, but along much of the eastern side of the North Island.

For information, contact: John Bevan (j.bevan@gns.cri.nz)
For more information on GeoNet, visit: www.geonet.org.nz

Natural hazards in autumn 2004

Floods and droughts	Weather extremes	Weather extremes - rainfall
Coastal hazards	Landslides	Earthquakes and volcanic activity

Tsunami near Gisborne

Damage from the 1947 tsunami, north of Gisborne
[photo: Gisborne Disctrict Council Collection]

On the evening of 17 May 1947, after very minor offshore earthquake, a tsunami hit a stretch of North Island coastline between Tonga Bay and Waiora. At its maximum, north of Gisborne, the tsunami was about 6 meters high. This tsunami was the second to hit this stretch of coastline that year - an earlier and higher tsunami occurred less than 2 months before, on 26 March. Scientists have suggested that both these tsunamis may have been produced by eruptions from offsore 'mud volcanoes'.

On the lookout for magma

The Taupo caldera (Lake Taupo) with the volcanoes of Tongariro National Park in the distance.

An illustration of S-wave velocity changes in cross-section across the Taupo Volcanic Zone (derivative of Vs with depth). The circles show the projections of earthquake hypocentres within a 5 km profile.

The Taupo Volcanic Zone contains a record of huge and violent eruptions of a scale that is almost impossible to imagine. Fortunately such events are infrequent – the last ignimbrite eruption occurred about 2000 years ago from Taupo. An eruption of the scale of the last Taupo event would have a significant impact on the economy of the country.

However low the probability of such an event, we still need to ask: how would we know if such an eruption was imminent? What would the signs be, and, more importantly, would we recognise their significance?

As part of the background to the routine geohazards monitoring, GNS has an active research programme aimed at answering these questions.

Scientists believe that magma is almost always present beneath the Taupo area, but usually it cools in a benign manner without ever reaching the surface. The heat it provides supplies energy to the numerous geothermal systems. However, before an eruption, magma starts to accumulate at shallow depths (less than 5 km) over time scales that may be as long as 100 years. Instability within the overcharged magma system could then lead to a catastrophic discharge or eruption.

Until recently, geophysical techniques could not detect the magma within the crust, let alone provide a quantitative measure of how it may change with time. This has now changed, and two complementary methods have been developed that have successfully imaged what are believed to be magma layers. Initially, measurements of the electrical properties of the earth (using the magnetotelluric method) successfully identified highly conductive zones within the crust beneath the Taupo Volcanic Zone that scientists believe can only have been caused by zones of connected melt. Corroboration has come from a technique that uses seismic waves generated by distant earthquakes to deduce changes in the shear wave velocity beneath a site from the wave forms measured at the surface. This technique, known as receiver function inversion, has identified horizontal zones of low shear wave velocity characteristic of melt components within the crust. These zones are found at a similar depth to the conductive zones previously identified.

These studies provide the first steps towards not just identifying zones on magmatic accumulation, but to determine the extent of these zones, and to measure their changes with time – the key component in understanding and monitoring the potential for Taupo’s massive eruptions.

Hugh Bibby (h.bibby@gns.cri.nz)

Understanding earthquake shaking potential in Whakatane

GNS has recently undertaken a micro-zoning study of the Whakatane urban area. We know from research and historical examples that ground response to earthquake shaking varies depending on the geology and geotechnical properties of the surface and subsurface materials. This has implications for the types of damage that will occur to the ground and to buildings and other infrastructure.

GNS used a variety of techniques to gather data from Whakatane including:

• searching Whakatane District Council and Environment Bay of Plenty files for geological and geotechnical information
• using Nakamura microtremor surveys to determine the ability of the ground to amplify earthquake shaking and the frequency at which amplification would occur
• cone penetrometer and seismic cone penetrometer probing to determine the geotechnical properties of the ground.

The study has also looked at earthquake sources that are likely to affect Whakatane. The Whakatane Fault, which lies beneath the town, will produce the strongest shaking. Other nearby faults will produce shaking similar to that experienced during the 1987 Edgecumbe earthquake. Using this and other data GNS has been able to construct a picture of the geotechnical properties of different areas within Whakatane and how they will respond to earthquake shaking. For example, on the Whakatane River flood plain a soft or weak soil layer will probably cause extensive damage to underground surfaces, but will partially isolate most buildings from the worst shaking. However, the opposite will occur on the hills with underground services likely to remain largely intact, but buildings and their contents likely to more extensively damaged. The Whakatane District Council and Environment Bay of Plenty will be able to use this information to assess the likely impact of different levels of earthquake shaking on different parts of the local infrastructure and plan appropriate mitigation measures to help to reduce the losses from future earthquake shaking in Whakatane.

After the February floods: what about next time?

Scientists from GNS and NIWA are beginning work on a multi-million dollar model to predict the risks of natural hazards.

The programme will be coordinated by Andrew King of GNS and Rob Bell of NIWA. Dr Bell said the new model will offer a wide range of organisations better information to plan and prepare for floods, earthquakes, and other natural hazards.

‘The model will show in detail the sort of damage and casualties a hazard would inflict on a region. We hope to include data on the people, the buildings, the terrain, roads, bridges, and other infrastructure.

‘The model should help local authorities, utilities companies, emergency managers, and others to reduce the impact of natural hazards, and improve the response to events such as the floods in February. That’s because they’ll know how much and where damage and disruption are likely to be.

‘It’ll also help them prioritise spending when faced with choices like whether to build higher stop banks to protect land from floods or to earthquake-strengthen a vital bridge.

‘The risk model will be built in stages. We hope to have a prototype working for three regions by June 2006’, said Dr Bell.

The project is being funded by the Foundation of Research, Science & Technology at a cost of $8 million over four years.

For more information, contact:
Dr Rob Bell (r.bell@niwa.co.nz)
Dr Andrew King (a.king@gns.cri.nz)

Coastal hazards guidance manual

A new manual, ‘Coastal hazards and climate change: a guidance manual for local government in New Zealand’ is about to be published by the N.Z. Climate Change Office, Ministry for the Environment.

The manual aims to strengthen the integration of coastal hazards and climate change within the land-use and development planning process.

The key objectives of the manual are:

• to provide regional and territorial authorities with information on the key effects of climate change on coastal hazards
• to provide a decision-making framework to assess the associated risks
• to provide criteria by which to appraise and decide on appropriate planning and resource management responses to the risks.

The manual was developed by a team consisting of NIWA, Beca Consultants Ltd, and DTec Consultants Ltd, with additional input from Tonkin & Taylor and Duncan Cotterrill.

The guidance manual is available on the web at www.climatechange.govt.nz/resources/local-govt/coastal-hazards-may04/index.html

For further details, contact: info@climatechange.govt.nz

Tsunami warnings workshop

Tsunami warning sign in Washington, USA developed by the US National Tsunami Program.

One of the outcomes of the recent review of the National Civil Defence Warning System was the need to reflect upon the contents of warning messages for the different types of events. MCDEM, working with GNS and NIWA, will hold a number of workshops to address this. The first of these, focusing on tsunami, will be held in Tauranga to coincide with the Natural Hazards Management Conference 2004 (www.naturalhazards.net.nz). The workshop is scheduled for the evening of 11 August, after the close of the conference and before the field trips the next day. The workshop will explore issues around developing local capability for responding to tsunami warnings. Other workshops, dealing with other hazards, are planned for later in the year.