Seismic Hazard Study Favours Transmission Gully Motorway Route

A report prepared for the Porirua City Council by the Institute of Geological & Nuclear Sciences (IGNS), and published on the Council’s website, has confirmed what many of us knew – upgrading the present route of State Highway 1 between Pukerua Bay and McKay’s Crossing to four lanes would be a waste of money.

Wellington desperately needs a high quality northern motorway route that will operate in all weathers and conditions – something the coastal SH1 and Rimutaka SH2 routes cannot achieve. As recently as January 8th Wellington has been cut off from the rest of the country by storm-induced flooding and landslides on these roads.

The IGNS report confirms that the steep and unstable slopes north of Pukerua Bay township could collapse in a strong earthquake producing large landslides which might take MONTHS to clear. These landslides would be much larger than the single slide which closed the Manawatu Gorge road for several months last year.

Wellington currently faces three large earthquake threats which could trigger landslides on SH1 & 2 and other roads.

The best-quantified threat is the one-in-eight chance of a magnitude 7.5 earthquake occurring on the Wellington fault within the next 30 years. Prepared from evidence of past fault movements, this is the most quantifiable risk to the city and its suburbs.

Less well-known is the risk imposed by the movement of the South Island’s Alpine Fault. The Alpine Fault produces large earthquakes, involving horizontal movement of up to 8 metres along fault segments tens of kilometres in length. Current thinking is that such an event is overdue, as historical evidence indicates that large earthquakes occur every 100 to 300 years. The last Alpine Fault events occurred ca 1620 and 1720.

For Wellington and much of the South Island, the massive Alpine Fault is the “sleeping giant” as such a movement could occur as a series of perhaps three or four magnitude 7 earthquakes over a period of minutes, resulting in a combined energy release in excess of Richter magnitude 8.

The northern extent of the Alpine Fault passes along Wellington’s west coast offshore from Wellington, Kapiti and Horowhenua, on the western side of Kapiti Island. If this submarine section was to move in conjunction with the alpine section, there would be extensive ground shaking on the western side of the lower North Island. Any resulting tsunami would simply add insult to injury.

Sound familiar? Wellington’s last experience with a magnitude 8 plus earthquake was 150 years ago, in 1855, when the Wairarapa fault ruptured, causing 9 metres of lateral movement along a 70-odd km section of the fault.

Wellington’s third, but poorly understood threat is the possibility of a subduction thrust event on the plate boundary to the south-east of the city. It is known that the collision interface between the Pacific and Australian plates is locked under Marlborough and may have been so for millennia – hence the deformation in the upper South Island.

However, the plate interface to the north of Marlborough, where the Pacific plate is still sliding under the Australian plate has been stalled for many years – perhaps 90-100 years according to one source. If this section of plate boundary was to release the strain that has built up, then the Pacific plate would abruptly slide an unknown distance beneath the overlying Australian plate in what is known as a subduction (diving under) thrust earthquake. However, whilst there is evidence that such earthquakes do occur, seismologists are still trying to calculate how often; and face a daunting task in trying to build up a picture of past events. There are clues, but the fact that all the “fun” happens underwater, means that the picture is slow to emerge.

Clearly Transit New Zealand faces considerable challenges in addressing the poor roading infrastructure in the lower North Island and upper South Island. Like the coastal highway north of Wellington, the coastal section of State Highway 1 at Kaikoura in the South Island is a high maintenance piece of road and at risk from the same earthquake scenarios. Both these roads follow routes that were defined during nineteenth century pioneering days, when four-lane carriageways were not envisaged – the delight of having a track wide enough for a horse-drawn cart in fine weather was quite sufficient.

Even more modern roads have been found wanting in terms of design. Wellington’s urban motorway was opened in the 1970s using excavation and slope management techniques that met the standards of the time. Further additions such as the Terrace Tunnel and Ngauranga Interchange were made in later years, as funding allowed.

Following the collapse of overhead spans of expressways as a result of earthquakes in Los Angeles in 1994 and Kobe in 1995, roading authorities here acted with remarkable alacrity in strengthening the overhead section of the Wellington urban motorway at Thorndon. Some supporting columns were given kevlar jackets to reduce the impact of concrete fracture and increased footings to reduce misalignment when the soft soils liquefied. At the top of the columns, the insubstantial hips upon which the sections of roadway are hung were lengthened to try and reduce the chance of a section falling should the horizontal movement of the Wellington fault increase the distance between supports by moving them apart.

These measures, if successful, will not prevent the motorway becoming temporarily unusable following a sizeable earthquake on the Wellington fault. However, they should make it possible to quickly bridge any gaps provided a complete span does not drop.

As early as 1990, engineering and council experts were highlighting the instability of the steep slopes above the coast road north of Pukerua Bay, and their susceptibility to failure as a result of earthquakes. The main trunk railway line also traverses this unstable slope through a series of tunnels and landslide barriers. At one active section of slope, a landslide detection system controls train movement, and rail passengers who care to look can see the buildup of loose material that forms against the barriers between maintenance visits.

Attempting to expand the capacity of the Wellington coastal route along unstable narrow corridors is false economy. The land for Transmission Gully is available and progressive establishment of a road designed to modern standards and expectations should begin in earnest.

An alternative to the coastal section of State Highway 1 at Kaikoura also needs to be developed. This section of road has been closed by accidents, floods and landslides on several occasions in the past 3 years. It is a major corridor for road transport carrying freight to and from the interisland ferries.

These two sections of road are not only vital to the local communities; they are also crucial to manufacturing, tourism and service industries nationwide.

Improvements will not appear with the waving of a magic wand but, with adherence to a coherent plan and progressive funding, useful sections of improved roading can be made available in a co-ordinated fashion. A long-term vision is sorely needed.

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