Understanding Wellington’s Vulnerability to Earthquakes

A project which aims to better understand the Wellington region’s vulnerability to earthquakes was launched at Te Papa on December 11th.

Funding for the 7-year $3.5 million project is being provided by the Earthquake Commission, the Accident Compensation Corporation, Wellington City Council and Greater Wellington (the regional council).

The oddly-named “It’s Our Fault” project, which will be led by GNS Science, is to better define Wellington’s earthquake risk using the latest geological techniques and sophisticated computer modelling. It is hoped that the project will help Wellington become better prepared for and safer from earthquakes.

There are four major active earthquake faults in the region – named Ohariu, Shepherd’s Gully, Wellington and Wairarapa – as well as several others both on- and off-shore. Of the major faults, the Wellington Fault is thought to be the biggest local seismic hazard for Wellington, and it has been given a one-in-eight chance of causing a large earthquake within the next 30 years.

Risk from the other major faults is assumed to be lower, with the Ohariu Fault thought to be some 300 years from its minimum return period and the Wairarapa Fault thought to be a thousand or more years from its return period since its last rupture in the magnitude 8.1 Wairarapa Earthquake of 1855. The Shepherd’s Gully Fault, which runs to the west of the Ohariu Fault, is more of a problem and requires further study to determine when it last ruptured – it’s thought to rupture every 2500 to 5000 years.

However, the interaction between these earthquake faults is not fully understood. The project is expected to provide more information on whether a large earthquake on one of the regions many faults will advance or retard earthquakes on neighbouring faults.

The project media release from GNS Science states that the project will improve knowledge of individual faults, but it is not clear whether it will study Wellington’s other major local risk – the possibility of a subduction-thrust event nearby. These events occur when the Australian and Pacific tectonic plates, which are strongly locked together near Wellington, move abruptly as a result of accumulated strain. Some research suggests that these events happen at 800 to 900 year intervals, but evidence of the last event is difficult to find.

There are four main strands to the research – the likelihood and frequency of large earthquakes, the expected size, the physical effects, and the social and economic impacts. There are knowledge gaps in all these areas.

Since the 1980s, the Wellington region has been addressing some of the known risks by tightening building codes and strengthening key infrastructure. Buildings that do not meet modern earthquake-resistant codes have been demolished, strengthened or flagged for attention. The raised carriageway which carries the urban motorway above parts of Thorndon has been strengthened by applying kevlar jackets to support columns and increasing the width of hips should sections move apart. This work was carried out after a similar structure built above similar poor soils collapsed during the Kobe earthquake in Japan. It is hoped that the modifications will prevent one of the large spans dropping, should the Wellington Fault move the columns further apart, thereby allowing the road to be quickly re-established using bridging techniques to cross gaps in the roadway.

Nevertheless, much work still remains to be done. Constant bickering over the Transmission Gully highway proposal leaves the region reliant on two low quality routes northward. Both of these routes, Centennial Highway (State Highway 1) and the Rimutaka Hill Road (State Highway 2) suffer closures caused by bad weather almost every year, and are expected to be blocked for an extended period after a major earthquake.

Many residents have been tardy in storing water and other essentials required to survive a major quake. Maintaining a drinkable water supply following a major earthquake is probably the biggest challenge facing local and regional councils, with feeder pipelines crossing the Wellington Fault at several locations.

There seems to be a belief that government will address all post-quake issues rapidly. However, just how this will be achieved is not clear. Wellington airport, which has been built on land thought to have risen out of the harbour in the Hao Whenua earthquake of the mid-1400s, is expected to face major difficulties following a major quake, and may find its runway shorter than normal due to failure of reclaimed land. The ability of large ships to enter the harbour may also be limited if a major quake on the Wellington Fault raises the sea-floor by as little as a few metres at the harbour entrance.

It is to be hoped that the project will contribute to understanding and planning for some of these outstanding issues.

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