During earthquakes, the land on one side of a fault may suddenly move horizontally or vertically by up to several metres. When this occurs, any buildings or other structures that straddle the fault will be torn apart and severely damaged. Such ruptures are often not a narrow line, but may be a zone up to tens of metres wide.
New Zealand has a number of cities and towns with buildings on or close to an active fault, many built long before the risk of fault movement was known. The Greater Wellington region, including Wellington and the cities of Lower Hutt and Upper Hutt, has many buildings on or close to the Wellington Fault – it runs through the grounds of the prime minister’s official residence. Other centres built on active faults include Franz Josef, Hanmer Springs, Blenheim, Nelson, Porirua, Waikanae, Waverley and Waipukurau.
Planning close to active faults
While it is difficult to protect existing buildings, some local authorities are now controlling new development on known active faults, especially those that move frequently. In 2003, the Ministry for the Environment produced guidelines for the development of land close to active faults. It was recommended that any new building be set back 20 metres from a known active fault zone. Many local authorities have now adopted this guideline.
To enforce such regulations, the exact position of a fault must be known. Some faults are simple linear features, with a scarp that is only a few metres wide. Others have deformed broader zones of land, tens and even hundreds of metres wide.
Many active faults have scarps on the ground’s surface that can be mapped using ground studies and aerial photographs. However it is not straightforward to locate the fault line in some areas. For example, parts of the Wellington Fault are under water, covered by river sediment, or modified by urban development, and complex investigations are required to accurately locate it.
Totara Park – suburb on a fault line
The Wellington Fault runs through the Upper Hutt suburb of Totara Park. This area has been planned with a number of special features to protect residents. One section that the fault traverses, California Park, has been set aside as a recreation reserve. Through the rest of the suburb, the fault line runs down the centre of California Drive. This street has two lanes separated by a wide grassed berm that covers the fault trace. No house is closer than 20 metres to the fault. Very few service lines, such as water, gas and sewer systems, cross the fault. Those that do cross it have flexible joints to withstand ground shaking.