Earthquake risk zones
Despite the large number of active faults in New Zealand, some regions are more prone to earthquakes than others. An iso-seismal map is used to determine the hazard for a particular location in the seismic design standard that engineers use. To simplify the design and construction of light timber-framed and smaller concrete block masonry buildings in these regions, New Zealand’s construction standards divide the country into four earthquake risk zones. Structures in high-risk zones must be built to more stringent requirements than structures in low-risk zones.
The level of earthquake shaking that is considered when designing a structure is related to historic seismicity and the distance from known or expected active fault lines. In general, greater shaking is expected in regions closer to active faults. However, geological characteristics, regional topology of the ground and presence of soft soils at the site can significantly influence the type of shaking a building is likely to experience during an earthquake.
For the purposes of constructing low-rise timber and concrete block buildings, New Zealand’s earthquake risk is divided into four main strata (zones 1–4) according to the relative level of seismic risk. Areas close to the Alpine Fault carry the highest risk (zone 4), while the northwest North Island (including Auckland) and southeast South Island (including Dunedin) carry the lowest risk (zone 1).