Enforcement of seismic provisions in the nation's building codes is credited with dramatically reducing the impact of the most damaging earthquake to hit New Zealand for nearly 70 years.

A magnitude 7.1 earthquake struck Christchurch, New Zealand's second largest city, at 4.36 a.m. on Sept. 4, causing extensive damage to unreinforced masonry buildings, not engineered to resist sesimic loads, but no significant damage to major buildings.

Investment in seismic retrofits appears to have saved some of Christchurch's most important historic buildings, including the Anglican Cathedral in the city's center and the Catholic Basilica.

The major message from the earthquake is that the New Zealand building standards are heading in the right direction, says Win Clark, executive Officer of the New Zealand Society for Earthquake Engineering. "There are things we can do better, but it's clear" that good codes and code enforcement make a difference, he says.

The event was centered 40 kilometers west of the city. It was relatively shallow; occurring at a depth of 10 km. Monitoring by Geological and Nuclear Sciences showed peak ground accelerations produced were strongly directional but not particularly high.

Seismic energy was distributed through short rather than long-period motions says Clark. This had a greater impact on stiff low-rise structures than it did taller, more-flexible structures, which were not challenged anywhere near their design capabilities, he says.

Widespread ground failure occurred along with significant incidences of liquefaction.

Christchurch is built on estuarine sediments deposited at the confluence of two large rivers. River sand, silt, and gravel deposits are distributed unevenly (depth, type, extent) in the area due to the heavily braided river channel topography. This uneven sediment distribution modified the ground acceleration, velocity and displacement experienced in different parts of the city. In some areas, the earthquake vibrations were amplified relative to those experienced in areas built on bedrock.

Water and sewer lines bedded in soft alluvium were stressed and pipe-joint displacements occurred. Waste had to be pumped into the streams flowing through the city. However, by nightfall, two-thirds of the city had water.

While the inertial loads on heavy electrical equipment at the city's substations did not affect supply, the system automatically shut down in some areas. Cable faults caused blackouts, and supply was further disrupted as water entered damaged cables.

The rural area surrounding Christchurch is New Zealand's largest dairying area. Several historic brick homesteads were destroyed but the extent of the damage to rural infrastructure, such as the electricity network and irrigation systems, remains unknown.