The earthquake was one of New Zealand’s deadliest peacetime disasters. It was also the costliest in New Zealand, and the third costliest earthquake worldwide, with damage estimated from $16 billion to $24 billion. About 5,000 homes were destroyed. Hundreds of buildings in the central city were either destroyed or so badly damaged they would have to be demolished – a process that is still continuing. Streets and infrastructure were severely damaged, and hundred of thousands of tons of silt and debris had to be removed.
The city was initially divided into zones of red (uninhabitable); orange (marginal); green (mostly undamaged); and white (awaiting assessment). Army patrols maintained tightly controlled cordons in the city center.
One surprise was the magnitude of the quake: at 6.3, much less than the September quake. So why was it so damaging? The answers, and the implications for engineering and building design, are significant.
The first reason was the depth of the quake, at just three miles, and its proximity to the city center. Another reason was the time it struck, when the city was full. The other main reason was the kind of shaking that occurred: Displacement was both vertical and lateral; in other words, everything shook up and down and side to side at the same time. You certainly felt it. People reported seeing tall buildings swaying more than six feet. The vertical thrust was also powerful. On the Modified Mercalli Index (MMI) the quake measured between level VIII to over X. This index, which relies on locally felt effects, much like the Beaufort scale for weather, is a more telling indication of how damaging an earthquake is in a particular area than the better-known Richter scale.
In Christchurch, peak ground acceleration (PGA) exceeded twice the acceleration of gravity in some areas – one of the strongest recordings anywhere in the world, and four times greater than the Haiti quake.
(Even the far deadlier magnitude 9.0 Japan quake, which struck on March 11, recorded an intensity VII on the MMI scale, “equivalent to very strong shaking with the potential for moderate to heavy building damage,” according to the United States Geological Survey (USGS). Most of the estimated 18,000 deaths were the result of tsunami waves.)
The next reason was the nature of the ground itself. The worst damage occurred in areas, especially in the east, with sandy soil where the water table lies only feet beneath the surface. A network of underground rivers runs beneath the city. Christchurch, before being settled by mostly British immigrants in the mid nineteenth century, was mostly swamp, used for food gathering by the indigenous Maori.