The M6.7 or 6.6 Northridge California quake had accelerations near 1 g and partially collapsed a parking garage. It nearly collapsed a 20 (?) story steel frame building (Heaton and others, 1995 (?)). If the shaking had gone on for a few more seconds, that building might have collapsed (along with others?). The strong shaking was about 20 seconds, maybe a bit more in deep sedimentary basins. You might expect that The Christchurch quake, being smaller, would have only shaken 15 seconds or less (unless there is a deep sedimentary basin beneath downtown). So, why the collapses?

At the Sept 2010 SCEC meeting (I think) or possibly a TV show, or both, they talked about construction in Chile. They showed a very large shake table with a scale model of a tall building (the model may have been 20 m-high). They showed the difference between construction where the concrete was crushed and not held in place: such a building, if it did not collapse, would probably have to be torn down.

But, they also showed a video of a model building where the concrete pulled completely apart on some supports, and then when the shaking was done settled back down in place. Maybe all you would see was a big fat crack with a little crushing around the edges. Or, if inaccessible, you might see nothing.

HD spent almost 6 months on the 5th floor of an apartment building in Santiago Chile from June to Dec 2010, and they felt really strongly some distant aftershocks. I wondered whether the building might have been damaged in the M8.8.

I wonder if the buildings in Christchurch were damaged in the Sept M7.1 and the M5+ aftershocks. Did this cut down on the amount of shaking a building could withstand before collapsing? What do they do next for any steel-frame concrete buildings that might still be standing and usable? Other construction?

These issues will come up in California again one of these days.
Chris