First, if you have some familiarity with what happens to building during earthquakes, you might want to skip my ramblings and go straight to the pictures.
Third, a disclaimer. The closest I’ve ever been to New Zealand is The Navigator.
Finally, I’m sorry that this is so clinical. Unfortunately, the only way structural design really moves forward is by analyzing failures and that needs to be done before the sites are cleaned and the evidence hidden or destroyed. It’s second nature for me to do this and no disrespect is intended.
Seismic loading is fundamentally different from other ordinary structural loads, as it depends on the design of the structure itself and not simply outside effects. Gravity loading is based on the building’s dead weight and the load of its occupancy, regardless of whether the building’s weight comes from concrete slabs or fill over wood joists. Wind loading depends on the wind speeds expected and the building’s shape, but is not affected by whether the wind’s sideways pressure is resisted by a frame or by masonry walls. Seismic loading, on the other had, is determined both by the local ground accelerations that are expected (measured as a percentage of gravity acceleration) and by various measures of how stiff the building is. Two buildings that look similar but have different structural types (one a steel braced frame, the other a concrete moment frame, for example) will have different design seismic load because they will react differently to ground acceleration. More flexible frames generally perform better because their flexure absorbs energy while stiff buildings are damaged by their own rebound.
Beyond the analysis of how buildings resist sideways acceleration, the greatest issue is quake design is continuity. Brittle materials (unreinforced concrete and masonry) develop cracks that break single elements (walls or slabs) into smaller, weaker elements. Poor connections between elements (wood joists pocketed in, but not tied, masonry walls, for example) lead to the elements moving differentially because of their different inherent frequencies.
On to the evidence:
The Anglican cathedral, first page of AK’s link. The stone spire has collapsed where it changes from a relatively rigid base to the more flexible portion where the large windows begin. Note that the small cross on the sanctuary gable end has also collapsed. In modern construction, the masonry would be a skin, not the structure, so its stiffness would be less important. There’s little that can be done to retrofit this type of structure short of base isolation (putting shock absorbers below the entire building) or inserting a steel frame within the tower.
The CTV Building, third page of AK’s link.
As far as I can tell, a reinforced-concrete frame that failed in the worst way possible: the columns gave out. That type of failure leads to pancaking (officially known s “progressive collapse”) and is a symptom of lateral loads that were simply too large for the frame capacity. This could be because of reduction in capacity from last fall’s earthquake, an older building designed under less stringent codes, codes that underestimated the local ground acceleration, or misdesign. The last is possible, but least likely.
Knox Presbyterian Church, page 4 of AK’s link: Classic failure of the connection between the masonry gable-end walls and the wood truss roof. The roof appears to be in good condition, but there was (a) not enough of a tie between the walls and the truss members and (b) no ductility in the unreinforced masonry.
The Catholic cathedral, fifth at AK’s link:
The fact that the damage appears to be confined to one side of the building is odd and may indicate differing soil conditions across the site. In any case, the relatively weak but heavy towers survived long enough before collapsing to transmit movement to and damage the walls below. Again, classic damage to unreinforced masonry.
The Pyne Gould building, ninth at AK’s link.
Note that the columns and beams at the edge of the collapse are straight. The failure was in the beam-to-column connections, which were not sufficiently ductile for the load. Otherwise similar to the CTV building.