Building damage can be divided into two broad categories: damage that was caused solely by earthquake shaking; and damage that resulted from ground deformation including liquefaction, lateral spreading or landslip.

While shaking damage to dwellings has been observed on the flat, the February and June 2011 aftershocks in particular caused significant shaking damage to hillside houses. The observed high vertical accelerations were responsible for severe damage sustained by tile roofs and brick veneers, and unreinforced foundations were often severely cracked.

Liquefaction effects on buildings

Liquefaction-induced ground movement has caused stretching, hogging, dishing, racking/twisting, tilt, differential settlement, differential displacement or any combination of the above to buildings.

The severity of the damage is dependent on the damage type, the type of building, the building geometry and the amount of foundation movement that has occurred.

Differential settlement cases

Parts of the foundation settle by different amounts resulting in uneven slopes in the floor. Differential settlement is the most difficult behaviour for which to set acceptable limits.

Lateral stretching

Lateral stretching of a foundation may occur when the ground beneath it spreads laterally during the ground shaking. This action is often accompanied by liquefaction and associated ground settlement. If the floor plate of the dwelling is not strong enough, then the lateral spreading will cause an extension of the floor plate (i.e., the concrete floor slab will crack or the timber floor will fracture generally at joints between framing members). Combinations of any of the above settlement cases and also combinations of settlement and stretching are possible.

Source: Ministry of Business, Innovation and Employment