Safe can be looked at in two ways. The first is the most obvious - anyone inside the building during an earthquake will be protected from injury or death. The second is financial: is the house a safe investment? Will it experience major damage and require expensive repairs, or an even more expensive rebuild? Until now this has not been seen as important, however it is likely that insurance companies may set premiums for some properties on the basis of their perception of how well it will stand up to an earthquake. Insurance excesses are likely to increase, and that will be another financial risk.
There is no officially published checklist of good and bad features to be aware of when assessing a house, which makes house hunting difficult (Maybe CERA, BRANZ and DBH could knock one up in a couple of months?). Sometimes the value of the good features are undermined by the bad. For example: timber houses with timber cladding and roofing iron are light, flexible and strong. Clearly a safe structure. Put it onto a concrete pad foundation that has a shape prone to cracking (anything that is not a square, rectangular or circular e.g. T shaped foundation pad, L shapes, H shapes) and the risk of structural damage is increased.
For existing buildings it cannot be assumed that because they have survived more or less intact so far each one is as good as the day it was built. Many older styles are at risk because of construction materials and techniques, weakened structure (rotting timber framing, inadequate foundations) and lack of maintenance. Also consider that a "Green Stickered" house is not necessarily a safe or sound one. The Green sticker only means the building was considered safe to enter and appeared to be in much the same structural condition as prior to the earthquake. This conclusion was reached as the result of a very brief inspection, often without going inside the house.
All styles of building have a finite life and a few may be near their use by date. This is likely to be a factor mainly where you are attracted to older style houses made out of bricks or other forms of masonry. See here about the Royal Society of New Zealand's paper on building deign in the wake of the Christchurch earthquakes and the section on a building's "earthquake life".
New houses designed and built to the new earthquake standards can be expected to be safe, however "exotic" designs such as lots of glass or large and non-square concrete pad foundations are likely to be areas at risk of damage. Some of the plans currently advertised by building companies have exotic designs that may attract higher insurance costs.
There are qualified people who can do a better job of assessing a property than we can, and banks and perhaps insurance companies will want to see reports from them before they agree to do business with us. However that costs money, so a certain level of DIY assessment skill will be needed to weed out the obvious risks. The following links take you to good places to start. Some of them are overseas and so legislative requirements will be different, however the basic information remains valuable. If you have time to read only one then the BRANZ initial observation is it.
- Building Research Association of New Zealand (BRANZ) quoted here on initial observations on what failed and what survived the February 22nd earthquake.
- Royal Society of New Zealand's brochure THE CANTERBURY EARTHQUAKES: Answers to critical questions about buildings A bit light on useful stuff, it does give some handy background. Here.
- Canadian Government's website How would your home stand up? is designed for homeowners to identify structural issues in their own homes. Here.
- California's Association of Bay Area Governments website Housing Vulnerable to Damage in Earthquakes looks at construction types and the type of damage that might arise. Here.