Interesting discussion. I note Chris Dempsey's reference to the blog I posted a couple of days ago about the Christchurch earthquake risk assessments done by EQC and scientists at NZ Institute of Geological and Nuclear Science in the late 1980's and early 1990's. Here's a concluding extract:

"....This information cites four earthquakes that did severe damage in and very close to the City of Christchurch (1869, 1901, 1922 and 1987). Based on this information the EQC report predicted a return period for another equally devastating earthquake of 55 years. Given the Cheviot earthquake in 1901, and then the Christchurch February 2011 earthquake - exactly 110 years later - with a couple of big ones in between, perhaps their predictions are conservative. New Zealand was advised this earthquake sequence was likely. So whose fault is it that authorities didn't act?...."

You can see this blog at: http://joelcayford.blogspot.com/2011/06/faulty-thinking-about-christchurch.html

There is another post immediately following that one about plate tectonics and what's happening. User friendly.