Not many details yet, but the USGS Earthquake list shows a M7.2 earthquake ~25-30 km from Christchurch, New Zealand. I’ll post more details as I find them, but here is a Google Map of the epicenter – UPDATE: the refined location puts it further from Christchurch, to the northwest. The earthquake appears to be relatively shallow – 16-30 km deep, but that is a preliminary estimate. I don’t know much about the tectonics/structure around Christchurch, but you can see that Christchurch is to the south of the termination of the subduction zone between the Australian and Pacific plates. The historic seismicity for the area implies this isn’t a likely location for a big earthquake. There is no active volcanism on the South Island of New Zealand (that I know of … anybody know better?)
And here are some of the first details from Radio New Zealand: It was also felt in other parts of the South Island and in Wellington, where it was felt in a rolling motion which lasted for about 40 seconds. Radio New Zealand listeners report particularly strong effects in Christchurch, with one resident saying the turned his house upside down, with crockery breaking. Another said the power had gone out.