Here are some facts about the earthquake which struck the Canterbury region on Saturday.
It measured 7.1 on the Richter scale. At first we were told it was 7.4. That would mean nearly three times stronger. Each 0.2 means double the strength. On average, 18 earthquakes between 7.0 and 7.9 occur each year somewhere in the world.
The earthquake occurred on a fault line that had been hidden for 16,000 years. We knew about the Alpine Fault line running along the west side of the Southern Alps but nobody knew about this one. It had been covered by gravel for thousands of years, building up pressure. The fault line is about 22 km long. Photos from a plane show the ground moved where it crossed the fault line on Saturday. A row of trees in a field was in a straight line last week. Now part of the row has moved 4m.
There was a pre-shock of 5.4 a few seconds before the main earthquake on Saturday. This probably woke people and gave them time to move to safety.
We have had about a hundred after-shocks since the big earthquake. Most are between 3 and 4 on the Richter scale but last night there were two strong ones measuring 5.4. After-shocks could continue for several weeks but will become weaker and occur less frequently.
Listen to July 18th 2009 and July 22nd 2009 to hear about the Fiordland earthquake last year.
Questions
1. Is any place in the world safe from earthquakes?
2. What other places in New Zealand could have hidden fault lines?
3. What does this mean for new buildings?