M7.1 Darfield Earthquake

Did I mention violent gloaming? Well, maybe the Earth misheard and decided to unleash some violent moaning. On Saturday morning at 4.35am, Christchurch was blasted by a M5.4 foreshock followed by a M7.1 mainshock occurring on a previously unmapped and unknown fault. The epicentre was about 30km West of our house at a depth of 10km. The earthquake generated the highest peak ground acceleration (1.25g) ever recorded in New Zealand; peak accelerations where we lived were at least 0.25g- that's about 2.4 m/s.

I knew that because the first arriving P-wave wasn't big enough to throw us out of bed, our house would be ok. I researched the effects of previous earthquakes on Christchurch, looked at liquefaction maps of the city, and studied the subsurface geology before we bought our house. We chose to live in the suburb on the highest "flat" ground, and almost all of the houses in our area are undamaged. We regained electricity in 5 hours and have had no water or sewage problems.

So, knowing our house wouldn't fall down when those first P-waves arrived was a big relief, and we just stayed in bed. The surface waves and S-waves generate a violent rocking and rolling and twisting motion because they travel in 2 directions orthogonal to each other. These waves made our walls shake all around and it felt like we were caught in a washing machine. All the doors and windows shook, plates and cups rattled in the kitchen, and my coffee percolator fell and smashed into a million pieces, but the dog didn't make a sound, and Anna never woke up. The shaking lasted a long time, quietly dying away. Since then we have had over 90 aftershocks, many of them stronger than M5.

Estimates are now that as many as 3 in 5 houses in Christchurch need major repairs or demolition, but we are fortunate to live where we do. A lot of people are suffering from trauma and still live without power or water or roofs over their heads. Trauma occurs when the unexpected happens, especially when it happens suddenly. I think about earthquakes every day; I study their effects. We have always thought about what would happen to our house in an earthquake. That's why we removed the central brick chimney 2 years ago and replaced it with a structural timber beam. Timber houses shake, but they usually do not fall down.

Schools are closed for the week, so while other geologists are eagerly mapping the new fault and deploying seismometers to detect aftershocks, I am at home taking care of Anna. We organized her crafts today and made some new Shrinky Dink bracelets, thankful to carry on as normal. I suppose I would be even more jealous of the other geologists out mapping and gaping if I didn't have pneumonia. Happily, I had enough energy yesterday morning to see the rupture trace near Darfield. Here are some photos of the fault.






Thanks so much to everyone who has sent best wishes and said prayers; we are so thankful not to have been greatly affected by this dramatic event.