When an earthquake strikes, the normal, and reassuring, sense of terra firma vanishes instantly. This is caused by not one, but often two, potentially devastating impacts of the earthquake: the shaking itself and, if the subsurface is unconsolidated and water-logged sand, liquefaction. I have written about this phenomenon, the complete loss of strength and support for the foundations of buildings, before, in the context of the Loma Prieta and Haiti earthquakes. And now, through one of those odd series of connections that occur far less rarely than one expects, I have received a very personal account of liquefaction associated with the recent Christchurch earthquake.
I’m working in Jakarta at the moment, and recently got together with an old geologist friend whom I hadn’t seen in a long time. As the catching-up conversation hurtled from one topic to another, it turned out that, until not long ago, he had lived in Christchurch, and that a mutual colleague still does - and has experienced major liquefaction events in both the earthquake of last year and this latest, tragic, one. Specifically, sand volcanoes had erupted on his property in both earthquakes. This, of course, led me to exclaim (for once, that word seems quite appropriate) that this topic was of some considerable interest to me, and I have now been in touch with my erstwhile-colleague in Christchurch. The account and the photographs that he has sent me are extraordinary, and he has kindly (given how much else of somewhat greater importance he has on his mind right now) allowed me to reproduce them here.
Admirably, he ran a brewery in Christchurch – I say “ran” because it was only a couple of blocks away from the collapsed building that tragically trapped so many people. Here’s how he described the event:
I was in the brewery weighing out hops. The building is still standing, though we have sustained some structural damage, parapet walls down and large cracks in the walls (not to mention all the fallen bottles and barrels and broken glass) was all that I could see before we evacuated the area. All the customers and staff were uninjured though it was terrifying and the mess that I glimpsed before I left is much much worse than last time.
In a later email he describes how he has had to make his staff redundant and has “No idea when if ever we will be allowed back into the CBD.” The Central Business District (CBD) remains cordoned off.
As for the liquefaction and sand volcanoes at his home, the effects are incredible:
It seems churlish in a way to moan about the state of our garden while people are still being pulled from the rubble, but ironically just this last weekend Lisa's parents finished cleaning up the last of the sand from the September earthquake and now it is all back in EXACTLY the same places. Another 100 tons or so to remove! Our wooden house has survived well again, although I am sure it must have sunk and bent a bit more, but we can still live in it.
Aftershocks continue to shake the house as I write this email. I still have dust and bits of masonry in my hair!
“Another 100 tons or so”! But then, when you look at the extent of the eruptions, this seems like a possible underestimate. Here are a couple of photos of the last clean-up, followed by more of the current mess.
I wrote to my old colleague that “While I deeply regret the spontaneous deposition of tons of my favourite material yet again in your garden, your photos are fascinating” – I hope readers will agree. I guess that geologists must be phlegmatic about natural processes impacting their lives, but this would push me (and most certainly my wife) to the limit. So thanks, Martin, and very best wishes for re-building (and re-clearing).
[See a compelling video of liquefaction in a wheelbarrow as the results of arenaceous volcanicity are cleaned up in Christchurch; Chris Rowan at Highly Allochthonous has a kept an excellent series of posts on the earthquake updated, here and here. See also an excellent and graphic summary of Christchurch liquefaction as a pdf by the local government, and New Zealand Sciblogs have a description of the process and of sand “boils.”. The New Scientist has a good summary of New Zealand tectonics. As for what might conceivably be done to reduce the effects of liquefaction, see my post on the remarkable skills of bacillus pasteurii.