I know earthquakes are no laughing matter. I know because a few days ago I was woken at three in the morning by a huge roar that sounded like a lorry had driven into the bedroom, and the house shaking so badly that things fell on the floor. I thought it was all going to collapse on top of me and I admit I was scared. I didn’t get much sleep after that.
I was frightened even though I’m used to experiencing smaller quakes, that sometimes make the house shake for a couple of seconds. This was something much much bigger – as it turns out, it was 4.9 on the Richter scale, which makes it a ‘light’, but very nearly ‘moderate’, size quake.
Because of mining activity in Kiruna we’re used to hearing an explosion, and sometimes vibration, after one every night when the mine blasts the iron ore. In addition there are occasional small quakes, when part of the rock in the mine falls. The company apparently can’t say when these falls will happen, but they say they ‘expect’ them, so they’re called ‘expected seismic events’. They are more numerous nowadays, as every year the ground around Kiruna comes closer to collapse. A tremor can come at any time of the day, and some come with a bang as well as a shake, so they are always a bit of a shock, for a second.
But this week’s earthquake was presumably not ‘an expected seismic event’. The mine is still partly closed, almost a week later, and investigations continue. Apparently there was quite a lot of damage there, though thankfully no-one was hurt. We’ve also experienced a few more bangs and shudders in town over the last few days.
The reason the small quakes happen, LKAB explain, is that the iron ore sits in a chunk that is set at an angle to the ground, so as iron ore is removed from beneath there’s a ‘hanging wall’ left above it, and eventually parts of this wall fall. It’s a point of wonder, or even suspicion, that long ago the company’s office was built just the ‘right’ side of where the ore sits, even though at that point mining was above ground and little was known about the position of the ore underground.
Although earthquakes are no laughing matter, there is just the smallest shadow of a smirk on my face as I write this. It turns out that the epicentre of this sizeable, unexpected, earthquake was not under the ‘hanging’ wall but under the relatively stable ‘footwall’ – right beneath LKAB’s office.