When is the High Peak Radio slot? I might try to listen in, if I can get it in Sheffield.
Papa and Mama Haku were about 15 miles from the epicentre and described it as like an enormous train passing very close to the windows. We had a 5.6 out here earlier in the year and if you're not used to them (which I'm not) its a pretty weird feeling.
It's not like I'm a geologist, but what I recall is that limestone faults and fractures far more easily than sandstone (it's full of cracks, joints and bedding planes already, it doesn't take much to move bits apart), so it transmits a lot more movement in earthquakes -- rather than the energy being absorbed by breaking the stone, the stone breaks with much less energy and passes the rest on. Sandstone, while not as immobile as granite, is a more solid than limestone.
If you're sitting on top of granite, like we are, you wouldn't feel a thing. So much energy is expended against the rigidity of the stone it doesn't go far.
Extensive mines might well affect how easily the ground moves. The general rule of thumb I recall is, the less solid the ground, the more easily earthquake motion is transmitted. It may be that having a lot of relatively empty spaces which allows stone to squish (yeah, that's a technical term) or displace more easily would end up transmitting more of the motion to the surface.
When buildings are made earthquake-proof, it's not by building them more rigid, because that means that more of the energy of the earthquake is expended in the structure of the building itself. It's done by making sure that the buildings have several degrees of free movement, so that they can essentially move and sway and pass the energy through the structure, instead of stopping it there. My guess is that mines would do that for the ground -- give the area more chance to pass the movement on through.
For what it's worth. You might want to check that with a real geologist, though.
~Actually, I just looked it up. I have the idea correct, but the reasons at least partly wrong. Hard stone has a high impedence, which results in a high velocity and low amplitude of shear waves. The softer the stone, sediment or soil, the lower the impedence, the lower the velocity of the shear waves, but the higher the amplitude -- much more side-to-side, which is how the buildings catch it.
Neil - I'm only on every other weekend, at about 1 o'clock on a Sunday morning! Giant dino-eating frogs have been scheduled in for the morning of 10 March, so that means the Market Rasen earthquake will be 24 March at 1am-ish! We keep it current but not very tightly topical, just in case the schedule changes. Reception might be a bit poor (we had issues in Castleton), but looking at the HPR website they're now advertising very prominently on their front page that you can listen live online!
Luna - that all makes sense. I'm sure when the US gets online they'll be able to confirm this. I was aware that flexibility was all important for earthquake-proofing buildings. A mine shaft would certainly provide breathing room.
As I understand it, most of the collapses that cause this sort of earthquake occur quite deep, so I'm not sure that there's too much risk of a house collapsing into a sinkhole as a result.
I seem to recall that most differences in shaking magnitude on bedrock are more controlled by specific local geology (existing faults, landforms focussing surface waves etc.) rather than the particular rock type. Plus, sandstones and limestones have a large range of different competencies, so I'm not sure whether you could say that there's a clear systematic difference between the two.
Soft sediments do amplify surface waves. Topography also has an effect. If you are a sucker for seismograms I've another on my blog ...
http://hypocentre.wordpress.com/...sen-earthquake/
We have quakes up here all the time, and they're never that news-worthy. And it's funny to say that--they're usually in the high-4 to mid-5 range, and they generally occur in the Interior, so we just get the outer ripples in Anchorage.
I remember the 2001 'quake as I was awake at the time wondering around the house in Cambridge. I like to think it was responsible for my falling down a step but at the time I thought I was just tired and uncoordinated. Which is really a more likely explanation as it was nearly 1am.
Cool - thanks everyone for the explanation. Think Mum will be happier now!
Hypocentre - I am dead impressed that you felt the p-waves first. Now, everyone who's experienced it thinks I'm completely nuts for being jealous, but I really want to know what that feels like.
Spoke to my dad last night, who was driving home when it happened, and he commented that there was loads more roadkill on the motorway than there usually is. Just a load of terrified animals not knowing where to run next?
I was up late lesson planning and had just gone to bed, so was awake and felt it.
It felt a bit like when upstairs' / next door's (?) washing machine is on and shaking the lounge, but stronger. Also stronger than when trains went past and shook my old house near the viaduct in Durham. However not so strong that I was afraid the roof was going to fall in: the most dramatic thing was the doors rattling.
Ah, so maybe Ben did wake up with it - he said he woke when you got into bed, but seemed to have put it down to just you getting into bed doing it. What a strange sensation it must have been. Still feel gutted that I missed it.
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