After last night's 'quake, which I felt most strongly, I can't understand how this much shaking results in no discernible damage, not even new hairline cracks.
Is it because the whole "plate" on which the area is situated moves in one lump, so to speak? So the whole house moves this way and that, but doesn't actually flex within itself? Is that how it works?
Or is it down to build quality and would older or newer properties be at greater risk?
MM
I doubt it's build quality, otherwise the new estate a couple of miles away from here would have been flattened
I'm as far from a guru as it gets but one opinion I can pass on about really old houses is that when bombs dropped in WW2 they just sort of lifted up as separate bits and settled back down again in roughly the same place :-)
On Wed, 27 Feb 2008 09:11:16 +0000 someone who may be MM wrote this:-
That's partly it. If one part of the land on which the house is built moves one way and the rest of the land moves the other way then the house will be re-arranged.
At the low energies of this relatively minor earthquake. At higher energies more buildings are not able to cope with the shaking.
Not all buildings are the same, but as a generalisation the better built will stand up better. In an areas prone to large earthquakes some buildings have now been built with active systems to lessen the effect of earthquakes, for example some tall buildings in Japan.
AIUI it's a combination of local soil conditions & foundations (how much quake energy is transferred to your building), the natural resonant frequency of the building compared to the vibrational frequency of the quake (certain sizes and shapes of buildings are more vulnerable) and the strength and elasticity of the building's structural elements and the connections between them (timber good, unreinforced masonry bad).
As another poster pointed out, this is a very small shake - insufficient to crack even old mortar on poorly maintained buildings - apart from a few chimneys that were probably in a state where they could have been lifted off the building by hand. (And I'm rather glad I dismantled a chimney in that condition within the last few months).
You would be surpised at how elastic soil, and even concrete steel and brick structures can be, let alone timber frames.
I remember watching the MOT over the clay soil here going down about 4" when a 30 tonner backed in the drive, and then coming back up again afterwards..
A couple of years ago I was watching the demolition of a large Victorian building (Derby Carriage Works C Shop, for those who knew it).
The gable ends were brickwork about 500 mm thick and 15 m high, and the demolition excavator had to work very gingerly to bring it down piecemeal in the right place. Simply nudging near the apex caused the entire wall to flex to and fro quite alarmingly. Perhaps it was largely held in place by gravity.
Chris
Yes, I realise that this must be the case, but a brick building just seems so solid when one is standing right next to it.
MOT?
MM
Well if it used lime mortar, it certainly wouldn't have been held together by that ;-)
Largely I suspect the way blockwork developed, was that early man piled up stones , then plugged the gaps with sand, then discovered the sand washed out. But a bit of lime in the sand stopped it washing out.
Which is really all lime mortar is. Sand that doesn't wash away.;-)
Ministry Of Transport Type I road base material. Essentially crushed limestone.
Even more solid when it falls on you :-(
Mary
>
This is why timber houses are preferred in earthquake zones in the states. Thing is, if you are also in a tornado/hurricane zone, then your light timber house entirely blows away. Simon.
Yes, I can believe that. But having experienced that tremor as the first in my life, I just marvel at the way mother nature can gather enough energy to wobble an entire country a few inches this way and that. It is to me as fascinating as trying to explain television to Charles Dickens would have been. There is nothing comparable that is man-made. Explosives would merely result in devastation. Imagine trying to mount, say, a whole village (just a village, not an entire country!) on some kind of manufactured plate, then inventing some device that could extert the energy needed to shift it like an earthquake can. I don't think human beings could ever devise such a device. Totally mind-boggling to me, that tremor last night.
MM
on 27/02/2008, MM supposed :
The Brum one a few years ago, despite being further away and not as big, seemed to be much stronger here. That one managed to wake me up from a very deep sleep and gave me quite a start.
This one managed to wake me, but I was just at the dozing off stage so I caught the whole thing. A quiet rumble initially which gradually built up then just stopped.
I suppose the difference might have been the subterrainian rock strata and the direction it lays in.
Probably my most startled moment from a natural phenomena whilst in a deep sleep - was several years ago when the church steeple a 100 yds away was severally damaged by a lightning strike. I actually think I was so startled I lifted off the bed :-) The pulse from the strike caused us quite a bit of damage to our electronics.
Totally mind-boggling to me, that tremor last night.
Sorry to highjack your post, but am I alone in 'Tremors' being my favourite all time film?
I agree entirely with your sentiments on this. The amount of energy needed to do such a thing is incredible! I thought about a simple man-made task say of shoveling some soil for a few feet - then to think of most of the whole country wobbling for a few seconds - gives some practical idea of the scale/energy involved. Amazing.
Almost inspired me to start building one of these...
Well, interestingly enough, not necessarily.
If you're trying to shift rock, eg for road building or quarrying, you use explosive. Put it near the surface and you get a bang and lots of little bits of rock flying anywhere - but not much volume shifted. Put it deep, and it looks a lot less spectacular - but shatters rather more rock, provided you put enough in.
So if you go deep enough, you'll end up with a shock wave, ie earthquake. See underground nuclear tests - how do you think they're detected? Locally stuff is pulverised, but futher away it's more like what you want.
But yes, it is an interesting reminder of the scales involved in plate tectonics - which is significantly larger than what we experienced last night.
cheers, clive
On Thu, 28 Feb 2008 02:26:30 -0000 someone who may be "Clive George" wrote this:-
The same was true in Ye Olden Days with depth charges. If it made a big spout of water then it was likely to have been set off for a film or photograph. Ones used against submarines made little disturbance on the surface of the water.
First earthquake I noticed was the Lleyn Peninsula one back in 1984. The building I was in (had got in to work very early) was a concrete 1950s thing in Fleetwood. When trolleys were pushed up and down the corridors the whole building sounded (and to a very minor extent, felt) like an earthquake. Funnily enough, the earthquake sounded and felt like a trolley... but a very big one. :-)
I saw it many years ago knowing absolutely nothing of it beforehand. Brilliant. (Not so sure I would wish to view it again - case of right film, right time.)
Damn sight better than Dune. Which should have been sunk in Waterworld.
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