How far are the effects of earthquakes felt?

[Chocolate fireguard]

Couldn't quarry blasting also be a cause of any movement in underground boulder chokes, or other factors. I just don't see your connection of specific boulder movements to earthquakes as anything other than speculation, particularly where loss of life is involved.

About 30 years ago we were granted access to a cave beneath a quarry above Bradwell, on the way up to Bradwell Moor. Just a short pitch led into what had obviously once been a well decorated passage, but all the stal was in bits on the floor. We understood this was due to blasting, but it really was very close to the action!

 

[pwhole]

We're digging in a boulder choke very close to a quarry with regular massive blasting, but we haven't detected any movement at all. My theory is that we're too close. And our installed scaffolding is also superb

 

[Pitlamp]

Couldn't quarry blasting also be a cause of any movement in underground boulder chokes, or other factors. I just don't see your connection of specific boulder movements to earthquakes as anything other than speculation, particularly where loss of life is involved.

Yes - quarry blasting is indeed a factor - e.g. Skirwith, where the old show cave path disappears into a huge boulder pile thanks to the big quarry below the road - or all the snapped and laterally displaced stal columns in Black Rabbit Cave (Peak District).

No, the earthquake effect is not speculation. There have been too many incidences of movement having been observed in our caves immediately following recorded earthquakes for it to rely on coincidence. I think there's a topic on this somewhere else on here IIRC but really busy at the mo, so can't look. You may find it? There's also an article in Descent from 3 or 4 years ago about the huge boulder that was found to have fallen down the 15 metre entrance shaft a few days after a moderate quake. (Picture of the boulder below.)

You make a good point though; it would be useful to draw all these incidences together in one place, so folk could make their own comparisons. I'll stick it on my list of stuff to research and write (but it's a very long list so don't hold your breath). If someone else wants to do this first, fine by me.

 

[thehungrytroglobite]

At present there is an experimental seismometer set up at Ingleborough Cave (at Clapham, in the Dales).

The following resource has been flagged up on here several times in the past:

Earthquakes around the British Isles in the last 60 days

The British Geological Survey provides up-to-date information on recent and historical earthquakes, educational resources, and seismic hazard services

www.earthquakes.bgs.ac.uk

It records earthquakes and gives the location of the epicentres. A small quake (only Mag 2.5) occured near Buxton in the Peak District on 2nd January this year. So I asked the owner of the seismometer if he'd picked it up in the Dales. The answer was a definite "Yes" and he sent me this screenshot of what he'd captured.

What we can learn from this is that the effects of even small(ish) earthquakes are detectable a long way away. This may mean that loose areas in caves a considerable distance from their epicentres might need special care after a quake. (Buxton is something like 80 miles from Clapham).

I do remember a fairly dramatic event in a Dales cave that was almost certainly caused by an earthquake offshore from Grimsby (a lot further away than Buxton).

It's probably best to be aware that it's not just local earthquakes that need to be taken into account.

(Disclaimer; I'm no seismologist, just a caver with a healthy respect for loose boulders.)

View attachment 18260

Am I correct in thinking this means the seismometer picked it up about 30 seconds *after* the earthquake? By my calculations, the epicentre was 99.69km away as the crow flies. So that means, the earthquake (or tremors or whatever it is exactly that is picked up) took 30 seconds to travel 100km through the rocks...
Please let me know if this is incorrect or if I'm not understanding it properly. If I'm right, then that's really amazing and just goes to show how interconnected everything is. The butterfly effect and all that

 

[huwg]

Am I correct in thinking this means the seismometer picked it up about 30 seconds *after* the earthquake? By my calculations, the epicentre was 99.69km away as the crow flies. So that means, the earthquake (or tremors or whatever it is exactly that is picked up) took 30 seconds to travel 100km through the rocks...
Please let me know if this is incorrect or if I'm not understanding it properly. If I'm right, then that's really amazing and just goes to show how interconnected everything is. The butterfly effect and all that

Yup. Velocity of seismic waves in rock is on the order of Kms per second so sounds about right.

 

[AlanClark]

Yup. Velocity of seismic waves in rock is on the order of Kms per second so sounds about right.

Just looked it up, 4.5 to 8km/s for the different wave forms but at that 30 seconds seems a bit on the slow side

 

[thehungrytroglobite]

Just looked it up, 4.5 to 8km/s for the different wave forms but at that 30 seconds seems a bit on the slow side

Looks like that's roughly what the graph says

 

[LadyMud]

Blasting creates a shock wave with strong acceleration and velocity, but very little displa

.ficant

Dodgy internet signal in Antarctica?

 

[ChrisB]

Oops! Hope a mod can tidy that up. [Previous post deleted as requested - moderator comment]

Blasting creates a shock wave with high frequency vibration, so strong acceleration and velocity, but little displacement. Earthquake vibrations have a bigger low frequency component. Think of that as the earth moving sideways by a bigger distance, but moving more slowly. So a boulder can roll out of a normally stable position.

As I mentioned above, the low frequency carries further, like the bass music in a passing car, so earthquakes can affect boulders much further away than blasting can.

 

[thehungrytroglobite]

Oops! Hope a mod can tidy that up.

Blasting creates a shock wave with high frequency vibration, so strong acceleration and velocity, but little displacement. Earthquake vibrations have a bigger low frequency component. Think of that as the earth moving sideways by a bigger distance, but moving more slowly. So a boulder can roll out of a normally stable position.

As I mentioned above, the low frequency carries further, like the bass music in a passing car, so earthquakes can affect boulders much further away than blasting can.

I see. So is it shock waves that the graph is measuring?

 

[andrewmcleod]

'Shock waves' are still displacement i.e. something moving back and forth. Just (in this case) moving back and forth a small amount instead of a large amount. The power of the wave is proportional to both the amplitude (max displacement) squared and the frequency squared, so you can have a wave with a large amount of displacement but a low frequency (e.g. earthquake ground waves where buildings sway backwards and forwards a meter or so over a fraction of a second), or have a wave with a smaller displacement but a high frequency, and have the same power in the wave.

Seismographs measure displacement against time, so you get both the amplitude and the frequency. Ideally I presume you want to measure the displacement separately in each dimension/axis.

There are several kinds of mechanical waves that are relevant.

P-waves are longitudinal waves, like sound waves. The motion of the particles in a P-wave is backwards-and-forwards along the line of travel of the wave. They can travel through a fluid, and so are able to pass through the outer core of the Earth (which is liquid). They are the fastest seismic wave, so arrive at detectors first.
S-waves are transverse waves, like electromagnetic waves. The motion of the particles in an S-wave is at right angles to the line of travel of the wave. Because fluids have no shear strength, S-waves can't travel through a liquid (because when one particle moves at right angles to another, that doesn't pull nearby particles with it). Therefore, they do not pass through the outer core of the Earth (leaving a shadow on the opposite side of the Earth). S-wave are slower than P-waves and thus are detected later.
There are various sorts of ground wave that travel along the surface of the Earth, like waves on the sea. Gravity is important for these waves (unlike P and S waves) because it affects how the surface moves. Because they are confined to a thin layer, they don't weaken as much with distance as P and S waves which spread out in three dimensions. They are typically slower waves (although I think some are faster than S waves). I think these can be some of the most destructive waves in an earthquake?

 

[rm128]

Not sure if this will work, but hopefully you can access the following link:

WaveTypes.pptx

1drv.ms

It's a PPT that shows a couple of slides from a presentation I gave many years ago. The 1st slide shows the difference between P- & S-waves.

The 2nd is a bit more complex and shows various wave types in a 2D model. Consider this as a 2D section of the Earth, with the surface being at the top. One very significant type of "ground wave" is marked by the red arrows. Note the very strong amplitude over a shallow depth.

 

[TheBitterEnd]

This has got me thinking how bad my memory has become 🙃 I did my masters degree dissertation on forces caused by pile driving vibrations and I remember next to nothing about the magnitude of the forces.


(must be in for the most pointless post award 😂 )

 

[Nettlebed]

In general, sub-surface shaking is less than on the surface. Think of how much the top of a jelly shakes compared with the inside, away from the free surface. A large EQ in NZ (1940s?) wasn't felt much in a deep mine on the west coast, but those looking up the shaft could see daylight being eclipsed by the shaft bending.

In 2016 we had a complex magnitude 7.8 quake in NZ centered on the east coast of the South Island (Kaikoura). 150km away, a large section of the roof of the middle camp in Nettlebed fell off - would have been about 1.5x0.7x0.4m high, so probably weighing around a tonne. It fell on a sleeping platform and now has bits of sleeping bag poking out, but thankfully it was unoccupied at the time.

The vertical scale on the original screen grab is 0.0004 mm/s... a bit under a millionth of a mile per hour

 

[Pitlamp]

Thanks for posting Nettlebed; I bet you snigger at our pathetic little quakes in the UK, if you live in NZ!

I still think the cumulative effect of lots of small quakes on rickety boulders is not taken seriously enough though

 

[andrewmcleod]

A magnitude 7 earthquake releases a million times the energy of a magnitude 3 earthquake.

Of course, the magnitude isn't what matters; what matters is the intensity which depends on the depth and other factors. That said, earthquakes below magnitude 3 typically fall into the 'not felt' category; you have to normally get into 3-4 to get into the 'vibrations are similar to the passing of a truck, felt generally' category.

But apparently in 1865 there was a magnitude 2.2 earthquake in Barrow-in-Furness with a depth of about 1km that had a modified Mercalli intensity of VIII:
"Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Motorists are disturbed."

BGS - UK Historical Earthquake Database

Macroseismic data on historical British earthquakes, collected by BGS since 1980

www.quakes.bgs.ac.uk

The Barrow-in-Furness Earthquake of 15 February 1865: Liquefaction from a Very Small Magnitude Event - Pure and Applied Geophysics

—High intensity and liquefaction phenomena are usually associated only with relatively large magnitude earthquakes. An earthquake in 1865 in the northwest of England suggests that a sufficiently shallow small event can also produce liquefaction. The effects are well-documented in historical...

link.springer.com

 

[andrewmcleod]

PS my description was a generic description (copied off Wikipedia) of the types of things observed at intensity VIII, not what was observed in Barrow during the earthquake (just realised that might not be clear).

 

[Pitlamp]

This current topic on here may be related to this discussion:

Clatterway Dangerously Unstable

Clatterway has become dangerously unstable, one or more rocks in the roof that have to be squeezed past, in flat out crawls, are now loose, wobble and are likely to fall. It is strongly advised NOT to attempt Clatterway until DCA / PDHMS have inspected and hopefully made repairs.

ukcaving.com

 

[thehungrytroglobite]

In general, sub-surface shaking is less than on the surface. Think of how much the top of a jelly shakes compared with the inside, away from the free surface. A large EQ in NZ (1940s?) wasn't felt much in a deep mine on the west coast, but those looking up the shaft could see daylight being eclipsed by the shaft bending.

In 2016 we had a complex magnitude 7.8 quake in NZ centered on the east coast of the South Island (Kaikoura). 150km away, a large section of the roof of the middle camp in Nettlebed fell off - would have been about 1.5x0.7x0.4m high, so probably weighing around a tonne. It fell on a sleeping platform and now has bits of sleeping bag poking out, but thankfully it was unoccupied at the time.

The vertical scale on the original screen grab is 0.0004 mm/s... a bit under a millionth of a mile per hour

What is Nettlebed?

 

[paul]

A large cave in New Zealand: https://en.wikipedia.org/wiki/Nettlebed_Cave