How far are the effects of earthquakes felt?

[Pitlamp]

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.)

 

[huwg]

I personally wouldn't worry about a magnitude 2.5 in Buxton having any impact in the dales. Unfortunately I don't have any numbers to back this up, but I have worked on earthquakes for a few years. More of a hunch than being able to throw numbers at you. One scale that might be useful to give some idea of what the shaking really means, is the mercali intensity scale. This gives the amount of shaking but in a non numeric way specialist friendly way. From memory part of one of them is "plant pots may fall over".

For a bit of perspective, commercial earthquake hazard models don't bother modelling eqs with magnitude less than 5, and an increase in magnitude of 1 (e.g. 4 to 5) is approximately a 32 fold increase in energy release by the earthquake, so a Mag 2.5 is really small. I appreciate buildings and dodgy boulder chokes are not the same though! Perhaps there is some comparison to be made with buildings that have already collapsed and then are damaged by further smaller aftershocks?

From a more finger in the air approach, I have sat on a fault that recently ruptured and I think the smallest aftershock we felt was high Mw 3, but most that we felt were 4+. We would have been much less than 10km from the hypocentre of these events that we felt. Equally 20-30km away from a Mw 6.5 shook me enough to accept I may have had my chips.

This is not to say that earthquakes can't cause issues in unstable chokes. The accelerations recorded here are absolutely tiny though. For information, the Mw 6.5 produced accelerations in excess of 9.8 m/s (note the units!) close to the epicenter in a sedimentary basin.

I think it is great to get this instrumentation out there in the UK, I am a great believer in getting more quality data and keeping it open! Hope that is useful anyway.

 

[Pitlamp]

Thanks Huw - I never intended to suggest that specific (small) earthquake near Buxton would have caused problems here in the Dales. Just that the effects of earthquakes generally may be more widespread than some cavers appreciate. The other event I mentioned (offshore from Grimsby) dislodged a huge boulder which fell down the entrance shaft at Jingle Pot. It's written up in an issue of Descent. Sadly, the fatal accident to a northern caver in Bull Pot Kingsdale a few years ago was likely to have been related to a quake associated with the nearby Dent fault.

The other aspect perhaps worth bearing in mind is the cumulative effects of earthquakes. If you have a pile of books on a table and give the table a gentle shake, the books will stay in a pile but move about a bit so the pile is a bit less neat. The next shake will move each book relavive to its neighbouring books a bit more - and so on. Eventually even a very gentle shake of the table may topple the whole pile. In the same way, I suspect (as a non seismologist) that a similar situation applies in boulder chokes; a very small earthquake may have effects out of all proportion with the degree of shaking caused by that single event.

Bottom line is that a lot of cavers don't seem to take into account these small British quakes and a few of us feel that it's no bad thing to raise awraness.

So . . . take care folks!

 

[Bob Mehew]

Based on Huwg's comment about forces involved, surely the forces that one caver will apply to a boulder whilst passing by, be far more than a Magnitude 2.5 earthquake?

 

[Pitlamp]

Based on Huwg's comment about forces involved, surely the forces that one caver will apply to a boulder whilst passing by, be far more than a Magnitude 2.5 earthquake?

Depends how clumsy you are!

 

[ChrisB]

Something to consider is the the low frequency component of earthquakes travels furthest. The low frequency involves the greatest movement, but with slower acceleration and velocity. So close to the focus, there is more chance of fracture of the rock, further away there is more risk of balanced boulders being moved.

I'm an earthquake engineer not a seismologist (I designed buildings to stay up, when subject to the shaking the seismologists predict) but that does mean some understanding of the motion.

 

[huwg]

For a comparison you could hit the ground next to the seismometer with a hammer. Obviously it will be different frequencies but might give you some idea of the accelerations that are being observed. I'd be curious!

Also I'm not really a proper seismologist so if one does turn up they can probably be more helpful!

 

[alanw]

For a comparison you could hit the ground next to the seismometer with a hammer.

Or get a very large number of Mexican football fans to jump up and down:

Did Mexican World Cup fans' celebrations shake the earth?

Fans jumping for joy over goal caused an artificial quake in Mexico City, scientists say.

www.bbc.co.uk

"During the game, the Mexican team managed to score 35 minutes and seven seconds in, at this moment our monitoring systems detected a seismic movement with an acceleration of 37m/s2 picked up by at least two sensors inside Mexico City. These were very probably produced by the massive celebrations,"

 

[Jenny P]

I think I'm correct that the fall of boulders in the Canyon section of Little Neath River Cave, some years ago now, was attributed to an earthquake.

I can't recall the details or the year but do remember going along the passage before the event and admiring the high and relatively narrow "canyon". Then going along the same passage some time later and being awed by the mass of boulders which had fallen across the passage higher up but had hit the opposite wall and so stayed perched above our heads.

Can anyone with a better memory than me tell me when this was and if I'm correct that it was thought it could have been due to an earthquake.

 

[ChrisB]

I don't know the cave but the nearest event I can remember that was 'big' for the UK was at Bishop's Castle in 1990, magnitude 5.1.

Not particularly close but could have had an effect.

 

[andrewmcleod]

Without a robust statistical analysis of the average rate of observed rock movement and subsequent comparison to measured earthquakes, trying to work out the effects is subject to significant confirmation bias. Any individual example is meaningless without some estimate of the background rate - particularly since smaller earthquakes are (relatively) common.

 

[Bob Mehew]

The BGS web site reports

DATE 02/01/2024
ORIGIN TIME 17:30:43.5 UTC
LOCATION 53.279 -1.940
DEPTH 2 km
MAGNITUDE 2.5
LOCALITY BUXTON,DERBYSHIRE

see https://www.earthquakes.bgs.ac.uk/earthquakes/recent_events/20240102173030.html#page=summary

If you then click on the Macroseismic tab, you will see (if I have interpreted the map correctly), 4 reports that the event was felt located around Buxton. (They need at least 5 'felts' to calculate an intensity which is then displayed on the BGS page for Earthquakes around the British Isles in the last 60 days, see https://www.earthquakes.bgs.ac.uk/earthquakes/recent_uk_events.html under the heading "Int".)

That suggests there was a sufficient strong earthquake at Buxton to have 'moved the earth'. Does anyone know of any observations from the Peak District caves and mines since then which might indicate such an impact?

 

[Pitlamp]

Without a robust statistical analysis of the average rate of observed rock movement and subsequent comparison to measured earthquakes, trying to work out the effects is subject to significant confirmation bias. Any individual example is meaningless without some estimate of the background rate - particularly since smaller earthquakes are (relatively) common.

Indeed - but how do you also factor in the cumulative effects of multiple events?

There have been topics on here previously in which collapses in caves have been found soon after earthquakes in the "vicinity" (this word being relative, depending on the magnitude of the quakes in question). I suspect there's no shortage of examples.

 

[Pitlamp]

Something to consider is the the low frequency component of earthquakes travels furthest. The low frequency involves the greatest movement, but with slower acceleration and velocity. So close to the focus, there is more chance of fracture of the rock, further away there is more risk of balanced boulders being moved.

I'm an earthquake engineer not a seismologist (I designed buildings to stay up, when subject to the shaking the seismologists predict) but that does mean some understanding of the motion.

Hm, I foresee an interesting conversation and some brain picking in a certain tent on a certain hill, later this year!

 

[braveduck]

Thanks Huw - I never intended to suggest that specific (small) earthquake near Buxton would have caused problems here in the Dales. Just that the effects of earthquakes generally may be more widespread than some cavers appreciate. The other event I mentioned (offshore from Grimsby) dislodged a huge boulder which fell down the entrance shaft at Jingle Pot. It's written up in an issue of Descent. Sadly, the fatal accident to a northern caver in Bull Pot Kingsdale a few years ago was likely to have been related to a quake associated with the nearby Dent fault.

The other aspect perhaps worth bearing in mind is the cumulative effects of earthquakes. If you have a pile of books on a table and give the table a gentle shake, the books will stay in a pile but move about a bit so the pile is a bit less neat. The next shake will move each book relavive to its neighbouring books a bit more - and so on. Eventually even a very gentle shake of the table may topple the whole pile. In the same way, I suspect (as a non seismologist) that a similar situation applies in boulder chokes; a very small earthquake may have effects out of all proportion with the degree of shaking caused by that single event.

Bottom line is that a lot of cavers don't seem to take into account these small British quakes and a few of us feel that it's no bad thing to raise awraness.

So . . . take care folks!

Also you have to take into account the depth of the Quake .They vary in depth so greatly ! The shallower it is ,the more likely some effective damage locally.

 

[AlanClark]

Here's the link for the list of earthquakes of significance for the UK from the BGS, https://earthquakes.bgs.ac.uk/earthquakes/UKsignificant/index.html 2008 near Market Rasen was the last above M5, I remember the ground shaking reports map being quite interesting for that one, plenty of reports from local to Lincs but a swath up the west of the Pennines at the boundary of the Carboniferous and Triassic strata and a far south as Bedford(at least), certainly got my attention when it shook the living room at home. Showed that the felt effects can occur at a reasonable distance.

 

[Badlad]

How does quarry blasting affect nearby caves? Especially the cumulative effect of regular blasting. Do these register locally - on the seismometer at Ingleborough Cave for example.

 

[Pitlamp]

I felt that Market Rasen event here in the Dales, as I just happened to be up late. It made the glass panels in the barograph jiggle, alarmingly.

Yes, seismometers certainly do pick up shock waves from quarry activity. However it ‘s a very different signal, so distinguishable.

 

[Badlad]

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.

 

[braveduck]

How does quarry blasting affect nearby caves? Especially the cumulative effect of regular blasting. Do these register locally - on the seismometer at Ingleborough Cave for example.

Was once in Mud Hall GG ,when there was a very big blast at Horton quarry , Certainly heard it !