Author Topic: Earthquakes and boulder chokes  (Read 6537 times)

Offline Pitlamp

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Earthquakes and boulder chokes
« on: March 08, 2011, 07:10:27 pm »
Interesting experience at the end of Malham Cove Rising on Friday last week. Many will know that this has been an ongoing underwater dig for years. We're working underneath a wide expanse of roof which is crazed with cracks and has a habit of falling down in big lumps, hence all the scaffolding work. The slabby limestone overhead is closely associated with the Langscar fault, which is exposed at surface further up the Watlowes valley above the Cove.

The normal technique is to give the roof a right good donging with a steel scaff pole immediately beyond the last section of scaffolding completed then, when it's safe, start work on the next section of scaff. This is then repeated as necessary . . . .

The roof just in front had been tested very carefully and we were happy to work underneath it extending the scaff but two of us had quite large boulders fall onto us (separately) on Friday, together with numerous small fragments. It occurred to me that this was the first dive since the Ripon earthquake on 3rd January and I'm wondering if that has actually disturbed the roof enough to make it unsafe.

This part of the roof has now been scaffolded (today) but has anyone else ever experienced problems in loose areas following an earthquake? If this can happen I think perhaps we ought to try and monitor earthakes before diving at Malham in future - and perhaps be a bit more aware of it in other potentially loose ground in caves elsewhere.

(Mods - please don't move this to the "Yorkshire" page as it's a general question about earthquakes, rather than about a specific cave in the Dales.)

Offline gus horsley

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Re: Earthquakes and boulder chokes
« Reply #1 on: March 08, 2011, 07:23:35 pm »
I wonder if a combination of pre-existing loose rock and  possible confined water vibration generated by the earthquake could have been responsible.  Although earthquake waves don't really affect consolidated ground much, they do have quite a dramatic effect on water, as witnessed by phenomena such as liquefaction.  I'm not aware of any research which has been done on phreatic waters and earthquakes though.

Offline Peter Burgess

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Re: Earthquakes and boulder chokes
« Reply #2 on: March 08, 2011, 08:13:49 pm »
When you consider the phenomenon of water hammer in plumbing and how shock waves are transmitted in that case, it isn't too difficult to imagine what MIGHT happen in a cave passage full of water.

Offline Pitlamp

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Re: Earthquakes and boulder chokes
« Reply #3 on: March 08, 2011, 08:48:56 pm »
You both have an interesting point; any diver will tell you that vibrations travel very well in water.

Has anyone else noticed any effects of that earthquake in caves?

Offline Ed W

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Re: Earthquakes and boulder chokes
« Reply #4 on: March 08, 2011, 09:06:39 pm »
Not quite eye witness stuff, but two caves we explored and surveyed in Iceland in 2005 had significant changes to them in 2007 which were probably due to earthquakes.  In one a boulder choke underwent significant collapse, totally blocking the main way on.  In the other a new "skylight" entrance was formed where a section of solid (but fairly thin) roof collapsed.  In between the two visits the general area had seen a period of frequent earthquakes (up to 300 a day), though the vast majority of these were of magnitude 3 or below (there were about 5 to 10 earthquakes in the 3 to 3.5 range in total).  It is highly likely that these earthquakes had some part to play in the alterations to the caves, though freeze/thaw may well have contributed to the latter case.

On the other hand, a trusted source regailed me with the story of being subjected to a Magnitude 6 earthquake whilst in a boulder choke in a lava tube in Mexico.  In this case the damage was more to his laundry bill than the cave.

Offline Pitlamp

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Re: Earthquakes and boulder chokes
« Reply #5 on: March 09, 2011, 07:25:43 am »
Crikey - rather him than me. Thank goodness we don't have "proper" earthquakes like they do in Iceland.

Offline zippy

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Re: Earthquakes and boulder chokes
« Reply #6 on: March 09, 2011, 08:55:08 am »
In the early 50's there were a couple of earthquakes centred around Winster, Derbyshire.  Some hypothesised that these were due to mine collapses, so Op Mole & others got to go and "inspect" several sites.  Interesting enough though, there was a report in the local paper (not sure which one) stating that "miners working at the nearby ...xyz...  mine felt nothing of the eathquake"

The point being, that just 'cos you feel it on the surface, you may not underground - apparently.

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Offline Duncan Price

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Re: Earthquakes and boulder chokes
« Reply #7 on: March 09, 2011, 09:21:10 am »
A large section of (obviously loose) roof peeled away at the bottom of the King's Road in Daren Cilau (area of T with main streamway).   IIRC it is reported in Descent/CDG NL.  The debris was largely washed away into the sump creating a temporary sand bar where you surface.  The event followed the 2002 earthquake in Dudley.

Offline barrabus

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Re: Earthquakes and boulder chokes
« Reply #8 on: March 09, 2011, 10:44:50 am »
The point being, that just 'cos you feel it on the surface, you may not underground - apparently.

I seem to remember reading somewhere that some miners working underground during the 1906 San Francisco earthquake returned to the surface unaware that it had happened.
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Offline Burt

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Re: Earthquakes and boulder chokes
« Reply #9 on: March 09, 2011, 01:47:24 pm »
The point being, that just 'cos you feel it on the surface, you may not underground - apparently.

Z.

But I distinctly remember when surveying cables in a tunnel under Royal Portbury docks feeling the whole thing shake as the old power station nearby was being demolished by big diggers.
Needless to say, as these tunnels were under water as well as underground, I had a similar "laundry" moment as Ed W's mate!
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Offline pwhole

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Re: Earthquakes and boulder chokes
« Reply #10 on: March 09, 2011, 02:47:17 pm »
Apparently Mam Tor had a bit of a shake a few weeks ago, partly down the cold weather we've had, but as it's on a natural fault, it's quite possible it was enough to give Odin Mine a bit of a twist, and hence cause the recent collapse within the backfill. Obviously the fault also produced the mineralisation that made the mine so successful, so it's often a double-edged sword!

The Louth quake a few years ago was very strong in Sheffield, and that could well have caused some damage in the near Peaks too - one mine I visited recently had a significant collapse which was 'definitely not there' ten years before when previously explored by others, and it's possible that a Mag 5 could shake things enough - my flat got a serious rattling when it happened.

Offline mike barnes

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Re: Earthquakes and boulder chokes
« Reply #11 on: March 09, 2011, 03:36:07 pm »
Hi Pitlamp,

I had a bad collapse in Heavens Door underwater. Luckily it was between visits. I had seen a downstream continuation which I went into. The vis that day wasn't great, but went in for about 10m where I could see an open route. It was all in a very loose area. When I went back some weeks later in better vis, the whole lot had collapsed and the line lay buried with the open route now completely blocked. I recall there was an earthquake somewhere near/in South Wales around that time. It seems the only other thing that may have caused the collapse was the line moving in the current, though I'd cable tied it to the roof to prevent it moving any rocks. Or pure coincidence that the collapse was due to happen when it did, even if I'd never gone there.

Could the 'right good donging' just have weakened the roof still further, then your bubbles being a 'dong' too far. Another incident in Heavens Door, such an appropriate name, occurred this time above water. 3 blocks in the roof looked a bit iffy, so I gave them a light tap with a hammer, but all sounded fine. Many months later, having crawled under them numerous times, 1 of them decided to add some excitement to the day by dropping down just a short distance from me. I'll never know if I hadn't  tapped it, would it then have fallen.
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Offline Pitlamp

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Re: Earthquakes and boulder chokes
« Reply #12 on: March 09, 2011, 05:00:59 pm »
Some really interesting stories there - excellent information; thanks very much.

Offline paul

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Re: Earthquakes and boulder chokes
« Reply #13 on: March 09, 2011, 05:21:07 pm »
Simon Brooks showed me a boulder filled depression in the Traligill Valley in Assynt where he and J-Rat had been investigating a bit of passage through a clearing in the boulders. They returned to the surface as J-Rat wanted to have a fag. Just then there was a bit of a tremor, and the whole thing collapsed!
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Offline Pitlamp

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Re: Earthquakes and boulder chokes
« Reply #14 on: March 11, 2011, 05:26:51 pm »
After this morning's news headlines I'm right glad I don't go caving in Japan!

Offline The Old Ruminator

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Re: Earthquakes and boulder chokes
« Reply #15 on: March 11, 2011, 05:57:58 pm »
Shatter Cave and part of Withyhill show a whole generation of smashed and upturned formations. The Leaning Tower of Pisa is one. It has long since repaired itself. Clearly there was a considerable disturbance along the Withybrook Fault a long while ago.

The Leaning Tower of Pisa.



Smashed and upturned stalagmite boss ( mid/left ) Ring Road, Shatter Cave.



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Offline The Old Ruminator

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Re: Earthquakes and boulder chokes
« Reply #16 on: March 11, 2011, 06:03:40 pm »
Obviously the forum has been set up to mis spell DARK  ---   PLACES. Like I said very silly.

Offline Bob Mehew

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Re: Earthquakes and boulder chokes
« Reply #17 on: March 11, 2011, 08:42:59 pm »
Today's earthquake in Japan reminded me of this resource at  http://www.earthquakes.bgs.ac.uk/ .  I don't know if one could make some attempt to correlate observations with their alerts at http://www.earthquakes.bgs.ac.uk/recent_events/uk_special/alert_info_uk.htm

Offline ChrisB

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Re: Earthquakes and boulder chokes
« Reply #18 on: March 11, 2011, 09:55:09 pm »
I would expect earthquakes to bring down loose rock and extend cracks, but not to create loose rock where it wasn't. Ground shaking would make anything loose fall down there and then - so in Pitlamp's original post, I would be surprised if the insecure stuff had resulted from the quake.

Chris

Offline graham

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Re: Earthquakes and boulder chokes
« Reply #19 on: March 12, 2011, 08:12:07 am »
Shatter Cave and part of Withyhill show a whole generation of smashed and upturned formations. The Leaning Tower of Pisa is one. It has long since repaired itself. Clearly there was a considerable disturbance along the Withybrook Fault a long while ago.

Ain't necessarily so. I don't remember the reference but I'm pretty sure I read a paper years ago where someone did a number of U/Th dates on stal in a situation like that and demonstrated that the damage was not contemporaneous and could not, therefore be attributed to an earthquake. if it could have been attributed to a number of quakes then it would have been quite remarkable how each one knocked over a different stal.

It is clear from a number of sites that damage which, superficially, might be regarded as quake damage has other causes, frequently subsidence in unconsolidated fill beneath teh stal.
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Offline Peter Burgess

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Re: Earthquakes and boulder chokes
« Reply #20 on: March 12, 2011, 08:41:15 am »

It is clear from a number of sites that damage which, superficially, might be regarded as quake damage has other causes, frequently subsidence in unconsolidated fill beneath teh stal.

I was going to say much the same.

We shouldn't always think of the most dramatic reasons for why something moves out of the vertical. Think about the normal world around us. What is the most common reason for buildings, trees, and other structures keeling over? Subsidence, soil creep, and undermining by water courses are three that immediately come to mind. Why should it be so different in the underground environment?

Offline Pitlamp

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Re: Earthquakes and boulder chokes
« Reply #21 on: March 12, 2011, 08:57:13 am »
For ChrisB; h'm, I understand your point. However we've been active at Malham regularly for 20+ years and never noticed anything like this before. The roof is certainly already loose; it's a big expanse of slabby shattered limestone. This is why the project has taken so long to do safely. The scaffolding is extended in sections only once everything which can be brought down in front by hitting and prising with scaffold poles has been done. That area of roof had been checked as described on the dive prior to the earthquake and it was whilst we were constructing the next section of scaff (first dive post earthquake) that two of us separately had "interesting" experiences with stuff falling down unprovoked.

The Langscar fault is responsible for this shattered area. By general geological standards it's only a very minor fault zone. (What cavers call a big fault is usually insignificant to a proper geologist!) However, I think that earthquake on 3rd January this year must have caused enough movement within the shatter zone to loosen blocks which had previously been jammed or at least well chocked in.

(Maybe we need to do a thorough energetic check of the roof immediately in front on every dive in future!)

Maybe cavers generally should take earthquakes a bit more seriously if visiting a loose area (and not assume that everything is necessarily as it was when previously visited). Really my main reason for the original posting was to raise awarenes of this possible safety issue to cavers but it's brought up some very interesting information in the process, so thanks to all those who have contributed above.

Offline The Old Ruminator

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Re: Earthquakes and boulder chokes
« Reply #22 on: March 12, 2011, 09:49:36 am »
I actually have an open mind about the Shatter Withyhill damage. I would say, though, that most of it was not caused by subsidence or collapse as some of the broken and recemented formations are up on the wall. There is a classic one in Shatter broken in several places and recemented back against the wall. Look carefully and they are pretty much all over the place. I also read somewhere of a localised earthquake in the area some centuries ago. The stal deposition in Shatter seems to occur quite quickly so maybe that was the cause. A proper geomorphicological study of the caves is long overdue but I hear the resurvey project is nearing completion.

Offline The Old Ruminator

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Re: Earthquakes and boulder chokes
« Reply #23 on: March 12, 2011, 09:51:53 am »
Well a bit from the net.

Earthquakes in Somerset, did I hear you say? - some will know of these occasions but I suspect many more will not. During the first few years of the 20th century; a gentleman W.G. Willis Watson F.R.Hist.S decided to compile a Chronology of Somerset, he had already produced three books on Somerset life and was collecting Somerset events from wherever he found them which were printed in the "Notes and Queries" column of the Somerset County Herald. At the instigation of Somerset Folk Publications he was persuaded to continue this area of research, the results of which were published in two books by that publisher in 1925 entitled "A Chronological History of Somerset".

The periods covered are from BC 1200 to BC 200 and AD 43 to AD 1925. Part 1 displays a copy of John Speed's map of Somerset of 1610 (as reproduced by Bassett in 1666) and Part 2 displays a copy of Richard Blome's map of Somerset of 1673, reproduced c.1698.

The writer recorded earthquakes for the following years AD:

534 - 543 - 974 - 1081 - 1122 - 1185 - 1199 - 1200 [At Montacute] - 1276 [Great Earthquake at Bridgwater] - 1284 - 1318 [Great Earthquake]

Offline The Old Ruminator

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