Earthquakes and boulder chokes

[Peter Burgess]

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?

 

[Pitlamp]

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.

 

[The Old Ruminator]

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.

 

[The Old Ruminator]

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]

 

[The Old Ruminator]

See also -

http://www.earthquakes.bgs.ac.uk/earthquakes/historical/historical_listing.htm

 

[The Old Ruminator]

From the above link.

21 December 1248 SW England



This earthquake evidently affected chiefly the south-west of England. Much discussion has taken place about the reported damage to Wells cathedral, described to Matthew Paris by the Bishop of Bath (William of Bytton). This was the fall of the tholus, apparently some large stone decoration in the course of erection above the nave, or possibly the lantern of the central tower. This report is somewhat undermined by the facts that (a) the chapter records make no mention of the earthquake; (b) Bishop Bytton was in Rome at the time and (c) modern examinations of the fabric have shown no trace of the damage to the stonework.



Apart from what happened at Wells cathedral, it is said that the tops of chimneys, parapets and pillars were thrown down, and that large cracks appeared in walls (at unspecified places) suggesting a maximum intensity in the range 7-8 MSK. The earthquake was probably also felt at Exeter (Polwhele 1793).

That was the one I was thinking about.  To be honest it seems quakes are far more prevalent than we might think and where we have a structural weakness like the Withybrook Fault effects might be more widely felt.

 

[Les W]

I seem to recall a plaque at the top of Glastonbury Tor that claims the church that was built there was destroyed in an earthquake. Only the tower survived.

 

[Bob Mehew]

The UK experiences trivial earthquakes quite frequently.  In my old job, one of the entertaining tasks for some of our specialists was to come up with the size of an earthquake likely to affect a nuclear installation on a frequency of in 10,000 years.  I recall one piece of work being done was to go and get information on the number, size etc of grave stones which had toppled over in an earth quake.  I wonder if one might be able to do some fancy calculation on whether a straw of a certain length might break into bits due to being vibrated by an earthquake?  Or we could install our own earth quake detectors such as http://en.wikipedia.org/wiki/File:EastHanSeismograph.JPG 

 

[potholer]

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.

When we were in Slovenia, shortly after there'd been a decent-sized earthquake nearby, there were a few small aftershocks that people didn't seem to notice while underground (~-400m) even though they were noticed on the surface.

Though to be fair, I guess that things noticed on the surface in small shocks (furniture rattling, etc) don't exist underground.
The couple of small quakes I've felt in Stafford in recent years while sitting or lying down indoors, I'm not sure I'd have noticed if walking outdoors, hanging on a rope, etc.

 

[gus horsley]

It's down to the mechanical strength of the rock.  Hard compact rocks like granite and massive limestones don't convey the shock waves the same way as weaker rocks and unconsolidated sediments.  The Mexico City quake created the most amount of damage in buildings built on the site of an old dried up lake, forming an oval zone of devastation.

 

[Pitlamp]

We had three of us in the water at Malham yesterday and nothing fell down. That section of roof is now safely scaffolded (phew!).

 

[gus horsley]

We had three of us in the water at Malham yesterday and nothing fell down. That section of roof is now safely scaffolded (phew!).

That's good news.  maybe it was the earthquake that did a bit of "gardening".

 

[Pitlamp]

Just noticed there was a magnitude 4.0 quake in the North Sea, just over a week ago (Saturday 22-2-20). Although some distance away, anyone planning a visit to anywhere loose in Dales caves should bear it in mind. 4.0 is quite big for the UK. If I was one of those actively digging in the North York Moors caving area, I'd be especially careful.

 

[mikem]

183 quakes were recorded worldwide that day - noone seems to have reported feeling our one:
https://www.volcanodiscovery.com/earthquake/news/98691/Earthquake-report-world-wide-for-Saturday-22-February-2020.html

There was also one on Teesside on 23rd Jan (which was noticed) & Taunton 5th December:
https://earthquakes.bgs.ac.uk/earthquakes/recent_uk_events.html

Although a lot of broken formations, that were previously assumed to be earthquake damage, have now been shown to be caused by ice expansion under the surface.

 

[Cavematt]

Hi Pitlamp ? If our current dig in the North York Moors was large or deep enough to be troubled by an earthquake, I would be a very happy man