• The Derbyshire Caver, No. 158

    The latest issue is finally complete and printed

    Subscribers should have received their issue in the post - please let us know if you haven't. For everyone else, the online version is now available for free download:

    Click here for download link

Earthquake, may have affected Peak District caves

Ian Ball

Well-known member
As I age the effect of seismic activity on the underground environment is becoming more intriguing.

With the fragility of formations, I'd always assumed that nature was in a kind of equilibrium, and the places at risk from seismic activity would be human excavated.

Just stay away from mines, the advice of my Cornish mining great uncle.
 

Brains

Well-known member
Side thought... In the deep ocean even the biggest tsunami are small waves at surface and presumably just a pressure wave at depth. They only assume large size when they come ashore. Earthquakes are a combination of wave types, but again mainly seem to displace surface sediments. At depth they are just pressure waves, other than the area of the actual fault activation. Of course, what counts as a surface sediment relative to caves and mines is an issue. I seem to recall accounts of explorers surfacing after a quake having felt nothing, while the surface had significant disturbance. Curious, to me anyway ..
 

ChrisB

Active member
Although triggered by earthquakes, tsunami waves are different from the waves in the rock. In deep ocean, tsunami waves are gravity waves with a low height above the surface, a long wavelength and high speed. They therefore have a lot of energy. Water being fluid, the passage of the wave doesn't dissipate significant energy. When they enter shallower water, the long wavelength cannot be maintained, but the energy is still there, so the wave height has to increase.

Earthquake waves in rock are mainly compression or shear waves (that's a simplification) and dissipate with distance from the fracture; the higher the frequency the faster they attenuate. The soil column above the bedrock tends to amplify the motion; depending on soil type and depth, the natural vibration frequency will resonate with some components of the earthquake vibration. The motion at the surface is therefore greater than at the bedrock. Sometimes it can be a lot greater, and in addition there will be movement on unstable slopes or where saturated ground has temporarily liquefied.

So we might expect that near surface shafts (particularly dug shafts) might be damaged, but caves and mines within the rock will see less movement. Anything unstable, however, may well move; if the ground acceleration is 0.1g, that's the same as tilting it about 6º. I haven't calculated the natural frequency of typical stal but my guess is that it will be rigid enough not to resonate.
 

Flotsam

Active member
I been on an expedition where there had been a R6.5 earthquake in the locality a couple of months before. 900 people killed and thousands of buildings destroyed There were no signs of any disturbance underground. There was apparently a significant aftershock around R5 while we were underground but none of us noticed.
 

Pitlamp

Well-known member
The sad fatailty a few years ago in Bull Pot Kingsdale was very likely associated with an earthquake on the Dent fault, which happened 2 or 3 days beforehand. We had a huge boulder fall down the entrance shafrt of Jingle Pot during a week when there was an earthquake. (It fell down the line of the rope, fortunately when we weren't there.) We've also had large boulders rain down right next to us in a loose area of Malham Cove Rising, in an area thoroughly checked not long beforehand, immediately after an earthquake.

Despite the physics (which is certainly interesting) direct experience flags up the need for cavers to be vigilant and expect things previously deemed safe before an earthquake to be potentially unsafe. That's really the reason I started this topic. Take care!
 
Top