Earthquakes?

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Earthquakes?

Postby bccguide » Jul 15, 2007 10:21 pm

I was wondering about earthquakes and the dynamics with caves. I am a new tour guide for a privately owned cave and I get a lot of questions about earthquakes from guests. How it was explained to me was that the shock wave from the epicenter passes threw and doesnt disrupt the cave because of Newtons Law of Motion. For example with the five steel balls that swing and hit each other, the first ball hits the the second but passes that energy threw to the last ball, with the middle ones never moving. My problem with this explanation is that if you were to put a hollow ball in the middle with the walls being paper thin, representing a cave, the energy from the first ball would crush the middle hollow ball. Any thoughts?
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Postby Grandpa Caver » Jul 15, 2007 11:10 pm

I dont know the answer either but I've heard pretty much the same thing adding that most earthquake damage occurs when the shock waves roll through softer ground. Bedrock only shifts horizontally giving it a good shake but no destructive rolling motions.

I get asked this a lot myself and I'd like to hear the facts from "those in the know" myself...How bout it folks?
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Re: Earthquakes?

Postby Teresa » Jul 16, 2007 9:40 am

bccguide wrote:I was wondering about earthquakes and the dynamics with caves. I am a new tour guide for a privately owned cave and I get a lot of questions about earthquakes from guests. How it was explained to me was that the shock wave from the epicenter passes threw and doesnt disrupt the cave because of Newtons Law of Motion. For example with the five steel balls that swing and hit each other, the first ball hits the the second but passes that energy threw to the last ball, with the middle ones never moving. My problem with this explanation is that if you were to put a hollow ball in the middle with the walls being paper thin, representing a cave, the energy from the first ball would crush the middle hollow ball. Any thoughts?


The analogy doesn't follow, because a cave's walls usually aren't paper thin.

Earthquake motion is very complex, with all sorts of wave frequencies and pressures, but there are three basic motions: P or compressive waves (like squeezing a spring from end to end) S or secondary waves (up and down, like shaking out a rug) and surface waves (a rolling, tumbling motion like ocean waves, or drawing directional curlicues along a plane).

P waves move through rock fairly easily, since they are linear. S waves may cause some vertical slip at fault zones, but they usually have less energy than P waves. It is the rolling surface waves that people primarily pier buildings against, since in essence the earthquake has a bucking motion, and whatever is not tied down on top will be tossed.

As long as the cave walls are solid, solutional structures, P waves aren't going to be a problem unless you are so close to the epicenter that the energy transmitted will overcome the rheology (physical integrity) of the stone that it literally shakes it apart. It takes some pretty intense movement to shake most rock apart grain by grain. S waves may be a problem if you are in a structurally unsound, or heavily faulted or jointed cave, if the walls can move independently of each other. Again, in a cave with only a central joint, but mostly solutionally enlarged, this is likely not a big problem.

Surface waves may cause loose rocks to fall in a cave, but the cave, being underground, will tend to move in circles, with the movement, not fight it as will an unpiered or unreinforced (especially brick) building, unsecured water heater, book cases , and other stuff which tumbles during an earthquake.

If the earthquake epicenter is so near and so intense and the cave thin-rooofed enough that stuff crashing on the surface can come through, you might have a problem. There is more likelihood of a manmade entrance collapsing because of a surface slide than the cave itself being hurt.

I always explained this in terms of riding out a quake inside some spongecake. If you hit the entire spongecake (P waves) the spongecake transmits the energy around your air bubble. If there is a crack in the cake, the S waves (up and down waves) might cause some movement on one side of the crack, but not the other. However, these waves aren't usually that strong. The rolling surface waves might make you queasy, or grab for something, or you might not even notice them. It's the surface waves which cause liquefaction, and most earthquake damage, and the place most vulnerable to them would be at a man-made entrance.
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Postby Rick Brinkman » Jul 16, 2007 1:43 pm

Then how do you explain broken speleothems in virgin caves??

Lewis and Clark Caverns (Montana) has a LARGE stalagmite (15' tall) that has toppled over like a tree. What else would do that other than an earthquake?
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Postby Teresa » Jul 16, 2007 2:47 pm

I did put a caveat in the original, unaltered post about earthquakes bringing down unsecured rocks. A stalagmite, unless it is cemented to a flowstone floor, is unsecured. Bats are known to break soda straws by knocking against them. Ceilings collapse, and knock stuff down with them.

Other ways stalagmites can be tumble and be broken are: pushed over by a bear stumbling in the dark, undermined by water, becoming unsteady as the clay floor beneath dries out, shrinks and compacts (perhaps unevenly.) Clay floor creep can even break columns because of sideways shear.

I've seen stal which have slid down a clay cave wall under its own weight, and crashed. It doesn't take much to topple an unsecured stal. Was on an investigation once of a LOT of broken stuff in the cave stream-- and no footprints where any perps would have had to be to pull them over. We came to the conclusion the clay bank got too mushy to hold them during an in-cave flood event.

Note too: I did not say that earthquakes NEVER move rocks in caves, (just look at some near San Andreas fault caves) but I was talking about the effect of the ground movement on the cave's existence itself -- not all of its contents. Obviously (again a caveat in the original post) you have a cave in flakey rock, developed along joints and faults directly over an epicenter, and you have a major (6.5 or greater) earthquake loose things are going to jumble around-- but again, that depends how deep the quake is, what the bedrock is, how solid is it, and the depth and configuration of the cave.

I got my info from someone with the SLU Earthquake Center-- the people who have a seismic station inside of my favorite local caves. The primary evidence for earthquakes not being that damaging to caves are the caves of the eastern Ozarks. They rode out 2000 shakes in 4 months from 1811-1812, From 100 miles away and there isn't much evidence of recent shake and break loose in them. Even better evidence comes from caves only about 50 miles as the crow flies from parts of the NMSZ.
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Postby bccguide » Jul 16, 2007 7:30 pm

good information. the cave is a solution cave, and the surrounding rock is marble. but it sounds as if you would feel some motion if you were in the cave during a close earthquake, is this correct? possibly from the P waves in the circular roll.
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Postby gillip » Jul 16, 2007 8:51 pm

One major speleo crasher that has not been mentioned is flooding. I have seen (well I didn't actually see it happen, but I saw the evidence) stalactites, stalagmites, draperies, and even pendants of limestone wiped out under high flow.
Also, changes in the environment of the cave can cause instability of calcite fabric. A wet cave drying out can affect the strength of speleothems. Also, if a cave has an opening sufficeint for the slightest environmental change, speleothems that formed under a certain chemical equilibrium may be unstable under the new conditions. Certain calcite fabrics are stable under certain conditions.
I am working in a small cave that is very open by means of a large sinkhole. The formations are all trashed, and not by man or animal. The cave is witin 100 m of a fault, so being a green geologist I thought movement along the fault might have caused the damage. Moments later I came to the realization that the fault is late Mississippian to early Permian and no movement has occured in recent geologic history. Some experiments that I have been doing indicate that wetting and drying (relative drying, submersion to 80% humidity, which does occur in the cave) of the speleothem causes spalling. Flake off a few cm's arround the base of a stalagmite and a high flow event could easily tip it over. I am working on replicating this at a larger scale to see if it still holds true. This is an extreme case, but environmental changes in the cave definately have an impact.
Since many caves form along joints, an earthquake could do significant damage if it caused movement along the fracture. We shouldn't worry too much though, because as Teresa pointed out many caves in the Missouri, Kentucky, and Tennessee survived 3 earthquakes greater than 8.0 and thousands of aftershocks in 1811/1812. An 1895 earthquake in the New Madrid siesmic zone the registered 6.8 was a little better documented. It affected a huge are in the midcontinent, shown on a map at:
http://quake.wr.usgs.gov/prepare/factsh ... on1895.gif
If we experience anything more than an 8.0, caving will be our least concern.
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Postby Teresa » Jul 16, 2007 9:46 pm

bccguide wrote:good information. the cave is a solution cave, and the surrounding rock is marble. but it sounds as if you would feel some motion if you were in the cave during a close earthquake, is this correct?


I've never personally been in a cave during a close/big earthquake. Some people who have have told me that rather than motion, you hear the sound (rather like a distant freight train) and maybe a little rattling noise (if there are loose rocks on other rocks in the cave). I've heard that noise above ground during a mild quake (about a 4). Remember, even though you are on a surface, you aren't on 'the surface'. If you're in a marble cave in the US, I'm guessing you're on the US west coast (I may be entirely wrong about this). The US west coast has more shallow epicenters to their quakes than here in the midwest, so you might have more intense effects. What sort of caves are around SF CA? Seems there are enough cavers in that region, you might ask them what underground effects happened during the 1989 World Series quake.

We've discussed caves and quakes on several of these NSS boards, and I've never heard of anyone being trapped in a cave because of a quake, or being injured from falling stuff...though I've heard of loose stuff falling. Of course, loose stuff can fall in any cave, at any time...no earthquake needed.

possibly from the P waves in the circular roll.


P waves are linear and compressional, not circular. I'll try again. Think of a Slinky toy. You can put a Slinky cylindrical side down lengthwise on a table like a tube, and induce a compressional wave (from one end to the other, with one side resting on the table) (extend, compress, extend, compress) by pulling on one end of the Slinky.

That motion is very different from an S wave (Slinky going up and down, like a sine wave, but not rolling side to side.)

The surface wave (the rolling destructive one) is orienting the Slinky at a 90 degree angle to you (like a rolling pin), setting a Monopoly house on the apparently flat top of the cylinder, and then rolling the Slinky along the curved surface. The house slides off almost immediately, if not wired on to the Slinky.

So P wave motion does NOT equal surface wave motion.

If this didn't help, and you have Shockwave installed, try this link:

http://www.geology.sdsu.edu/visualgeolo ... quakes.htm

Watch the particle.
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