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.