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I took a tour of a cave in northern California last weekend. The tour guide asked us, "If an earthquake occurred, what would we feel in here?"

My answer was, "fear," but she said we would feel nothing - that earthquakes are not felt underground.

Although I did not contest her assertion, I have my doubts about it - after all, don't earthquakes normally have their epicenter underground (well, all the time, I reckon, as they can't emanate from the air)? And if an earthquake happened to erupt from within a cave[rn], wouldn't it most definitely be felt there?

I would think one might even end up with a stalagtite stuck in his noggin.

UPDATE

And, if I'm right (that there could be a whole lotta shakin' goin' on while on a journey to the centre of the earth), has anybody lived to tell the tale and really done it (told the tale)? A first-hand account could be riveting.

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    $\begingroup$ The epicentre is on the surface by definition, but the hypocentre is certainly below the surface. $\endgroup$
    – gerrit ♦
    Commented Jul 14, 2015 at 15:45
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    $\begingroup$ I think this question needs a quantitative answer... it would be nice to see a model, or some data from seismometers at vs beneath the surface. The present answers are rather arm-wavy, since the short, direct answer to the question (not to mention the intuitive one) seems to be No — indeed, this report indicates that there can be substantial damage underground if peak ground acceleration at the surface is high. $\endgroup$
    – Matt Hall
    Commented Jul 14, 2015 at 23:36
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    $\begingroup$ Earthquakes are definitely felt in basements. I can't imagine any mechanism which would keep them from being felt in natural caves. $\endgroup$
    – keshlam
    Commented Jul 15, 2015 at 3:30
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    $\begingroup$ Tunnelling and Tunnel Mechanics by Kolymbas have a short chapter on earthquakes. The author states that; 'Experience shows that underground structures, especially deep ones, are far less vulnerable to earthquakes than superficial ones' and list reasons and a few references. link.springer.com/chapter/10.1007%2F3-540-28500-8_18 $\endgroup$
    – user2821
    Commented Jul 15, 2015 at 13:12
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    $\begingroup$ When the 2011 earthquake hit in the northeast US, I was in NYC in the subway when the earthquake hit. I don't know if it was the trains shock absorbers or the tunnel, but I felt nothing. I didn't realize there was an earthquake until I left the subway and saw a sea of people on the streets that had evacuated the sky scrapers. $\endgroup$
    – Christian A Soto
    Commented Aug 24, 2017 at 0:57

3 Answers 3

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Ground motion results due to passage of elastic waves. Now there are different kinds of waves, e.g., P waves, S waves, surface waves, etc. Most of the shaking (and therefore damage) is caused by surface waves. So if you are in a deep cave or mine then the amount of shaking you might experience can be much lower than on the surface. This of course assumes that the mine or cave is not right on the fault, but some distance (10s of km) away.

Having said that, there are some situations (based on the type of earthquake and local geology) in which waves can interfere constructively to cause significant shaking even at large depths, but this only happens in certain areas and not everywhere.

In many ways elastic wave propagation is similar to acoustic or water waves. Imagine a small explosive source in a lake/pond, let's say 50 m underwater. After the explosion there will be more 'shaking' on the surface, but not much underwater unless you're extremely close to the explosive source.

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    $\begingroup$ As a visual example: faculty.gvsu.edu/videticp/wave_animation1.GIF The water analogy is more-or-less exact (for Raleigh waves, anyway) in this case. $\endgroup$
    – Joe Kington
    Commented Jul 14, 2015 at 18:43
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    $\begingroup$ Local geology, as @stali mentions, have an often underestimated impact of the intensity. I'm no speleologist, but most caves I know about are in limestone or hard igneous rocks. Shaking hazard is much more severe in soft sediments: earthquake.usgs.gov/regional/nca/soiltype $\endgroup$
    – user2821
    Commented Jul 14, 2015 at 19:38
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    $\begingroup$ I would expect, from @Tbbe 's comment, that caves in regions or materials that feel earthquakes aren't likely to last long enough to be observed -- instead being collapsed by earthquakes. Worst case would to be in a newish cave when a long-quiet fault becomes active, then. $\endgroup$
    – cphlewis
    Commented Jul 21, 2015 at 22:08
  • $\begingroup$ @cphlewis Caves can be common in some tectonic active regions and there are examples of rather old systems, but yes, e.g. a cavity situated under unconsolidated lake bed sediments would probably collapse rather quick (even without an earthquake). Interested in how caves are formed should look up karst forming and developing processes. $\endgroup$
    – user2821
    Commented Jul 22, 2015 at 6:23
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    $\begingroup$ Still, as answers are pointing out, surface waves have the highest amplitude on the surface, that's where the strongest impact is. Think about the analogy of shaking a completely full and a half full bottle. The micro-environment in the full bottle would be much more healthy than being in a bottle where the water is not hold in place by the walls, even if acceleration is the same. $\endgroup$
    – user2821
    Commented Jul 22, 2015 at 6:26
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There are three main types of waves produced during an earthquake: P, S, and L waves, which stands for Primary, Secondary, and Love. (There was a mnemonic I read many years ago that went P=pressure, S=Shear, and L=Long.) The P and S waves are body waves. That is, they propagate inside the earth. The P wave is basically a sound wave; as the mnemonic goes, a pressure wave. The S wave is a shear wave and is perpendicular to the direction of fault slip. The L wave, which has the highest amplitude is a surface wave and, as such, propagates along the surface, not inside.

If you were inside a cave when a nearby earthquake occurred, you would not feel the L waves, since they propagate along the surface. Whether or not you would feel the P and S waves would depend mainly on two things: the magnitude of the original earthquake and the type of fault that produced it. If the fault were a thrust fault, the S wave would be up and down, which a standing person probably wouldn't feel unless the earthquake was quite large (>5.5?). However, if the earthquake were along a strike-slip fault, the S waves would be side to side, which a standing person would likely feel.
(I realize this answer duplicates some of that given by @stali, but I was already typing this when I saw that answer come across, and I thought I had some additional thoughts in this answer. Sorry for the duplication.)

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Googling leads to a very old document: EFFECTS OF AN EARTHQUAKE IN A MINE AT TOMBSTONE, ARIZONA stating that in many cases, mine workers did not notice earthquakes which were felt above ground.

A newer book, Earthquake effects on tunnels, check the "Look Inside" option for the introduction) confirms this. According to this source, a possible reason is that overground structures are less constrained in their movement while inside solid rock, the displacement cause by the earthquake is much smaller.

Apparently your guide was right.

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    $\begingroup$ I still think you would feel the quake if you were close to where it occurred - how could you not? I agree that somebody in a cave in Volcano, California, may not feel a quake in San Francisco, whereas it may be felt on the surface, but I believe those in the cavern would feel a quake whose epicenter was in Volcano itself. IOW, if the statement was chaned to, "an earthquake felt on the surface may not be felt below the surface," I'll buy that. $\endgroup$
    – B. Clay Shannon-B. Crow Raven
    Commented Jul 14, 2015 at 17:02

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