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Earthquakes are shock waves moving through solid rock and soil. At the surface is where they release their energy. After the last large earthquake in San Francisco I toured the area and saw buildings and freeway bridges damaged and in some cases collapsed. But the underground train system, BART, was unscathed. Similar things have been noted in the press from other recent large quakes in Japan and Turkey. In fact, in Turkey there are underground cities more than a thousand years old that were undamaged even after the nearby above-ground cities were devastated.
In Arizona, I had heard of damage in Colossal Cave, near Tucson: one pillar connecting formations extending from the top and bottom was sheared. I had heard of some soda straws, very delicate, vertical, hanging formations, being broken by earthquakes in Arizona's Kartchner Caverns. And in Coronado Cave I also saw several very large pillars sheared near their tops. No damage to the parts connected only to the floor. But top to bottom connecting features were broken apart near the ceiling, and some features connected part way up the wall were either broken or just fractured.
My interpretation (wild guess) is that shock waves do not necessarily travel vertically. Like ocean waves, they have different effects at the surface then at depth where the wildest crashing wave may be a mere tug and may be later than the surface expression. So they may begin movement at an ever so slightly differing time at the bottom of the cave compared with that same wave's arrival above it in the ceiling. There must be something about a shock wave moving through the earth that makes it (again not unlike a sea wave?) ride through at an ever shallower angle until it plays itself out. So the floor and the roof of a cave may not see the shock wave at exactly the same time, or even at the same intensity, meaning that something inflexibly attached to both the floor and the roof would get broken at its weakest point (typically the narrowest point in a column). Sure enough, columns that were attached to both floor and roof were broken, near the top, and columns attached part way up the side of the cave were either broken or fractured: a lesser discrepancy between the time of the shock's arrival at the top and bottom of the feature, perhaps?
But in the photos below, first I show large breaks in columns attached firmly to top and bottom:
Next are some examples of breakage and fracturing of things attached below the roof of the cave. Here the damage was more subtle, and were it not for the column breaks that are so obvious, the interpretation could also have been the result of other forces such as the formation itself growing too heavy. But the fracturing looks like a stress-induced brittle fracture, a break, as if produced by a shattering blow.
At the monument headquarters I asked when the big earthquake was, and the lady at the desk thought it was in 1870. But she suggested I do some research rather than quoting her, and I did. It was actually 1887.
For those that are curious, here is what I believe explains the damage seen in the cave:
The central line shows north-south groundbreaking resulting from 1887 earthquake. The line traces vertical breakage of ground from 1 to 7 meters (about 3 to over 20 feet). Dates and plus signs show when and where a 1989 earthquake and its aftershocks were located, and speculation has it that these may just be aftershocks from the 1887 earthquake, which is the kind that happens about every 100,000 years and may have aftershocks for more than a hundred years. This is the largest earthquake ever felt, historically, in the southern Great Basin.
The epicenter may have been a monstrous XI to XII on the historical earthquake damage scale, with the Coronado Cave area receiving perhaps a IX- to maybe even X-sized series of jolts, and the Kartchner Caverns and Colossal Cave areas receiving perhaps VIII and VII intensity shaking, respectively. This could explain why there seems to be some serious damage in the Coronado Cave, and only very minor damage in the other two. Even in Tucson the earthquake and its aftershocks were bad enough to cause the city council to refuse to have meetings in its brick city hall.
On the other hand, one can see that this particularly monstrous earthquake hardly affected the Carlsbad area, where there are also caves (see my Carlsbad pages on this site), which lies northeast of El Paso.
Figure and discussion taken and condensed from website by Michelle Hall-Wallace (click to go there), which site is in turn based on an article by Terry Wallace and Philip Pearthree in Arizona Geology, vol. 19, no.3, Fall 1989 pp. 6-7.
Now, are you ready for the breeze and exit?