Earth Waves

I recently e-mailed a number of geologists and asked their opinion of how an underground structure would fare in a severe earthquake and here's the response I got. Here's what I wrote:

To whom it may concern:

I am doing a research project and was wondering if you could help me. How do you feel an underground structure would fare against a major earthquake? By major I mean at least 9 on the Richter scale. This underground structure would be made of reinforced materials, and be constructed in a dome or geodesic dome shape, because of it's amazing strength. Do you feel such a structure would fare better or worse than an above-ground structure? Also, if you do feel it could survive a 9 point earthquake, do you think it could also survive a 15 point quake?

Thank you for your help.

Here's the response I got:

Hi there. Interesting question. The first thing I have to explain to you is the earthquake measurement problems. The most common measurement scale is the Richter Magnitude scale. On the Richter scale, quakes of 2.0 or less are usually not felt by people. The largest quakes in the world have had magnitudes of 8.8 to 8.9. Although there is no limit to the upper end of the Richter scale, the theoretical limit is about 9.5 simply based on the fact that rocks experiencing enough pressure to break that violently will actually bend or melt before they break. The Seismic Moment measurement is becoming the more often used (and more accurate) form of measurement. The largest known quake on this scale measured at 9.6. There is no scale or measuring method which could result in a "15 point quake" as you suggest.

OK, now on to your specific question. There are a lot of assumptions. Are you building this structure in or near a fault? Faults as you may know are the locations of highest stress on the rocks. I would be willing to guess that any structure, no matter how reinforced, could withstand the pressures of an earthquake at or near a fault zone. Think about it ... quakes occur from two plates "bumping" or "sliding" along each other and getting stuck. If the forces pushing these plates can push up mountains, move entire contents, break miles of rock ... how well can even the strongest steel fare?

If you are talking about building this structure in an area where there are no "crushing" forces (that is somewhere a bit distant from the fault or epicenter), you have to now think about the surface waves of an earthquake. These are waves that roll along the surface like waves on water. People have seen surface waves which are several feet from top to bottom during a quake. It may be theoretically possible, I believe, to build a structure to remain intact through these waves, but I would not want to be inside it getting tossed around!

One last thing...you are thinking of building a structure that is solid and sturdy - and a geodesic dome is a good idea, but modern engineers have given up this approach for the most part, and are looking at the problem of building bridges and buildings differently. Bridges, buildings (particularly tall ones), and other structures are now being built to be flexible! They have found out that it is better to let the structure rock along with the waves rather than try to resist them. Something to consider. I hope I have answered your questions. Please feel free to contact me directly at cleger@mines.edu if you would like clarification or if you have any other questions.

Thanks for using the Ask-A-Geologist program!
Chris Leger

Offered by Peter.