Citizenkane - Blast events are broadband - they excite all frequencies. I've done quite a bit of airblast testing on structures and many modes get excited, but typically only the first few modes see any substantial deflection. You will see a similar effect in the earth. Knowing the frequencies near the blast won't tell you what your dam is doing. You need to have monitoring instruments on the dam to know for sure. There are lots of companies that do blast motion monitoring for relatively low dollars.