Here's an article on some geotechnical work they are doing at a dam. Basically they are seeing if the sands below the dam are loose enough for liquefaction. If the sands liquefy during an earthquake, the whole dam could slide, allowing the water to overtop, and fail the dam.
I remember investigating a dam with a similar concern, although this time it was on Leda Clay. The dam was part of the St. Lawrence Seaway. The central part of the dam went right over a band of this clay, which is known to fail suddenly, or turn into a liquid during an earthquake. We came to a conclusion that the dam was so remote, that even if it failed, it wasn't going to do much, and such a giant earthquake would cause worse problems.