Considering that seismic vibrations have long wavelength, several kilometers (they can reach hundreds of kilometers), two adjacent points, like the feet of the rack, vibrate with coherently (with the same phase). The rack only suffers from a vertical displacement movement, the wave never gets inside of the rack in order to excite it's modes of vibration. The amplitude of these waves are so small, we are not talking about a earthquake, that are neglectable for a audio rack. The problems involving the interaction of a seismic wave and a rack are several orders of magnitude smaller than the traffic induced noise or children playing through home. I dont see how this is the biggest problem to deal with. Please explain.
One can control the ressonance frequency trough mass, stiffnes or damping. You manage this low ressonant frequancy, only by using springs?
Low ressonant frequency is not the only usefull parameter, what is the settling time of your rack?
Energy spectrum is also usefull.
You seem to use a pendulum system that I have allready seen in some very tall building projects, to minimize oscilations cause by enviromental phenomena, such as wind driving forces. I've also seen it in a prroject to isolate an interforemeter in a gravitacional wave experiment facility.
IMHO, I think that a rack should adress other more important issues, like struture borne sound (generated inside and outside of the rack) or air borne sound, than seismic wave problems.
