This could be tough depending on how much sand you are quaking. May I suggest using pneumatics to pressurize something like innertubes? The problem is that the waveform would be full of compression/ pressurization artifacts, the advantage is that its pretty cheap.
Ummm, Servos don't have a negative signal, rather the position is maintained relative to the pulse width via the feedback circuit to the position sensor (which is usually a mechanical potentiometer). So you send out pulses at some regular cycle length, eg 50 Hz, and the width of the pulse...