bolting speakers to floor?

i have a pair of gershman speakers, that are very top heavy (on thick carpet over plywood). i live in earthquake country. i've tried several spike set-ups and they still seem unstable. how will it affect the sound to drill through the base and bolt them directly to the subfloor?
I would not recommend this because your speakers would become seismometers, transfering every tiny bit of mechanical energy of vibration to the voice coils and would degrade the sound. As an alternative, I would seek out a speaker stand that has a wide footprint where the spikes are located well outside the speaker footprint. You could then bolt the speakers to the stand. This may be a DIY project that other members have done.
Check out the speaker/amp stands by Sistrum. They would provide precisely what Blueswan is suggesting. Robert at is very helpful and could offer additional suggestions, I'm sure. It's even a toll-free number if I'm not mistaken. Good luck.
Since I have a baby issues I constructed a 1Mx1M MDF platform with spikes at the corners that is being flexibly bolted onto the platform so that I could actually rotate the speaker(since it's originally stands on the ball bearings) and it turned out to be the great tweak. In addition the speakers are loaded with 50lB of a lead shots
If they keep getting moved, yes, I would bolt them to the floor.
Blueswan and Jctubes are correct. Bolting your speakers to the floor may keep them from hitting you in the head during an earthquake but will screw up the sound the other 99.95 of the time. Sistrum platforms will transfer at a high speed, all extranous noise to ground. If properly sized your speakers will remain stable resting upon Sistrum Platforms and will at the same time become more musical.
My friend offers an engineered composite stand solution using (removable) adhesives. This assembly could be either spiked or bolted to the subfloor. If you use this solution you would not have to penetrate your speaker cabinet. An added benefit is that it increases the rigidity of the speaker. Contact me via email if interested.