All they have to do is compile a list of buildings that the City deems to be unsafe, and the owners will be sufficiently encouraged to make the upgrades (or lose their present tenants.) No subsidies, no tax breaks, no cost to the city.
Ah yes, the magic of the free market. There's absolutely no cost associated with moving, and there is a ready supply of housing that offers everything that the unsafe housing does, minus the lack of earthquake readiness.
Folks: the U.S. government (or any part thereof) can't just march in and force property owners to change their property. Government has to compensate the owners for any taking of a property-owner's rights. If the City of L.A. wants to march in and say "you don't get to use your office building because it isn't earthquake-proof", then the City has to buy the property at fair market value.
Yes, because enforcing building codes constitutes a "taking". I'm sure you absolutely wouldn't do something like blame the government if buildings collapse in an earthquake due to lax building codes or lax enforcement.
The really sad part isn't that you actually believe this, it's that you're not the only one.