I hate these "it wouldn't help" arguments regarding most things in this world, including this one. Case in point here, even a siren blasting 0 seconds beforehand would be useful to reduce the inevitable delay between when you feel something shaking and when you realize it's really an earthquake. I'm not sure where you're getting the idea that it wouldn't be useful, because as far as I know, the reason we don't have it is lack of funding, not lack of usefulness.