The response I got from USGS was this:
> You can read our FAQ on that at https://www.usgs.gov/faqs/how-fast-does-earthquake-informati....
> You may also be interested in ShakeAlert - see https://earthquake.usgs.gov/research/earlywarning/.
So in short, no, they don't really have it yet. I wonder though if we (hackers) could create a realtime system for the bay area since USGS has been on it for years without results. About 20-50 sensors scattered in homes around the bay would probably be enough for an early warning system. Warnings can be delivered via a mobile app, or some kind of realtime API that anyone can use for their own purposes (e.g. stopping machinery, sounding building alarms, ...).