To be clear, the detectors in Napa felt the tremors then transmitted the data to Berkeley. They got a 10 second warning because the tremors travel much slower than the speed of light. Being 10 seconds away means the quake is much less intense. This is helpful, but what we need is a warning for people within a few seconds of the epicenter.
The various types of seismic waves travel at different speeds. Luckily the most damaging are slowest, which travel at about half the speed of the fastest (~5 km/s). You can detect the fast waves and trigger warnings immediately, without having to wait for the damage to start.
California is a little bit of a special case, in that quake damage falls off rapidly with distance (the rock is kind of broken up here and doesn't transmit energy well). Earthquakes east of the rockies, while rarer, also have a much bigger radius of damage. In areas like the New Madrid Seismic Zone you could get useful warning times on the order of minutes.
Any early warning system is worth its weight in gold if it's automatically linked to infrastucture like gas mains and train signals (or data centers!). Even a few seconds warning can save lives. Unfortunately, like any other kind of public infrastructure besides roads, getting a decent warning system in place is politically impossibe in the US.
