FWIW, forgot to mention, the typical reaction time we have is around one minute. Usually too little to have everybody evacuated from premises... But we have history on this regard, and it has proven a very important development.
One minute is not a lot of time, but if you get a warning, you can do a lot of things in that minute that will save lives and reduce the disaster.
For example - gas pipelines can be depressurized and isolation valves closed, so any fires that start from the gas is limited to a very small amount of gas.
Elevators can be set to stop at the next available floor so passengers can get out. (one minute is more than enough for an elevator to go to a floor and evacuate its passengers).
Electricity can be shut off as well to limit the amount of fires that get sparked. Emergency power off functions may be activated in data centers to save critical data and perform quick power off to reduce fire risk.
Trains and public transit can be slowed down or stopped - depending on the system, it may not be possible to evacuate the trains in time, but once stopped, there's a much lower chance of derailing
And humans can go and hide in the strongest part of their building for safety.
One minute is not a lot of time, but a lot can be done to minimize the amount of risk and even reduce post-earthquake fires.