ShakeAlert is a potentially very valuable system of early warning of earthquakes. How much warning you get depends on how far you are from the epicenter. For example, for a quake on the Seattle Fault (right under the city), there would be no warning at all. However, for a major quake offshore on the Cascadia Subduction Zone Seattle might get as much as 50 seconds of warning before damaging waves arrived. Since really powerful quakes in the M8 to M9 range effect large regions, a system like ShakeAlert could help many people over a wide area.

One big issue raised up-thread is false alarms. In a fully developed and fully operational ShakeAlert system this is addressed by using a dense array of many sensors. Software analyses the phase and amplitude of first arrivals at many stations. By analyzing the combined input from a large number of stations, the software can sort out small local quakes from large damaging ones. For a geek level discussion of this see the Virtual Seismologist Algorithm being developed at Cal Tech.

We don't yet have a sufficiently dense sensor network for a fully developed system. That will require additional funding. However the system is developed enough for beta testing. One thing that is being done in Seattle is to connect shake alert to a number of valves on the city's main water pipes. The fear is that in a large quake, if one or more of the main lines were severed, the water could rapidly drain out of the reservoirs. This is water that would be desparately needed in the days/weeks after the earthquake. There are currently shut off valves on the system, but they weren't designed to cope with an area wide disruption of the system. ShakeAlert could start closing all the key valves as soon as there is indication of a large quake. The beauty of this for beta testing is that if there were a false alarm, the valves could reopen quickly enough that there would be little or no significant effects for the city. At worst you might notice a brief drop in water pressure.