The system is currently being tested using the US Pacific Northwest Geodetic Array: hundreds of GPS receivers placed along the North American coast between Northern California and British Columbia in Canada.
While conventional seismometers provide similar information, they run into trouble with earthquakes of magnitude 7 or higher. This is partly because in big quakes, the ground may shake for longer, but not significantly harder. GPS has no such problem, because it directly measures the movement of the ground.
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