GPS may be reliable for early warning of larger quakes. Both seismic waves and seismometers saturate at magnitudes above M7.5. That is to say its hard to tell a 7.5 from a 8 or 9 because appear similar size waves. It takes sophisticated computer analysis of a global set of seismographs to computer true quake size. This initial confusion happened in the disastrous 2011 Fukishima quake.
On the other hand working with GPS displacements gives an answer in less than a minute for larger quakes and more warning time.
On the other hand working with GPS displacements gives an answer in less than a minute for larger quakes and more warning time.
https://eos.org/features/seismic-sensors-in-orbit-2