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Rate sensitivity graph.
Deformation of the Earth's crust is measured with a variety of instruments. The sensitivity (smallest possible measurment) depends on instrument type and the time needed to make the measurement. For example, in the graph above, we see that Real-time GPS can detect movements (strain rates) as small as 100 PPM/yr (parts per million per year) over one day, but only 20,000 PPM/yr over one hour's time. Over longer intervals, the borehole strain meter at 200 m depth is more sensitive, capable of detecting 0.1 PPM/yr over 1 year's time.