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.
