Many geophysical effects produce variation in surface gravity on the order of 0.1 to 1 μGal. These include change in ground water level by ~0.1 m, underground magma formations near a volcano, daily
evapotranspiration from a
deciduous forest, yearly change in ground height due to
subsidence, the
free oscillations of earth excited by major
earthquakes, etc. For example, the maxima precision of a superconducting gravimeter is sufficient to measure groundwater level change of 1 mm, and can detect the onset of the
2011 Tohoku earthquake 510 km away from the epicenter. By combining data from many measurements, the sensitivity of gravimetry can be decreased further. 100 days of measurement with a superconducting gravimeter reached in precision, which was sufficient to detect the hum of the earth. ==Conversions==