Ground-based optical atomic clocks as a tool to monitor vertical surface motion

According to general relativity, a clock experiencing a shift in the gravitational potential ΔU will measure a frequency change given by Δf/f≈ΔU/c2. The best clocks are optical clocks. After about 7 hr of integration they reach stabilities of Δf/f∼10−18 and can be used to detect changes in the gravitational potential that correspond to vertical displacements of the centimetre level. At this le...