Pacific Northwest Geodetic Array
Central Washington University

Slow Earthquakes in the Pacific Northwest

Michelson Tiltmeters

A tiltmeter measures rotation of the earth's surface relative to a horizontal datum defined by gravity. In 1919, the renowned physicist, A. A. Michelson, noted that the most favorable arrangement to obtain high sensitivity and immunity from temperature perturbations is to use the equipotential surface defined by water in a buried half-filled water pipe.

Tilt of the Earth's surface causes the water in long baseline tiltmeter to flow to maintain an equipotential surface within the pipe. The equipotential datum has zero drift except where mass changes occur near the tiltmeter, as for example, may occur near the magma chamber of an active volcano.

Tilt is measured by monitoring the changing depth of water at each end of the water pipe using sensitive float transducers. Tilt is the difference in height divided by the length.

In most water pipe tiltmeters, the length is ≈500 m, and water potentiometrics can be measured to an accuracy of better than 350 µm. Hence tilts smaller than 2 nanoradian (2 nrad) can be measured (equivalent to detecting a height change of a 3 tenths of an inch between San Francisco relative to New York). The electronic range of the water level sensors is typically 2 mm providing a tilt measurement range of 16 µrad. Solid earth tides are expected to be around 60nrad with slow earthquakes registering 3 times this value in the veritcal field.

Other designs for tiltmeters have been devised from time to time but none are immune to thermal signals as the configuration devised by Michelson. Uniform temperature changes do not effect the tilt signal because they raise or lower the water level equally at each end. Moreover, local variations in temperature result in convection in the water that do not perturb the measurement points at each end. The temperature of each transducer is measured precisely to correct the observations for secondary effects. The pipe is sealed and is therefore immune to pressure fluctuations in the atmosphere. Slow leaks in the air or water paths have no effect on the tilt signal.

To reduce sensitivity to surface effects like rainfall, snow loading and soil expansion or contraction, the measurement of water height is made relative to the upper surface of a deep, stable monument constructed from 3 inch, schl-40 rigid steel pipe driven 20-30 m into the subsurface.

The two design disadvantages of the Michelson tiltmeter are that the pipe must be installed approximately horizontal, and the fundamental resonant frequency in the pipe is 20-25 minutes (PVC pipes of 15-20 cm diameter are commonly used). The resonance period is more than critically damped in a 15 cm pipe, but Michelson tiltmeters are unable to detect short period horizontal accelerations, or tilts at seismic periods.