Measurement of Earth's Free-Oscillations using a Pendulum
  One of the best ways to measure low-frequency eigenmode oscillations of the Earth is to monitor a simple pendulum responding to tilt. A theoretical basis for the method is given, by investigating a particular, well-known standing wave-the one with 53 minute period corresponding to oblate/prolate spheroidal deviations from the geoid. 
Randall D. Peters