Folded Pendulum Measurements of Earth's Free Oscillations
  The nearly-incessant free oscillations of the Earth (not the larger, long-lived normal modes seen following intense quakes) were first observed by accident in the record of a tilt-sensitive instrument designed to study surface physics. Later tiltmeter studies demonstrated the usefulness of autocorrelation for the routine study of these normally short-lived eigenmodes. More recent studies suggest that many tilt-sensitive instruments are capable of observing these oscillations--if the low-frequency response of their electronics is not suppressed, as is customary with conventional designs. 
Randall D. Peters