Distant motions from a building vibration test

Horizontal ground motion generated by vibration tests of the nine-story Millikan Library Building on the Caltech campus was recorded on the surface of the ground in the Pasadena area at distances up to 3 miles from the building. Later it was learned that the vertical component of the motion also was recorded by the seismograph on Mt. Wilson, 6.7 miles from the Library and 4,800 ft higher in elevation. The magnitude of the acceleration varied from 2.04 × 10-2g at the excitation level on the ninth floor of the building to 3.2 × 10-7g at Mt. Wilson. Simple calculations show that multistory buildings are particularly well-suited for inducing large dynamic forces in the ground with relatively small equipment.