Introduction to Earth Sciences I
1.3.1. The Earth Rings Like A Bell
Given a sufficiently large earthquake, like the so-called Good Friday earthquake in Alaska in 1964, which had a magnitude of 8.6, the entire Earth actually vibrates. It is fairly intuitive that the amplitude and "tone" of the vibrations must relate in some way to the material properties of the Earth. Imagine three tuning forks, one made of aluminum, one of steel and one of titanium, each struck equally hard. The pitch of individual tuning forks will be different and the time it takes for the tone to die away will differ also. The aluminum fork will make a dull thud and the sound will die out quickly; the titanium fork will make a high pitched ring and vibrate for a long time.
In the Earth these vibrations are called "normal modes" or free oscillations. The normal mode period for a solid iron ball as large as the Earth can be calculated. In 1911 A.E.H. Love made the calculations and determined the period would be about an hour. That is, struck hard enough the whole Earth would vibrate in and out every hour. In 1954 Hugo Benioff announced that he thought he had detected a normal mode of 57 minutes from the Kamchatka earthquake of that year, but the first real proof came in 1961 by Bruce Bolt who clearly identified a 54 minute mode associated with the 1960 Chilean earthquake. He also saw much shorter period vibrations that represent harmonics or overtones of the fundamental long period mode.
The fundamental mode vibrations can tell us about the properties of the whole Earth. The harmonics or overtones give us insight into the deeper parts of the Earth. It is usual to calculate on a computer the expected harmonics for various Earth structures and compare them to observations. That is, the process follows a forward approach. Normal mode inverse methods are available also but they are difficult to implement.
As we started off at the beginning, we can think of this in terms of information types - we observe normal mode vibrations and using a theory of elasticity we can infer parameters associated with the Earth's strength.