|
so:text
|
After a large earthquake the earth "rings" like a bell; this motion can be observed on sensitive instruments up to a month after a large event. These oscillations have specific frequencies which are properties of the whole earth and which can be measured very accurately indeed. The lowest frequency oscillation has a period of about one hour. Any combination of seismic waves can be represented as an equivalent combination of normal modes. In practice the mode representation is most useful at low frequency — for seismic waves above about 40 s — since at higher frequencies the number of modes becomes prohibitively large. (en) |
