For the first time in history two scientists succeeded in modulating an earthquake. The theory behind seismic signals was well known, but so far nobody had been able to use earth’s rumbling to communicate between two points. The mode used to modulate an earthquake: AM.
Randall D. Peters, Physics Department, Mercer University, explains:
Amplitude modulation is expected to be visible during large earthquakes, which are known to excite free (eigenmode) oscillations of the Earth. A means for observing the modulation, as illustrated in the materials that follow, is to perform frequency domain calculations on the ‘rectified’ signal, as opposed to the customary calculations that are performed on the ‘raw’ signal.
It’s easy to add meaningful information to such a raw signal. In essence, the raw signal of the earthquake can be seen and treated as an ordinary carrier wave.
The evolution of the spectral content of a signal is easy to analyze in a cumulative spectrum, as shown in the image below: