The mass is free to move relative to the earth. A magnetic field is maintained around the mass. As the mass moves, changes in the magnetic field generate a signal. The signal, typically around 1 Hz., is amplified up to 90 Db and converted to frequencies in the voice range. Seismic data transmissions can be identified by the listener by their continuous tone.
Ground movement modulates the signal which changes the tone.
As the spring loaded mass moves in one direction the tone increases pitch, movement in the other direction decreases the pitch of the tone. The analog data produced can be displayed on the rotating drum seismographs that we see on the 11 o'clock news after an earthquake. VHF and UHF frequencies used to transmit the data are usually found in the US Government frequency allocations. Occasionally you may find telemetry on frequencies licensed to universities. Transmitter output power is typically less than 1 watt.