Volcanic Tremor - John Seach

Tremor is a continuous vibration of the ground around volcanoes.
The signal lasts for a period from minutes to days. The dominant frequency is 1-5 Hz.

Harmonic tremor is a low frequency sine wave with smoothly varying amplitude.
Spasmodic tremor is a high frequency, pulsating, irregular signal.
 

Volcanic Tremor
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Copyright John Seach