Tectonic tremor

Tectonic, non-volcanic tremor is a weak vibration of ground, which cannot be felt by humans but can be detected by sensitive seismometers. It is defined empirically as a low-amplitude, extended duration seismic signal associated with the deep portion (∼20–40 km depth) of some major faults. It is typically observed most clearly in the frequency range of 2–8 Hz and is depleted in energy at higher frequencies relative to regular earthquakes.