Modulation of Tremor and Modulation of Noise: Same Natural Phenomenon?

One of the high-profile developments in seismology over the past decade has been the observation of episodic tremor and slip (ETS) and nonvolcanic tremor (NVT) as a subset of the phenomenon (Peng and Gomberg, 2010; Vidale and Houston, 2012). Sensitivity of an NVT occurrence to very small, dynamic, low-frequency stress perturbations in the Earth’s crust has been documented. For example, Rubinstein et al. (2007, 2008) reported pulsations of the envelope of the highfrequency (generally above 1 Hz) tremors with much longer periods of Earth tides and surface waves from large earthquakes. The phenomenon has been termed modulation. In reading through the recurring reports of the phenomenon of triggered NVT, I was struck by the resemblance to much earlier observations that dated back to the 1970s and 1980s.