Repetitive signals of sustained amplitude are observed both in music and seismology; in music this is often described as the rhythm, whereas in seismology such signals are often labeled as tremor (e.g., Chouet 1996). Tremor is generated by a variety of sources including volcanoes, glaciers, and hydroelectric plants (for examples, see Ferrick et al. 1982) and is often characterized by narrowband spectral features that maintain their amplitude, sometimes for periods of hours or days (so-called harmonic tremor). In this paper we present observations from an unusual anthropogenic source of seismic tremor, generated at an interface between...

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