  There is evidence of triggering of tremor by seismic waves emanating from
distant large earthquakes. The frequency contents of triggered and ambient
tremor are largely identical, suggesting that tremor does not depend directly
on the nature of the source. We show here that the model of plate dynamics
developed earlier by us is an appropriate tool for describing the onset of
tremor. In the framework of this model, tremor is an internal response of a
fault to a failure triggered by external disturbances. The model predicts
generation of radiation in a frequency range defined by the fault parameters.
Other specific features predicted are: the upper limit of the size of the
emitting area is a few dozen km; tremor accompanies earthquakes and aseismic
slip; the frequency content of tremor depends on the type of failure. The model
also explains why a tremor has no clear impulsive phase, in contrast to
earthquakes. A comparatively small effective normal stress (hence a high fluid
pressure) is required to make the model consistent with observed tremor
parameters. Our model indicates that tremor is not necessarily a superposition
of low frequency earthquakes, as commonly assumed, although the latter may
trigger them. The approach developed complements the conventional viewpoint
which assumes that tremor reflects a frictional process with low rupture speed.
Essentially our model adds the hypothesis that resonant-type oscillations exist
inside a fault. This addition may change our understanding of the nature of
tremor in general, and the methods of its identification and location in
particular.
