Model of deep non-volcanic tremor part II: episodic tremor and slip
  Bursts of tremor accompany a moving slip pulse in Episodic Tremor and Slip (ETS) events. The sources of this non-volcanic tremor (NVT) are largely unknown. We have developed a model describing the mechanism of NTV generation. According to this model, NTV is a reflection of resonant-type oscillations excited in a fault at certain depth ranges. From a mathematical viewpoint, tremor (phonons) and slip pulses (solitons) are two different solutions of the sine-Gordon equation describing frictional processes inside a fault. In an ETS event, a moving slip pulse generates tremor due to interaction with structural heterogeneities in a fault and to failures of small asperities. Observed tremor parameters, such as central frequency and frequency attenuation curve, are associated with fault parameters and conditions, such as elastic modulus, effective normal stress, penetration hardness and friction. Model prediction of NTV frequency content is consistent with observations. In the framework of this model it is possible to explain the complicated pattern of tremor migration, including rapid tremor propagation and reverse tremor migration. Migration along the strike direction is associated with movement of the slip pulse. Rapid tremor propagation in the slip-parallel direction is associated with movement of kinks along a 2D slip pulse. A slip pulse, pinned in some places, can fragment into several pulses, causing tremor associated with some of these pulse fragments to move opposite to the main propagation direction. The model predicts that the frequency content of tremor during an ETS event is slightly different from the frequency content of ambient tremor and tremor triggered by earthquakes. 
Naum I. Gershenzon and Gust Bambakidis
physics.geo-ph