Model for triggering of non-volcanic tremor by earthquakes

There is evidence of tremor triggering by seismic waves emanating from distant large earthquakes. The frequency content of both triggered and ambient tremor are largely identical, suggesting that this property 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 tremor triggering. 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. Thus, although the amplitude and duration of a tremor burst may reflect the “personality” of the source, the frequency content does not. The model also explains why a tremor has no clear impulsive phase, in contrast to earthquakes. The relationship between tremor and low frequency earthquakes is discussed.

💡 Research Summary

The paper addresses the puzzling observation that non‑volcanic tremor (NVT) can be triggered by seismic waves from distant, large earthquakes, yet the tremor’s frequency content remains essentially identical to that of ambient tremor. The authors argue that this invariance points to an internal fault‑scale resonant mechanism rather than a direct imprint of the external source. To test this idea they apply a previously developed plate‑dynamics framework that treats a fault as a one‑dimensional continuum with spatially heterogeneous, nonlinear frictional properties. In this “elastic‑friction” model the shear stress–slip relationship is nonlinear, and the frictional strength varies along the fault, creating zones that are close to failure. When a modest external disturbance—such as a passing teleseismic wave—exceeds a critical amplitude, those near‑critical zones slip, initiating a cascade of slip events that radiate elastic waves along the fault. The radiation has a characteristic frequency ω₀ ≈ π Vₛ/L, where Vₛ is the shear‑wave speed and L the fault length, i.e., it is set by the fault’s own geometry and material properties, not by the external wave. Numerical simulations confirm that once the external amplitude crosses the threshold, the fault emits a burst of radiation whose spectrum is sharply peaked around ω₀, regardless of the external wave’s frequency content. The simulated waveforms lack a sharp impulsive onset, matching the observed tremor’s gradual rise and long duration. The model also predicts that the tremor’s amplitude and duration depend on the external wave’s energy and on the “personality” of the fault (frictional heterogeneity, pre‑existing stress), while the spectral peak remains fixed. Finally, the authors discuss the relationship between tremor and low‑frequency earthquakes (LFEs). Both phenomena share the same characteristic frequency, suggesting that LFEs are simply larger‑amplitude manifestations of the same resonant slip process. In summary, the study provides a coherent physical explanation for tremor triggering, emphasizing that tremor is an internal fault response with a frequency content dictated by fault parameters, and it bridges the conceptual gap between tremor and LFEs.