Universality of very low frequency signals behind tremors, explained by a stochastic model

Abstract:

We detected and located tectonic tremors in subduction zones such as Western Japan, Cascadia, Mexico, Chile, New Zealand, and Taiwan, and along the San Andreas Fault. Behind short-period (> 1Hz) seismic signals recognized as tremors, we often detect very low frequency (VLF) seismic waves in 0.02-0.05 Hz. At frequencies between these two bands, microseism noise hinders detecting any signals from deep sources. By stacking broadband seismograms relative to tremor timing, VLF signals are successfully recovered for many regions, as long as noise level is sufficiently low. These signals are useful to constrain the focal mechanism of deformation associated with tremors. Generally the focal mechanisms are consistent with the direction of regional plate motion, suggesting that these represent shear slip on the plate interface. For example, the focal mechanism of tremors in Taiwan has not been constrained well, but the VLF focal mechanism shows a low angle thrust deformation.

The universality of VLF signals behind tremors confirms the hypothesis that these phenomena are observable parts of shear deformation at least between 0.02 and 10 Hz. We may call this broadband deformation as slow earthquakes. The moment rate function and spectrum of slow earthquakes are simulated by a simple Brownian-walk stochastic model with a characteristic time constant. This constant controls the detectability of tremor and VLF signals, and only slow deformation without seismic signals would be found when the constant is very long.