Universality of Slow Earthquakes in the Very Low Frequency Band

Abstract:

Deep tectonic tremors have been observed together with signals in the very low frequency (VLF) band from 0.02 to 0.05 Hz, which have been identified as VLF events in limited regions of subduction zones. By stacking broadband seismograms relative to the timing of tremors, we can detect similar signals in all regions where tremors occur in western Japan. These signals are inverted to obtain the moment tensor, and the fault-normal and slip vectors are generally consistent with the geometry of the plate interface and the direction of plate motion. Therefore, these signals are probably radiated by shear slip on the plate interface. The ratio between the seismic energy rate estimated from the tremors and seismic moment rate in the VLF band is almost proportional, with a proportionality constant (i.e., scaled energy) of around 10⁻¹⁰. The spatial distribution of scaled energy may reflect spatial variations in the frequency-dependent characteristics of slow deformation.

This method is broadly applicable if abundant data are available. We demonstrate that focal mechanisms can be determined using VLF signals for Cascadia and Mexican subduction zones.