Very low frequency earthquakes within accretionary prisms are very low stress-drop earthquakes

We report characteristic P wave spectra and stress drops of very low frequency (VLF) earthquakes that occurred within the accretionary prism along the Nankai Trough, southwestern Japan. Although many VLF earthquakes showed no distinct phases on a raw seismogram, a few showed slightly distinct phases. The arrival times of some phases were consistent with the calculated P wave arrival times. The corner frequencies of the VLF earthquakes were calculated using the stacked P wave spectra assuming an omega-square spectrum. Stress drops were very low, i.e., in the range of 0.1 - 10 kPa, which corresponds to 0.1% - 1% of those of ordinary earthquakes. Such extremely low stress drops of the VLF earthquakes suggest that the fault strength within the accretionary prism may have weakened due to the existence of a fluid within the thrust fault system of the accretionary prism.