What is a characteristic time of nonvolcanic tremor in slow earthquakes?

Non-volcanic tremors is considered as a high-frequency component of slow earthquakes discovered in many places worldwide Several aspects of tremor are well approximated by a Brownian slow earthquake model governed by Langevin equation (Ide, 2008), which also presents an analytic form of moment rate, and seismic energy rate, spectra. The Fourier spectrum has a flat amplitude level below a corner frequency and a high-frequency fall-off of 1/f. The corner corresponds to a characteristic time, which is much shorter than the total duration of activity. Although every tremor resembles to random noise, the apparent duration of its active period is different. For example, it is much longer in Cascadia than in western Japan. Each tremor sequence may have a corresponding characteristic time that reflects regional geophysical conditions. In the present study, we develop an inversion method to estimate characteristic duration of tremor activity. Velocity records of tremor are high-pass filtered and squared to obtain seismic energy rate functions. We have to remove spiky seismic events and calculate synthetic spectra including background noise, which is approximated by a fractional Gaussian process. We apply the inversion method to tremor sequences observed in different regions, Kii Peninsula, western Shikoku in Nankai subduction zone, and Cascadia subduction zones. The characteristic time in Kii, Shikoku, and Cascadia are about 120, 700, and 1200 s, respectively, which may be related to the size of slow slip events in these regions.