Relationship between moment rate of short-term slow slip events and moment rate of tectonic tremors in a frequency range of 1–10 Hz

Simultaneous activations of slow slip events (SSEs) and tectonic tremors are observed along subduction zones, and collectively referred to as episodic tremor and slip (ETS). It is assumed that tectonic tremors represent the manifestations of SSEs in a higher frequency range. We estimated the band-limited moment rate of deep tectonic tremors in a frequency range of 1–10 Hz during short-term SSEs (S-SSEs) along the Nankai Trough subduction zone, Japan, where these events typically last for several days. We calculated the envelopes of short-period seismic records and integrated them to estimate the band-limited seismic moment rate of the tremors in a frequency range of 1–10 Hz, using a conversion coefficient derived from synthetic records. The band-limited moment rate was determined by dividing the total seismic moment of the tremors by the duration of the S-SSE. By comparing these results with moment rates derived from strain data, we found that the band-limited tremor moment rates were approximately 0.5–1.9 orders of magnitude smaller than those obtained from crustal deformation. This difference was significantly smaller than that expected from the spectral amplitudes of ordinary earthquakes. The Brownian walk model has been proposed to interpret the proportional relationship between durations and seismic moments of the slow earthquakes. Based on the Fourier spectrum of the simulated Brownian walk model, this small difference was interpreted as slow slip speed maintained during S-SSEs. However, the Brownian walk model could not quantitatively account for the observed small difference in moment rates.