Further evidence for the occurrence of tectonic tremor in the Japan Trench subduction zone

Abstract:

Tectonic tremors are observed worldwide and most in the hot young subduction zones. However, recent studies reported that tremors and slow slips (SSEs) occur in the Japan subduction zone which is rather cold or old subduction (Kato et al., 2012; Ito et al., 2015). Ito et al. (2015) showed the coincidental excitations of the tremors and the SSE before the 2011 Tohoku-Oki earthquake by examining the ambient noise level in the ocean bottom seismometer (OBS) records. However, the previous method is only based on amplitudes. In this study, we examine polarization and envelope amplitudes in different frequency bands of OBS data for validation and further detection of tremors beneath the Japan subduction zone.

The average particle motion polarization for every 10-minute time window is calculated on the basis of the three-component covariance matrix of ground motion. The degree of linearity and the polarization azimuth are measured from the ratio among three eigenvalues and the eigenvector corresponding to the largest eigenvalue, respectively (Jurkevicks, 1988; Wech et al., 2007). We apply the method to the continuous velocity seismograms for 5 months from November 2010 to March 2011 recorded at 17 short-period OBS network stations deployed in the Japan Trench axis area off Miyagi, northeast Japan.

The polarization results show three major tremor sequences which correspond to the tremor sequences reported in Ito et al. (2015) and several minor tremor sequences. The stable and nearly constant polarization azimuths in these sequences indicate similar focal mechanisms and epicenters. The azimuths slightly change toward the largest foreshock of the 2011 Tohoku-Oki earthquake, which may possibly indicate the migration of tremor sources. To validate these tremor signals, we apply another detection method based on the frequency scanning detection at a single station (Sit et al., 2012). For a given time window, the ratios among the averaged amplitudes of three bandpass filtered envelope waveforms from 2-4 Hz, 10-20 Hz and 0.5-1.0 Hz (tremor, local earthquakes and ocean noises) are used to distinguish tremor signals from other noises. The preliminary result shows good correlation with the result from Ito et al. (2015) and the polarization analysis, which strongly support the occurrence of tremors in the Japan subduction area.