Polarization evidence for the occurrence of shallow tremors in the Japan Trench subduction zone

Abstract:

Recent studies have shown that tremors and slow slips (SSEs) occur in the shallow part of the Japan subduction zone (Kato et al., 2012; Ito et al., 2013; Ito et al., 2015). However, the observation evidences are still fewer than that in the young and hot subduction zones, such as the Nankai subduction zone and the Cascadia subduction zone. Ito et al. (2015) showed the coincidental occurrence of the tremors and the SSE before the 2011 Tohoku-Oki earthquake by examining the changes of ambient noise level in the ocean bottom seismometer (OBS) records. However, the previous tremor detection was only based on the amplitude changes and it was clear only at one station closest to the trench. In this study, we further investigate the polarization of OBS data to validate and deduce further source information of shallow tremors beneath the Japan subduction zone.
We calculate the average particle motion polarization for every 10-minute time window on the basis of the three-component covariance matrix of ground motion. The degree of linearity is measured from the ratio among three eigenvalues and the dominant polarization azimuth is represented by the horizontal components of 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.
We obtain several long sequences of high linearity and nearly constant polarization azimuths associated with tremors from the records of at least three stations near the trench. Three major sequences correspond to the tremor sequences reported in Ito et al. (2015). The stable and nearly constant azimuths in these sequences indicate similar focal mechanisms and epicenters of tremors. The dominant polarization azimuth shows the angle of about 130 degrees clockwise from North, which is almost consistent with the subduction direction of the Pacific plate. Furthermore, the azimuths slightly change toward the timing of the largest foreshock of the 2011 Tohoku-Oki earthquake, which may possibly indicate the migration of tremor sources.