Micro low-frequency tremor near the Japan Trench before the Tohoku-Oki earthquake

Slow earthquakes, such as tectonic tremors (TTs) and slow slip events (SSE), are the most distinctive geophysical phenomena on the subducting plate interface, especially at the updip and downdip ends of coseismic slip areas. TTs and SSEs have been observed in the updip portion of the subduction zone near the Japan Trench [Kato et al., 2012; Ito et al., 2013, 2015]. Furthermore, TTs have been recognized in the shallower part of the subduction zone in southern Kyusyu using ocean-bottom seismometers (OBS) [Yamashita et al., 2015]. Ito et al. [2015] observed three possible TTs sequences from excitation of ambient noise amplitude accompanying SSE. The tremor signals in these sequences had very weak amplitudes and were observed at only one station, nearest to the Japan Trench. Here, we report on micro low-frequency tectonic tremor (mLFTs) activity near the Japan Trench using the frequency scanning method (FCM) at a single station [Sit et al., 2012], prior to the 2011 Tohoku-Oki earthquake. Sit et al. [2012] proposed a TT detection method of calculating envelope waveform ratios through different bandpass filters of broadband data in the Cascadia margin. We applied this analysis to seismic data recorded at 17 short-period OBS deployed in the Japan Trench axis area off Miyagi, northeast Japan. Three bandpass filters of 2-4 Hz, 10-20 Hz, and 0.5-1.0 Hz, corresponding to the dominant frequency band of TTs, local earthquakes, and ocean noises, respectively, were used between January 1st to March 9th 2011, prior to the largest foreshock of the Tohoku-Oki earthquake. The results of the FCM show three major tremor sequences, suggesting tremor activity in the shallowest part of the subduction zone. The sequences agree with the results of Ito et al. [2015]. Furthermore, we successfully detected TTs at landward stations in the second sequence. We estimated the energy release from the envelope amplitude of two horizontal components filtered to 2-4Hz after removing site effects. The energy estimate is typically very weak, compared with ordinal TTs observed in other regions. The energy released near the trench just before the largest foreshock is larger than at any other period, suggesting that mLFTs occurred next to the Japan trench. The mLFT activity likely increased during the three days before the largest foreshock of the Tohoku-Oki earthquake.