Detecting tectonic tremor through frequency scanning and polarization analyses at a single station in the Japan Trench subduction zone

Wednesday, 24 February 2016
Satoshi Katakami1, Yoshihiro Ito2, Kazuaki Ohta2, Ryota Hino3, Syuichi Suzuki4 and Masanao Shinohara5, (1)Kyoto University, Kyoto, Japan, (2)Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan, (3)Tohoku University, Sendai, Japan, (4)Tohoku University, Graduate School of Science, Sendai, Japan, (5)University of Tokyo, Bunkyo-ku, Japan
Abstract:
Slow earthquakes, such as tectonic tremors and slow slip events (SSE), are the most distinctive geophysical phenomena on the subducting plate interface and occur at both ends of updip and downdip of coseismic slip areas. Tremors and SSEs have been observed in the subduction zone at the updip portion near the Japan Trench [Kato et al., 2012; Ito et al., 2013, 2015].

 Ito et al. (2015) showed three possible tectonic tremor sequences from the excitation of amplitude of ambient noise accompanying SSE. The tremor signals in these sequences with very weak amplitudes were observed at only one station. Here, we applied two other methods at a single station to detect and validate tectonic tremors near the Japan Trench; we re-examined the frequency scanning analysis [Sit et al., 2012] of ocean bottom seismometer (OBS) data.

 Sit et al. (2012) proposed “the frequency scanning analysis” to detect tectonic tremors by calculating ratios of the envelope waveforms through different bandpass filters of broadband data at a single station in the Cascadia margin. We applied this analysis to the seismic data recorded at 17 short-period OBS network stations deployed in the Japan Trench axis area off Miyagi, northeast Japan. Three types of bandpass filters with frequencies of 2–4 Hz, 10–20 Hz, and 0.5–1.0 Hz, corresponding to the predominant frequency band of tectonic tremors, local earthquakes, and ocean noises, respectively, were adopted.

The results of the frequency scanning analysis show three major tremor sequences, which correspond to the tremor sequences reported in Ito et al. (2015), suggesting the occurrence of tremors in the subduction zone. Furthermore, we successfully detected tremor signals at another two sites, especially from the second tremor sequences. We concluded that the second tremor sequence probably occurred in a slightly far area from the Japan Trench, or with larger magnitude than the other two tremor sequences.

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