[SCG58-P40] Spatial heterogeneity of shallow tremor activity near the Japan Trench
Keywords:tremor, Japan Trench, array analysis, OBS
Here, we updated hypocenters of ambient tremors and dynamically triggered tremors using the Multi Beam Back-Projection method [Ghosh et al., 2009; 2012] to verify if tremors are occurring at the spatial gap. Ghosh et al. (2009; 2012) does not evaluate uncertainty of located tremor explicitly. In our study, we evaluate uncertainty of located hypocenters following a method proposed in Arnadottir and Segall (1994).
Succeeded in locating the tremors, we estimated their energy rate. The site amplification factor of each OBS is corrected between 2Hz – 8Hz by the coda normalization method (e.g., Sato and Fehler, 1998). After the site amplification factor is corrected for each OBS, we evaluate seismic energy of the tremors at each array. At certain array, time delay is applied to waveforms of the tremors according to their slowness and arrival direction acquired through beamforming. The delayed traces are stacked to suppress ambient noise. The stacked traces at each array is then collected for geometrical spreading and intrinsic attenuation to estimate seismic energy at the source. At last, the estimated energy is divided by duration of the tremor to convert to the energy rate.
The updated location still shows spatial heterogeneity within a shallow tremorgenic region. The ambient tremor activity is more frequent in the up-dip part of the region, and less frequent in the down-dip part. Although spatial distribution of the energy rate of the tremors is more or less homogeneous within the whole region, some of the tremors occuring up-dip have about 2 order larger energy rate than others. The frequent and potential of high energy rate activity of the tremor in the up-dip part may be related to the crustal deformation observed near the trench axis in offshore Fukushima [Iinuma et al., 2016; Tomita et al., 2017].
Acknowledgements: This study is supported by JSPS KAKENHI (19H05596).