Ambient seismic noise interferometry applied to dense OBS network off Ibaraki region, Northeastern Japan along the Japan Trench subduction zone
Abstract
A dense network of 32 ocean bottom seismometers (OBSs) was deployed off Ibaraki region with station intervals of about 6 km from October 2010 (11 OBSs started from February 2010) to October 2011. On March 11, the 2011 off the Pacific coast of Tohoku Earthquake (Tohoku Earthquake) occurred. This OBS network is located near the southern end of the focal area the Tohoku main shock. The largest aftershock (M7.8) occurred 30 minutes after the mainshock in the landward side of the OBS network, which activated very high seismicity of aftershocks. We successfully observed more than 10000 aftershocks that occurred just beneath the network. Precise determination of hypocenters and focal mechanisms of these earthquakes are of importance for understanding stress regime immediately after the huge earthquakes in detail, but there is a trade-off between the estimation of source parameters and that of seismic velocity structure. In particular, uncertainties of seismic properties of thick sediment layers with very slow seismic velocities complicate accurate determination of the hypocenters. Along the existing active-source seismic profiles, P-wave velocity structure has been derived (Mochizuki et al., 2008) but S-wave velocity structure is still unrevealed despite of its importance.
In this study, we applied the ambient noise seismic interferometry to OBS data in order to infer the S-wave velocity structure. We discarded data when the seismic activity was high to ensure the uniform distribution of ambient noise sources. We divided the data into 1-hour segments and removed trends. Then, we applied a 1-bit normalization and calculated cross-correlations for vertical-vertical components to retrieve Green's functions. We applied 5 band-pass filters between 0.0625 to 0.4325 Hz to obtain group velocities for each frequency range. The results indicate group velocity of ~0.3 km/s for each station pair, which is attributed to the thick sediment that covers the seafloor. Such sediment, as well as the bathymetry , which varies by about 1000 m in depth within the network, should be reasonably treated in simulating waveform propagation. In our future work, we will employ the technique of finite element three-dimensional waveform computation (Ichimura et al., 2017) to acquire reliable source parameters based on realistic velocity model.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.S11C0347Y
- Keywords:
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- 7212 Earthquake ground motions and engineering seismology;
- SEISMOLOGY;
- 7219 Seismic monitoring and test-ban treaty verification;
- SEISMOLOGY;
- 7255 Surface waves and free oscillations;
- SEISMOLOGY;
- 7270 Tomography;
- SEISMOLOGY