| 研究実績の概要 |
On the top of the seafloor, two layers hamper the analysis of body waves recorded by ocean bottom seismometers (OBSs). They are the layer of water mass above the OBS and the seafloor sediment layer right beneath an OBS. The seafloor sediment layer is generally thin, less than a kilometer, and the P-wave and S-wave velocities are very low. A large impedance contrast between the sediment layer and the basement basaltic crust is the cause of strong P-wave and S-wave reverberations. Effects of the seafloor sediment layer are predictable if the structure of the sediment layer is known.
Through this project, a new method that characterizes the seafloor sediment layer using body wave records of earthquakes. Records of the radial, vertical, and pressure components are used to invert the multi-layered sediment structure at the seafloor. The method can resolve the P-wave impedance, P-wave travel time, and the S-wave velocity using the Markov chain Monte Carlo approach. The method has strength in taking the noise level information to retrieve the absolute value of the parameters. Synthetic tests using various models and real data applications to two stations showed the ability of the method.
Moreover, a possible influence of the sediment layer on the OBS data is investigated. The reverberations in the sediment and water layer of the ocean bottom environment cause frequency-dependent correction terms for the shallow layers (water and sediment). Also, we could identify the potential of combining the new method with the S-receiver function and surface wave analyses.
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