[STT51-P02] Study on the cause of the misalignment of the incidence azimuth of the air gun signal at the S-net station estimated from particle motion
Keywords:S-net, particle motion, transmitted wave
The author estimated the orientation of the Ocean Bottom Seismometer (OBS) and seismic wave velocities in the sediment of seafloor surface at 12 stations of the large cabled observatory ”S-net” (Seafloor observation network for earthquakes and tsunamis along the Japan Trench), consisting of 150 stations in a vast area in the Pacific Ocean on the east coast of Japan from off Hokkaido to Boso Peninsula, which was installed by National Research Institute for Earth Science and Disaster Resilience (NIED). The result of the estimated orientation of the OBSs is consistent with Takagi et al. (2019). P-wave velocity is estimated to be around 1.8 km/s at most stations, and S-wave velocity is estimated to be less than 0.4 km/s at stations on landward slope and 0.3–0.7 km/s at those on the seaward slope of the trench.
However, the estimated incident azimuth of the air gun signal based on the particle motion does not always coincide with the actual azimuth of the air gun (ship azimuth) in time series, and misalignment is recognized. Some are caused by local seafloor topography such as a nearby seamount having a large relative height. On the other hand, however, misalignments are observed at some stations where such seafloor topography does not exist. Regarding the misalignment of the sound source azimuth, it was observed that in the sound source localization of an air gun using OBS off Kamaishi City in Iwate Prefecture the misalignment was up to approximately 9 degrees (Iwase, 2016).
The topographic feature common to the stations installed in the Japan Trench area is the slope. Therefore, the effect of the case where the fluid-solid interface is inclined was examined. However, the observed value cannot be explained by the inclination angle estimated from the 500 m mesh seafloor topographic data published by JODC of the Japan Coast Guard. Also, the slope estimated from the particle motion of the surface wave observed in the air gun signal is roughly equivalent to the seafloor topographic data.
When the relationship between the X-axis of the OBS and the azimuth misalignment was examined, the misalignment tends to be smaller in the azimuth of the X-axis of the OBS. This suggests the influence of the seismometer installation orientation and the coupling state with the seafloor.
I would like to thank and National Research Institute for Earth Science and Disaster Prevention (NIED), Earthquake Research Institute (ERI), The University of Tokyo, and Japan Oceanographic Data Center (JODC), Japan Coast Guard, for providing data.
References
R. Iwase, Jpn. Jour. App. Phys, 55, 07KG01, 2016.
R. Takagi et al., SRL, 90(6), 2175-2187, 2019.
However, the estimated incident azimuth of the air gun signal based on the particle motion does not always coincide with the actual azimuth of the air gun (ship azimuth) in time series, and misalignment is recognized. Some are caused by local seafloor topography such as a nearby seamount having a large relative height. On the other hand, however, misalignments are observed at some stations where such seafloor topography does not exist. Regarding the misalignment of the sound source azimuth, it was observed that in the sound source localization of an air gun using OBS off Kamaishi City in Iwate Prefecture the misalignment was up to approximately 9 degrees (Iwase, 2016).
The topographic feature common to the stations installed in the Japan Trench area is the slope. Therefore, the effect of the case where the fluid-solid interface is inclined was examined. However, the observed value cannot be explained by the inclination angle estimated from the 500 m mesh seafloor topographic data published by JODC of the Japan Coast Guard. Also, the slope estimated from the particle motion of the surface wave observed in the air gun signal is roughly equivalent to the seafloor topographic data.
When the relationship between the X-axis of the OBS and the azimuth misalignment was examined, the misalignment tends to be smaller in the azimuth of the X-axis of the OBS. This suggests the influence of the seismometer installation orientation and the coupling state with the seafloor.
I would like to thank and National Research Institute for Earth Science and Disaster Prevention (NIED), Earthquake Research Institute (ERI), The University of Tokyo, and Japan Oceanographic Data Center (JODC), Japan Coast Guard, for providing data.
References
R. Iwase, Jpn. Jour. App. Phys, 55, 07KG01, 2016.
R. Takagi et al., SRL, 90(6), 2175-2187, 2019.