講演情報
[P02-03]Sensor azimuths of the offshore segment of N-net stations based on teleseismic records
*Yadab Prasad DHAKAL1, Takashi Kunugi1, Ryota Takagi2, Tetsuya Takeda1, Shin Aoi1, Katsuhiko Shiomi1 (1. NIED, 2. Tohoku Univ.)
A seafloor network for monitoring earthquakes and tsunamis, known as N-net, has been constructed in the southern half of the Nankai Trough region. The network consists of 36 stations in a hybrid configuration of previous DONET and S-net, such that it can be expanded further (Aoi et al. 2023). The network consists of two segments, offshore and inshore, each with 18 stations. In the present study, we examined the azimuths of the offshore seismometers using the telesemic waveform records from earthquakes, having moment magnitudes greater than seven, including the 2025 Myanmar and Kamchatka mainshock earthquakes. We used the method that was applied to estimate the azimuth of the S-net seismometers in Takagi et al. (2019). In this method, the polarization property of the fundamental-mode Rayleigh waves is utilized. When the Rayleigh wave propagates along the great circle path, we can estimate sensor azimuth from the geometry of the earthquake and station pair. For the elliptical Rayleigh wave, the vertical and radial component waveforms have a 90° phase shift. First, we calculated the Hilbert transform of the vertical component waveforms. Then, cross-correlation coefficients were computed between the Hilbert-transformed vertical and radial component waveforms by changing the sensor azimuths in one-degree intervals between 0 and 360 degrees. Results with cross-correlation coefficients greater than 0.7 were considered. The cross-correlation coefficients were greater than 0.7 for most events except for the NAE10 station in the analysed range of 0.01-0.03 Hz. At the NAE10 station, the value was over 0.7 only for the Mw 8.8 Kamchatka earthquake. The NAE10 has been located near the Kyushu-Palau Ridge, which may have an effect on the waveforms. Mean azimuths were computed for each station. Transverse, radial, and vertical component bandpass-filtered waveforms (0.01-0.02 Hz) at the N-net and nearby F-net stations are shown in the accompanying figure for the Kamchatka earthquake. Site codes, epicentral distance in km, and peak amplitude in cm/s are included. Strong similarity can be seen between the N-net station records (black traces) and F-net station records (red traces). The azimuths have been documented well at the F-net stations. These observations may suggest that the estimated azimuths of the N-net stations are reasonable. A manuscript with detailed results is under preparation and will be published elsewhere.
References:
Aoi S, Takeda T, Kunugi T, Shinohara M, Miyoshi T, Uehira K, Mochizuki M, and Takahashi N (2023) Development and Construction of Nankai Trough Seafloor Observation Network for Earthquakes and Tsunamis: N-net. 2023 IEEE Underwater Technology (UT), Tokyo, Japan, 2023, pp. 1-5. https://doi.org/10.1109/UT49729.2023.10103206
Takagi R, Uchida N, Nakayama T, Azuma R, Ishigami A, Okada T, Nakamura T, Shiomi K (2019) Estimation of the orientations of the S-net cabled ocean-bottom sensors. Seismol Res Lett 90:2175-2187. https://doi.org/10.1785/0220190093
References:
Aoi S, Takeda T, Kunugi T, Shinohara M, Miyoshi T, Uehira K, Mochizuki M, and Takahashi N (2023) Development and Construction of Nankai Trough Seafloor Observation Network for Earthquakes and Tsunamis: N-net. 2023 IEEE Underwater Technology (UT), Tokyo, Japan, 2023, pp. 1-5. https://doi.org/10.1109/UT49729.2023.10103206
Takagi R, Uchida N, Nakayama T, Azuma R, Ishigami A, Okada T, Nakamura T, Shiomi K (2019) Estimation of the orientations of the S-net cabled ocean-bottom sensors. Seismol Res Lett 90:2175-2187. https://doi.org/10.1785/0220190093
