Evaluating sensor azimuths of the offshore segment of N-net based on teleseismic records

Abstract Seafloor networks detect earthquakes and tsunamis more quickly than those on the mainland in the subduction zones. N-net was constructed in the western portion of the Nankai Trough subduction zone to fill the gap in the seafloor observation system in the region. The network comprises 36 stations, grouped into offshore and inshore segments, with each segment consisting of 18 stations. The offshore segment has recorded several teleseismic events since its operation started in July 2024. Due to a complex installation environment, the 3D sensor orientations of the seismometers at the N-net stations have remained unknown. The accelerometer records can constrain the sensor orientations around the horizontal axes, but the azimuth, sensor orientation around the vertical axis, has not been evaluated. However, the sensor azimuths of the seismometers are important for various seismological studies. This technical report focuses on determining the sensor orientations of the offshore N-net seismometers using teleseismic waveform records from large-magnitude earthquakes, including the 2025 Mw 7.7 Myanmar and Mw 8.8 Kamchatka earthquakes. The estimation followed the Rayleigh-wave polarization method, which was successfully applied for S-net in a previous study. To validate the results, bandpass-filtered waveforms from N-net were compared with nearby F-net stations, which have well-calibrated azimuths. The transverse, radial, and vertical components of both networks showed substantial waveform similarity. The estimated azimuths were also consistent with the measured azimuths of the cable layout during installation operation. These correspondences indicated that the azimuths of the N-net seismometers were reasonably determined. Graphical abstract