[S15P-17] Relationships between the peak accelerations and static rotations of the S-net sensors in the Japan Trench area
S-net is a large-scale cable-linked seafloor observation network for earthquakes and tsunamis around the Japan Trench area, established after experiencing the devastating effects of the 2011 Mw 9.1 Tohoku-oki earthquake in northeast Japan (Aoi et al. 2020). The network comprises six segments with 150 stations equipped with seismometers and pressure gauges and has been operated by National Research Institute for Earth Science and Disaster Resilience. Out of the 150 stations, 41 stations in the shallow-water regions (water depth < 1,500 m) were buried approximately one meter below the seafloor, whereas the other stations were laid freely on the seafloor in the deeper-water regions. One of the main objectives of the network was to enhance the Japan Meteorological Agency (JMA) earthquake early warning (EEW) and tsunami early warning systems.
It has been reported that the cable-linked sensor houses at the seafloors undergo larger rotations with the increase of peak ground acceleration (PGA) during earthquake motions (Takagi et al. 2019) and can influence the estimation of earthquake parameters for earthquake early warning systems (Hayashimoto et al. 2019). In the present research, we estimated the threshold peak acceleration required in order to induce rotation of the S-net strong-motion sensors by fitting the observed static rotation amounts with the peak acceleration values at each station separately using a larger data set. Firstly, we computed the changes in the orientation angles (hereafter written as rotations) of the sensors by evaluating the roll and pitch angles of the sensors from the pre-event and post-event parts of the 120,112 recordings from 1,878 earthquakes with Mj values greater than four, recorded between August 2016 and June 2022. The observed maximum value of the changes in the orientations was for the roll angle with a change value of approximately 16o at the S2N06 station during the 16th March 2022, Mj 7.4 event. In our data set, changes in orientations by more than 1o were observed in 17 event data, out of which nine event data were from nine separate stations of the S2 segment. The PGAs recorded for these nine cases were between approximately 150 cm/s2 and 1,400 cm/s2, and the largest rotation experienced by a station was not necessarily associated with the largest peak acceleration recorded at the station.
By observing that the rotations of the sensors were smaller and relatively flat until some threshold accelerations, we fitted the changes in the orientations with PGAs with two-line segments: one with a constant rotation angle until a certain threshold PGA and the other with increasing rotations for higher PGAs. The analysis showed that the rotations increase log-linearly with the recorded peak accelerations after some threshold accelerations of values between approximately 5 and 50 cm/s2. No systematic differences were found in the threshold peak accelerations between the buried and unburied stations. However, the changes in the pitch angles were systematically smaller than those for the roll angles, and the threshold PGAs required in order to induce changes in the pitch angles were, on average, smaller than those for the changes in the roll angles at both buried and unburied stations. The detailed results and some discussions on the recorded peak accelerations for the 2022 Mj 7.4 event mentioned above are included in Dhakal et al. (2023).
References:
Aoi et al. (2020) Earth Planets Space, 72:126
Takagi et al. (2019) Seismol Res Lett, 90(6):2175–2187
Hayashimoto et al. (2019) Q J Seismol, 83(1):1–10
Dhakal et al. (2023) J Disaster Res, 18:7 (in press)
It has been reported that the cable-linked sensor houses at the seafloors undergo larger rotations with the increase of peak ground acceleration (PGA) during earthquake motions (Takagi et al. 2019) and can influence the estimation of earthquake parameters for earthquake early warning systems (Hayashimoto et al. 2019). In the present research, we estimated the threshold peak acceleration required in order to induce rotation of the S-net strong-motion sensors by fitting the observed static rotation amounts with the peak acceleration values at each station separately using a larger data set. Firstly, we computed the changes in the orientation angles (hereafter written as rotations) of the sensors by evaluating the roll and pitch angles of the sensors from the pre-event and post-event parts of the 120,112 recordings from 1,878 earthquakes with Mj values greater than four, recorded between August 2016 and June 2022. The observed maximum value of the changes in the orientations was for the roll angle with a change value of approximately 16o at the S2N06 station during the 16th March 2022, Mj 7.4 event. In our data set, changes in orientations by more than 1o were observed in 17 event data, out of which nine event data were from nine separate stations of the S2 segment. The PGAs recorded for these nine cases were between approximately 150 cm/s2 and 1,400 cm/s2, and the largest rotation experienced by a station was not necessarily associated with the largest peak acceleration recorded at the station.
By observing that the rotations of the sensors were smaller and relatively flat until some threshold accelerations, we fitted the changes in the orientations with PGAs with two-line segments: one with a constant rotation angle until a certain threshold PGA and the other with increasing rotations for higher PGAs. The analysis showed that the rotations increase log-linearly with the recorded peak accelerations after some threshold accelerations of values between approximately 5 and 50 cm/s2. No systematic differences were found in the threshold peak accelerations between the buried and unburied stations. However, the changes in the pitch angles were systematically smaller than those for the roll angles, and the threshold PGAs required in order to induce changes in the pitch angles were, on average, smaller than those for the changes in the roll angles at both buried and unburied stations. The detailed results and some discussions on the recorded peak accelerations for the 2022 Mj 7.4 event mentioned above are included in Dhakal et al. (2023).
References:
Aoi et al. (2020) Earth Planets Space, 72:126
Takagi et al. (2019) Seismol Res Lett, 90(6):2175–2187
Hayashimoto et al. (2019) Q J Seismol, 83(1):1–10
Dhakal et al. (2023) J Disaster Res, 18:7 (in press)