[S-SS02] Seismological advances in the ocean

[SSS02-09] Threshold PGAs for inducing rotations of accelerometers at the S-net sites: a preliminary site-specific analysis

*Yadab Prasad Dhakal¹ (1.National Research Institute for Earth Science and Disaster Resilience)

Keywords:Ocean bottom seismograph, S-net, Ocean bottom seismology, MOWLAS, Rotation of sensors, Strrong ground motions

A large-scale in-line type ocean-bottom seismograph network, abbreviated as S-net, has been set up in the Japan Trench area, consisting of 150 observatories equipped with seismometers and tsunami-meters after the Tohoku-Oki earthquake disaster (Aoi et al. 2020). Most stations at water depths < 1500 m were buried to a depth of about one meter while they were sited freely on the seafloors at greater water depths. One of the objectives to bury was to safeguard the stations from damages due to fishing activities. At the same time, it was expected that the burial would enhance the coupling between the cylindrical sensor vessels and the seabed. This study analyzed the relationship between the rotation amount of accelerometer sensors due to strong-motions with the peak acceleration (PA) recorded by the sensors. Two types of rotation angles were analyzed. Roll: rotation angle around the long-axis of the cylinder i.e. around the X-axis of the sensor, and Pitch: rotation angle around the Y-axis of the sensor. Two segment lines were fitted between the absolute values of the rotation angles and the maximum values of the PAs of three-component records. The number of records at each station ranged between about 150 and 800. The records were obtained for more than 1000 earthquakes having Japan Meteorological Agency magnitude values greater than four from the period between Feb 2016 and Sep 2019. Preliminary results showed that the sensors generally remain stable up to the PA of about 50 cm/s/s, and the rate of rotation increases with the increase of PAs irrespective of the buried and non-buried stations. It is found that the rotation angle around the X-axis is generally larger than the rotation angle around the Y-axis for a given PA after the threshold value. It is found that the rotation around the both axes, on average, started at the same threshold PA and the threshold PAs ranged between about 10 and 50 cm/s/s at different sites. The buried stations showed that the roll and pitch angle values below the threshold PGAs were generally similar, while they were different at the non-buried stations. However, exceptions were present in both groups of stations. These results are generally similar to those reported in previous studies (e.g., Takagi et al. 2019). It is not so clear how these rotations affect the true ground motions. At sites of expected high PGAs, offsets to acceleration records are found, especially dominant in the Y and Z component records. Visual inspection of the Fourier spectra and time-frequency evolution suggested that the lower frequency contents from about 0.1 Hz are considerably contaminated by the large rotations following the strong S-phase. Nakamura and Hayashimoto (2019) indicated that the X-component records from the epicentral area may be used for near real-time estimation of magnitude if the offsets are small. A preliminary comparison of the observed PGAs, PGVs, and absolute velocity response spectra at the land and S-net stations for the 2021-02-13 Mw 7.1 Off Fukishima Prefecture earthquake are discussed in a separate paper in this meeting (Dhakal et al. 2021). A comparison of PGA, PGV, and acceleration response spectra on land and ocean bottom from several earthqaukes is submitted elsewhere (Dhakal et al. 2020). This is an ongoing study, and further analysis results will be presented in future papers.



Acknowledgment
This study was supported by the "Advanced Earthquake and Tsunami Forecasting Technologies Project" of NIED and JSPS KAKENHI Grant Number JP20K05055.

References:
Aoi et al. (2020). Earth Planets Space 72, 126.
Dhakal et al. (2020). Bull. Seism. Soc. Am. (Under Revision).
Dhakal et al. (2021). Japan Geoscience Union Meeting, Session ID S-CG53.
Nakamura and Hayashimoto (2019). Geophys. J. Int. 216, 1413–1427.
Takagi et al. (2019). Seismol. Res. Lett. 90, 2175–2187.