[S-CG53] Reducing risks from earthquakes, tsunamis & volcanoes: new applications of realtime geophysical data

[SCG53-03] Preliminary comparison of strong-motion parameters at the land & ocean bottom sites for the 2021 Mw 7.1 Off Fukushima Prefecture earthquake

*Yadab Prasad Dhakal¹, Wataru Suzuki¹, Takashi Kunugi¹, Takeshi Kimura¹, Shin Aoi¹ (1.National Research Institute for Earth Science and Disaster Resilience)

Keywords:Ocean bottom seismographs, S-net, The 2021 Mw 7.1 off Fukushima Prefecture earthquake, MOWLAS, PGA, PGV, Long-period ground motions

The 2021 Mw 7.1 Off Fukushima Prefecture earthquake is the largest magnitude earthquake in the Japan Trench area that was successfully recorded by all the 150 stations of S-net (Seafloor observation Network for Earthquake and Tsunami along the Japan Trench) after the start of the network. The Japan Meteorological Agency focal depth of the event was 55 km. The moment tensor solution of F-net (National Research Institute for Earth Science and Disaster Resilience, NIED) showed that the earthquake was a reverse fault event. The distribution of hypocenters of the aftershocks suggested that the event occurred inside the subducting Pacific Plate (NIED, 2021). This event was recorded by about 1060 strong-motion stations of MOWLAS acronym for Monitoring of Waves on Land and Seafloor operated by NIED, including the 150 stations of the S-net (Aoi et al. 2020). The largest peak ground acceleration (PGA) of approximately 1425 cm/s² was recorded at site MYGH10, located at the epicentral distance of about 70 km. This PGA value was larger than the PGA value of 1030 cm/s² recorded at the same site during the 2011 Mw 9.1 Tohoku-Oki earthquake. It is known that the cylindrical casings of S-net stations are prone to rotate during strong motions, adding artificial noises and offsets to the acceleration records. However, the X-component that is aligned along the long axis of the cylindrical casing is found to be generally less affected by the rotation of the cylindrical casing. The maximum X-component PGA of about 1100 cm/s² was recorded at the S-net S2N02 site at the epicentral distance of about 7 km. The X-component peak ground velocity (PGV) at the S2N02 site was about 50 cm/s, while the PGV at the MYGH10 was about 75 cm/s. We compared the X-component PGAs and PGVs from all the S-net sites with those on land recorded by K-NET and KiK-net of MOWLAS, operated by NIED, with respect to the ground motion prediction equation (GMPE) by Si and Midorikawa (1999). We found that the PGA values at the ocean bottom and land sites were, on average, similar within the hypocentral distance of 200 km. On the other hand, the X-component PGVs at the S-net sites were, on average, larger at the S-net sites. Dhakal et al. (2020) pointed out that the larger PGVs at the S-net sites were most probably due to the lower S-wave velocity of the subsea sediments. We also compared the absolute velocity response spectra (AVRS) in the period range of 1 to 10 s at the land and S-net sites with respect to the GMPE by Dhakal et al. (2015). The AVRS were computed from the two horizontal component records (see Dhakal et al. 2015 for details). We used the J-SHIS (Japan Seismic Hazard Information Station, Fujiwara et al. 2012) model to correct the deep soil amplification effect at both land and S-net sites. Similar to the case of PGVs, we found that the observed AVRS values at the S-net sites were, on average, larger than those at the land sites, and the root mean square errors (RMSE) for the S-net sites were slightly larger than those for the land sites. The RMSE values' differences are generally expected as the velocity model at the S-net sites is not well resolved compared with the velocity model on land. We are currently working on larger data set to understand the features of S-net strong-motion records for real-time application and report the results in future papers.



References:

Aoi et al. (2020). Earth Planets Space 72, 126.

Dhakal et al. (2020). Japan Geoscience Union Meeting, SSS04-10.

Dhakal et al. (2015). Journal of Japan Association for Earthquake Engineering, 15(6):91-111.

Fujiwara et al. (2012). Technical Note of the National Research Institute for Earth Science and Disaster Prevention, No. 379.

NIED (2021). The 2021-02-13 Off Fukushima Prefecture Earthquake (in Japanese), https://www.hinet.bosai.go.jp/topics/off-fukushima210213/?LANG=ja, accessed 2021/2/17.

Si H, S. Midorikawa (1999). Journal of Structural and Construction Engineering Transactions of AIJ 523:63–70.