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\begin{document}

\title{A comparison of horizontal and vertical peak accelerations and
velocities recorded by S-net ocean-bottom seismographs in the Japan
Trench area with the values recorded by stations on the ground in and
around the Kanto Basin}



\author[1,1]{Yadab Dhakal}%
\author[1,1]{Takashi Kunugi}%
\affil[1]{National Research Institute for Earth Science and Disaster Resilience}%


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  \date{May 10, 2024}


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\begin{abstract}
A large-scale seafloor observation network, known as S-net, has been
operated by National Research Institute for Earth Science and Disaster
Resilience (NIED) in the Japan Trench area since 2016. The main
objective of the observation is to provide reliable earthquake and
tsunami early warnings. The network consists of 150 in-line-type
observatories with sensors housed in cylindrical pressure vessels. The
stations and cables were buried about one meter beneath the seafloor in
the region of water depth less than 1500 m while they were sited on the
seafloor in the deeper area. Previous studies have shown that the S-net
sensors rotate after some threshold peak ground accelerations due to
poor coupling between the sensor house and sediment, resulting in large
rotational noises. Another serious concern at the S-net sites is the
amplification effect of sediments. To minimize the various effects,
researchers have devised equations using the displacement amplitude of
vertical component of motions to estimate the magnitude for Japan
Meteorological Agency earthquake early warning (Hayashimoto et al.
2019). The vertical motions are affected as well by the presence of
water layer in the oceanic area. Thus, the analysis of seafloor records
is challenging due to the multiply effects of sediments and water layer.
In this meeting, we report the results of analyses of strong-motion
records at the ground in the Kanto Basin and adjoining areas and the
S-net sites from nearby moderate to strong offshore earthquakes. Strong
motions at ground and borehole sites were taken from K-NET/KiK-net and
MeSO-net, respectively. We have found that the horizontal PGAs and PGVs
at the S-net sites were, on average, similar to or larger than those at
the ground and borehole sites at equal distances. In contrast, the
vertical PGAs and PGVs at the S-net sites tended to be smaller than
those at the ground sites for the S wave. Notably, the PGAs and PGVs for
the P-wave parts on the vertical records of S-net were smaller than
those at the ground sites. A portion of the results was published in
Dhakal and Kunugi (2021). We are now analyzing a larger dataset and
report the detailed results elsewhere. This study was supported by the
Advanced Earthquake and Tsunami Forecasting Technologies Project of NIED
and JSPS KAKENHI Grant Number JP20K05055.%
\end{abstract}%



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