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Seafloor pressure observations mainly use precise quartz resonant sensors at the present due to its high sensibility. Recently, a new silicon resonant sensor using Micro Electro Mechanical Systems (MEMS) technology has emerged with low-power consumption, compact-size, constant quality, and high sensitivity. High stability is expected due to small size of a sensing unit, and individual difference of each sensors is believed to be small because of using the MEMS technology. Influence of movement of the sensor itself is small due to mechanism of pressure transmission. Nankai Trough seafloor network for earthquakes and tsunamis-net (N-net) will be equipped with the new sensors. Before routine observations using this new sensor, we evaluate performance of the new silicon resonant sensor for seafloor pressure observation.An existing Free-fall pop-up type Ocean Bottom Pressure gauge (OBP) has a quartz resonant sensor. We replaced the sensor to the new silicon sensor, and performed seafloor observations off Boso peninsula near Tokyo. We obtained pressure data from two new sensors. Ambient noise spectra of the new sensor was compared to that of the ordinary sensor which was installed at the same position. The new and ordinary sensors have identical noise level at periods longer than about 50 s. However, the new silicon sensor seems to have higher noise levels at periods between 50 s and 10 s. The new sensor observed sea level fluctuation by volcanic eruption in Tonga and The identical waveforms were obtained from the new and ordinary sensors., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

S-net is a large-scale seafloor network in the Japan Trench area, consisting of inline-type 150 observatories with seismometers and pressure gauges. We analyzed the strong-motion data recorded at the S-net sites from earthquakes with magnitudes between about 4 and 7 with focal depths lower than 70 km. To minimize the effects of nonlinear site response and weak-couplings between the seabed and sensor houses, recordings with vector peak accelerations lower than 50 gals were selected. More than 6,000 recordings with high S/N ratios from about 600 earthquakes were used. Using the spectral inversion technique, we obtained some fundamental properties of the earthquake source spectra, path attenuation, and site factors from the horizontal-component S-wave portions of the recordings. The source spectra followed the source model generally well, and the estimated magnitudes were similar to the catalog magnitudes. The stress drops increased systematically with the focal depths, and the stress-drop values differed between the earthquake types. The path-averaged quality factors were somewhat larger than those in the past studies. The peak site frequencies ranged between about 0.2 and 10 Hz, while the peak amplification factors were between about 10 and 50. A moderate regionality was also observed in the distribution of site amplification factors. At some stations which were not buried, spurious site spectra were recognized. We discuss the above results and their implication in the presentation., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)