[U-15] The 2024 Noto Peninsula Earthquake (1:J)

[U15-P42] Aftershock observation and site amplification factors at the port area of Wajima and Suzu cities

*Yosuke Nagasaka¹, Atsushi Nozu¹ (1.Port and Airport Research Institute)

Keywords:The 2024 Noto earthquake, Aftershock observation, Site amplification factor, Wajima, Suzu

The 2024 Noto earthquake (M_J 7.6) occurred at 16:10 (JST) on January 1, 2024. Large strong ground motions were observed in the areas of Noto peninsula including strong motion observation stations in Wajima and Suzu cities (ISK003 and ISK002, respectively). Populated areas of these two cities are located in small basin along the coast. Peak velocities exceeding100 cm/s were observed at both stations. ISK003 is located at the edge of the basin and the populated area of Suzu city extends more than 3 km along the coast. Because strong ground motions are strongly affected by the structure of basin and its effect can vary within a small area, it is not evident that the site factors at ISK003 and ISK002 can be applied to the whole area of each city. In this study, we conducted aftershock observation at the port area of Wajima and Suzu cities and compared the ground motions with neighboring strong motion stations. Aftershock observation sites at Wajima and Suzu cities are 800m and 3 km from ISK003 and ISK002, respectively. Portable seismographs (JU210) were used for the observation. Observations were conducted from January 21 to March 28, 2024. Ground motions from 11 earthquakes equal or larger than M_J 4.0 were obtained. Fourier spectral ratios of aftershock observation site and neighboring K-NET sites were calculated. The spectral ratios showed that our observation site at Wajima city had clearly larger site amplification between 0.5-3 Hz than that at ISK003. On the other hand, smaller site amplification was observed below 2 Hz at the port area in Suzu city (Iida port) than that at ISK002. Then the spectral ratios were multiplied with the site amplification factors at ISK003 and ISK002 to obtain the site amplification factors at the aftershock observation sites. Site amplification factor at Wajima port was larger than that at ISK003 in the frequency range that is important for structures. Absolute value of the amplification was also large. The difference of site amplification factor can be due to the shallow subsurface structure. In Suzu city, smaller site amplification factor below 2 Hz was estimated in the port area. It should be noted that site amplification at ISK002 is extremely large and that at Iida port is not so small. This implies that the deep subsurface structures at ISK002 and the port area are different. This difference can be related to the existence of caldera in this area, which was not confirmed but implied by gravity anomaly distribution.