[S-SS10] Strong Ground Motion and Earthquake Disaster

[SSS10-P06] Seismic intensity and its characteristics for T-phase generated by seismic events around the southwestern off Torishima Island

*Takeshi Sato¹, Kazumasa Kayano¹, Akio Kawai¹, Takahiro Oka¹, Masao Abe¹, Saya Kondo¹, Masashi Kiyomoto¹, Noriko Kamaya¹ (1.Japan Meteorological Agency)

Keywords:Seismic intensity, T-phase, Torishima, seismic activity

From 3:58 to 6:18 on 9 October 2023 (JST), some seismic events occurred around the southwestern off Torishima Island and were caused to tsunamis observed in the Izu islands and the Pacific coast of Japan Islands (Japan Meteorological Agency, 2023; Sandanbata et al. 2024). At the same time, at some seismic intensity observation points in the Kagoshima and Miyazaki Prefectures in Japan, seismic intensity 1-2 on the Japanese seismic intensity scale were observed. These seismic intensities are considered to be attributed to T-phases generated by seismic activity around the southwestern off Torishima based on the travel times of T-phases. These seismic intensities were observed at seismic intensity observation points, including the Miyazaki Tsuno-cho yakuba, Kawaminami-cho Kawaminami, Kijo-cho Takajo in Miyazaki Prefecture, and the Kikai-cho Takigawa in Kagoshima Prefecture. Except for the Kikai-cho Takigawa, all of these points are seismic intensity observation points other than those operated by the Japan Meteorological Agency (JMA). Only Kikai-cho Takigawa observation point in Kagoshima Prefecture observed T-phases with large amplitude among JMA's seismic intensity observation points along the coast and on islands surrounding the seismic activity region off the southwest of Torishima based on their acceleration waveforms. These results imply that T-phase with a small attenuation propagated from the source region to the west-southwestern and west-northwestern direction, and locally has a large amplitude. Furthermore, the acceleration waveforms of T-phases observed on October 9, 2023 (JST), exhibited differences compared to the T-phase waveforms observed from a typical earthquake that occurred at 10:59 AM on October 5 (JST). The T-phases observed on October 9 (JST) had larger amplitudes, included higher-frequency components, and were characterized by a pulse-like nature with a shorter duration. We will present the outline and characters of waveforms for T-phases generated by these seismic events, and discuss the cause of these characters. The short outline of this study was published on the JMA website (https://www.data.jma.go.jp/eqev/data/gaikyo/monthly/202310/202310tokushuu_1_4.pdf).