17:15 〜 19:15
[SVC36-P08] 2024年9月24日鳥島近海の地震に伴うT相
キーワード：T相、鳥島近海の地震、ケーブル型海底地震計観測網、スミスカルデラ
The earthquake near Torishima Island (JMA, 2024) that occurred near Sumisu caldera in the southern Izu Islands at around 08:14 JST on September 24, 2024, like the earthquake near Torishima Island that occurred in the early morning of October 9, 2023, is an unusual earthquake accompanied by tsunamis that is not expected for an usual earthquake of that magnitude, and would be related to volcanic activity. Although not as prominent as the latter earthquake, in which 14 major significant T-phases are observed, T-phases are also observed. Meanwhile, since the latter earthquake, the National Research Institute for Earth Science and Disaster Resilience (NIED) has installed a part of the new cabled seafloor seismic observation network "N-net" along the Nankai Trough, and has been releasing observation data from July 2024 onwards. In this presentation, the characteristics of T-phases observed with seismometers including the ocean bottom seismometers (OBSs) of the N-net will be reported.
The observation data used in this study includes data of S-net, which consists of 150 observation points along the Japan Trench from off the coast of Hokkaido to off the coast of the Boso Peninsula, broadband seismometers of DONET deployed from off the coast of the Kii Peninsula to off the coast of Cape Muroto, and velocity-type seismometer data from the Japan Meteorological Agency's (JMA's) cabled OBSs along the east Nankai Trough and some of those on land, as well as data from the OBSs of the N-net mentioned above. The sampling frequency of all observation data is 100 Hz. All data were downloaded from the website of the NIED. The N-net and S-net observation data of these are obtained using velocimeters, and the vertical component is calculated based on the direction of gravity measured by an accelerometer attached to the corresponding velocimeter, and used for analysis. The gravity components are calculated by time-averaging accelerometer data for three hours from 00:00 JST on September 24th. In analyzing the data, firstly the propagation time to each observation point is calculated, assuming that the signal source is located at the epicenter of the 08:14 earthquake (M5.8) listed in the JMA's epicenter list. Then the time axes of the observed waveforms for each observation point by the propagation time (subtracted from the observation time) are shifted and a graph is created in which the waveforms are arranged in order of propagation distance. The sound speed is assumed to be 1472 m/s. The waveforms arranged using the above method are shown in Fig. 1. The figure shows the two minute waveforms including the T-phase associated with the earthquake. A 3Hz high-pass filter is applied to suppress low-frequency noise. Black indicates the S-net stations, red the JMA stations, blue indicates the DONET stations, and green indicates the N-net stations. The horizontal axis corresponds to the origin time at the signal source (epicenter) of the signal detected at each observation point, and the vertical axis indicates the propagation distance (km). The vertically lined up signals at around 8:14 indicate the T phase from the signal source. In the T-phases associated with the earthquake near Torishima Island on October 9, 2023, whose epicenter is located near the Sofu Seamount to the south, a notable feature is observed in that the arrival of T-phases at the JMA's observation points, particularly those in the Amami and Ryukyu Islands region, is delayed compared to the S-net observation points (Iwase, 2024), however, this time no such delay is observed. A similar trend is observed for the T-phases associated with an earthquake with an epicenter at roughly the same location that occurred on September 24th at 15:00 after this event (Fig. 2).
In addition, T-phases are detected that are estimated to have originated from the same location at 08:12 on September 25th (Fig. 3), although the epicenter has not been determined.
The detection of small-scale events other than the main events due to T-phases detected with the cabled OBS network will contribute to elucidating the corresponding volcanic activity.
Acknowledgment
The author would like to thank National Research Institute for Earth Science and Disaster Prevention (https://doi.org/10.17598/NIED.0007, https://doi.org/10.17598/NIED.0029) and Japan Meteorological Agency, for providing data. The web site of Geographical Survey Institute is used to calculate propagation distances.
References
JMA (2024), https://www.jma.go.jp/jma/press/2409/24a/202409241020.html.
Iwase (2024), JpGU2024, SCG54-P10.
The observation data used in this study includes data of S-net, which consists of 150 observation points along the Japan Trench from off the coast of Hokkaido to off the coast of the Boso Peninsula, broadband seismometers of DONET deployed from off the coast of the Kii Peninsula to off the coast of Cape Muroto, and velocity-type seismometer data from the Japan Meteorological Agency's (JMA's) cabled OBSs along the east Nankai Trough and some of those on land, as well as data from the OBSs of the N-net mentioned above. The sampling frequency of all observation data is 100 Hz. All data were downloaded from the website of the NIED. The N-net and S-net observation data of these are obtained using velocimeters, and the vertical component is calculated based on the direction of gravity measured by an accelerometer attached to the corresponding velocimeter, and used for analysis. The gravity components are calculated by time-averaging accelerometer data for three hours from 00:00 JST on September 24th. In analyzing the data, firstly the propagation time to each observation point is calculated, assuming that the signal source is located at the epicenter of the 08:14 earthquake (M5.8) listed in the JMA's epicenter list. Then the time axes of the observed waveforms for each observation point by the propagation time (subtracted from the observation time) are shifted and a graph is created in which the waveforms are arranged in order of propagation distance. The sound speed is assumed to be 1472 m/s. The waveforms arranged using the above method are shown in Fig. 1. The figure shows the two minute waveforms including the T-phase associated with the earthquake. A 3Hz high-pass filter is applied to suppress low-frequency noise. Black indicates the S-net stations, red the JMA stations, blue indicates the DONET stations, and green indicates the N-net stations. The horizontal axis corresponds to the origin time at the signal source (epicenter) of the signal detected at each observation point, and the vertical axis indicates the propagation distance (km). The vertically lined up signals at around 8:14 indicate the T phase from the signal source. In the T-phases associated with the earthquake near Torishima Island on October 9, 2023, whose epicenter is located near the Sofu Seamount to the south, a notable feature is observed in that the arrival of T-phases at the JMA's observation points, particularly those in the Amami and Ryukyu Islands region, is delayed compared to the S-net observation points (Iwase, 2024), however, this time no such delay is observed. A similar trend is observed for the T-phases associated with an earthquake with an epicenter at roughly the same location that occurred on September 24th at 15:00 after this event (Fig. 2).
In addition, T-phases are detected that are estimated to have originated from the same location at 08:12 on September 25th (Fig. 3), although the epicenter has not been determined.
The detection of small-scale events other than the main events due to T-phases detected with the cabled OBS network will contribute to elucidating the corresponding volcanic activity.
Acknowledgment
The author would like to thank National Research Institute for Earth Science and Disaster Prevention (https://doi.org/10.17598/NIED.0007, https://doi.org/10.17598/NIED.0029) and Japan Meteorological Agency, for providing data. The web site of Geographical Survey Institute is used to calculate propagation distances.
References
JMA (2024), https://www.jma.go.jp/jma/press/2409/24a/202409241020.html.
Iwase (2024), JpGU2024, SCG54-P10.