5:15 PM - 7:15 PM
[SSS12-P13] Analysis of earthquakes and shallow tremors off the Hyuga-nada and Shikoku regions using the Nankai Trough Seafloor Observation Network for Earthquakes and Tsunamis
Keywords:earthquake catalog, N-net, shallow tremor, Nankai Trough
The Japan Meteorological Agency (JMA) compiles a unified earthquake catalog in cooperation with the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), utilizing data from JMA, the National Research Institute for Earth Science and Disaster Resilience (NIED), universities, and other institutes. Since September 2020, in addition to the land travel time table (JMA2001; Ueno et al., 2002), three types of offshore travel time tables have been incorporated for ocean-bottom seismometers. These include travel time tables for the landward region of the Japan Trench (JMA2020A), the outer rise region of the Japan Trench (JMA2020B), and the Nankai Trough region (JMA2020C). Furthermore, the offshore system of the Nankai Trough Seafloor Observation Network for Earthquakes and Tsunamis (N-net) was established by NIED in July 2024. Its integration into the unified earthquake catalog is being considered in conjunction with the currently under-development onshore system. In this presentation, we report on the analysis of earthquakes and shallow tremors using three-component velocity waveforms from 18 offshore N-net stations.
For automatic hypocenter determination, vector transformation was applied to correct the vertical component based on the direction of gravitational acceleration. Hypocenter determination was then performed using the PF method (Tamaribuchi, 2018). In this initial analysis, the velocity structure was set to JMA2020C, consistent with DONET, and the average station corrections of DONET (P-wave: +0.68 s, S-wave: +2.06 s) were applied to all stations. As a result, seismic activity near the trench axis of Hyuga-nada, previously determined to occur at a depth of about 40 km, was found to be significantly shallower, at approximately 10 km. This result suggests this activity around the plate boundary. Conversely, earthquakes outside the observation network, including the event off Hyuga-nada on August 8, 2024, were systematically determined to be deeper. This discrepancy may stem from the station locations and/or inadequacies in the station corrections.
For shallow tremors, a hybrid method combining envelope correlation and maximum amplitude (Tamaribuchi et al., 2022) was employed. Tremor detection and hypocenter determination were performed using three-component waveforms with a 0.5–5 Hz bandpass filter. As an initial approach, station amplification factors were uniformly set for all stations. This analysis revealed increased tremor activity offshore of the Hyuga-nada region following the M7.1 earthquake on August 8, 2024. The tremor distribution was consistent with findings from previous seafloor observations, such as those reported by Yamashita et al. (2021).
These findings indicate that N-net enables a more detailed understanding of earthquake and tremor activity near the trench axis along the Nankai Trough. Moving forward, we aim to incorporate N-net into the unified earthquake catalog and shallow tremor detection processes. This will involve refining station corrections and ground amplification factors for both offshore and onshore systems.
Acknowledge: We used the N-net waveform data from NIED and the unified earthquake catalog.
For automatic hypocenter determination, vector transformation was applied to correct the vertical component based on the direction of gravitational acceleration. Hypocenter determination was then performed using the PF method (Tamaribuchi, 2018). In this initial analysis, the velocity structure was set to JMA2020C, consistent with DONET, and the average station corrections of DONET (P-wave: +0.68 s, S-wave: +2.06 s) were applied to all stations. As a result, seismic activity near the trench axis of Hyuga-nada, previously determined to occur at a depth of about 40 km, was found to be significantly shallower, at approximately 10 km. This result suggests this activity around the plate boundary. Conversely, earthquakes outside the observation network, including the event off Hyuga-nada on August 8, 2024, were systematically determined to be deeper. This discrepancy may stem from the station locations and/or inadequacies in the station corrections.
For shallow tremors, a hybrid method combining envelope correlation and maximum amplitude (Tamaribuchi et al., 2022) was employed. Tremor detection and hypocenter determination were performed using three-component waveforms with a 0.5–5 Hz bandpass filter. As an initial approach, station amplification factors were uniformly set for all stations. This analysis revealed increased tremor activity offshore of the Hyuga-nada region following the M7.1 earthquake on August 8, 2024. The tremor distribution was consistent with findings from previous seafloor observations, such as those reported by Yamashita et al. (2021).
These findings indicate that N-net enables a more detailed understanding of earthquake and tremor activity near the trench axis along the Nankai Trough. Moving forward, we aim to incorporate N-net into the unified earthquake catalog and shallow tremor detection processes. This will involve refining station corrections and ground amplification factors for both offshore and onshore systems.
Acknowledge: We used the N-net waveform data from NIED and the unified earthquake catalog.