9:45 AM - 10:00 AM
[SSS12-16] Seismic Activity in the M7-Class Repeating Earthquake Zone of the Southern Hyuga-nada Using the Double-Difference Method
Keywords:Hyuga-nada, ocean bottom seismometer, seismicity
Hyuga-nada is a region where the Philippine Sea Plate subducts beneath the Eurasian Plate, resulting in high seismic activity. At the plate boundary in the southern Hyuga-nada (offshore of Miyazaki City and Nichinan City), M7-class earthquakes have occurred approximately every 30 years. Consistent with this pattern, an M7.1 earthquake struck the region on August 8, 2024. According to an earthquake catalog by the Japan Meteorological Agency (JMA), the aftershock activity migrated offshore. However, because the earthquakes in the JMA catalog for Hyuga-nada are analyzed using only data from land-based seismic stations, the accuracy of hypocenter determination is relatively low. In particular, discrepancies in depth estimates become more pronounced offshore when compared to existing plate boundary models. In this study, we report on the relocated hypocenters of earthquakes that occurred before and after the August 8, 2024, Hyuga-nada earthquake using the double-difference method (Zhang and Thurber, 2003). This analysis incorporates precisely determined hypocenters obtained during an ocean bottom seismic observation period.
For the accurately determined hypocenters during the offshore seismic observation period, we utilized the results of Hirata et al. (2024, Seismological Society of Japan Fall Meeting (SSJ)). They used data of 10 ocean bottom seismometers. These were obtained from ocean bottom seismic observation conducted above the transition zone between the shallow slow earthquake and the regular earthquake in the southern Hyuga-nada between 2021-2022. In addition, they incorporated data of 10 land-based seismic stations along the coast of Miyazaki Prefecture. They extracted earthquakes occurred in the area between the Miyazaki Prefecture coast and the seafloor seismic network during the seafloor observation period which were larger than M0 and relocated the hypocenters using the double-difference method with the 3-D velocity structure created from Nakanishi et al. (2018). Their result indicated that hypocenters of offshore events shifted significantly landward after the relocation procedure, with a maximum shift of approximately 10 km.
Some of the seismic activity identified by Hirata et al. (2024, SSJ) overlapped with the aftershock region of the August 8, 2024, Hyuga-nada earthquake. Given that the same land-based seismic stations used in Hirata et al. (2024, SSJ) were operational during the 2024 earthquake, it is expected that the hypocenters of the aftershocks can be determined with high accuracy using the double-difference method. Preliminary analyses suggest that the southern aftershocks were distributed along the plate boundary. During the 2024 earthquake, the offshore system of the N-net (Nankai Trough Seafloor Observation Network for Earthquakes and Tsunamis) (Aoi et al., 2023) was already operational, and its data were recorded. In the future, we will incorporate P- and S-wave arrival time data of the ocean bottom seismometers to further improve the accuracy of hypocenter determination along the east-west direction in the offshore area. Furthermore, to compare seismic activity before and after the 2024 event with the M6.9 earthquake in October 1996 and the M6.7 earthquake in December 1996, we plan to pick P- and S-phase onsets additionally of university land-based seismic stations in operation since that time and relocate the hypocenters before and after these earthquakes using the double-difference method. By comparing the resulting hypocenter distribution with fault models based on previous studies, we aim to gain a better understanding of M7-class repeating earthquakes in the southern Hyuga-nada.
Acknowledgements
This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its “The Second Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research)” and “Research Project for Disaster Prevention on the great Earthquakes along the Nankai trough”. We thank the crews of the T/S Nagasakimaru (Faculty of Fisheries, Nagasaki University) and the R/V Kaiyomaru No. 3 (Kaiyo Engineering Co.) for their skillful work and these voyage participants from Kyoto University, Kyushu University, Kagoshima University, Tokyo University of Marine Science and Technology, and Tokyo University.
For the accurately determined hypocenters during the offshore seismic observation period, we utilized the results of Hirata et al. (2024, Seismological Society of Japan Fall Meeting (SSJ)). They used data of 10 ocean bottom seismometers. These were obtained from ocean bottom seismic observation conducted above the transition zone between the shallow slow earthquake and the regular earthquake in the southern Hyuga-nada between 2021-2022. In addition, they incorporated data of 10 land-based seismic stations along the coast of Miyazaki Prefecture. They extracted earthquakes occurred in the area between the Miyazaki Prefecture coast and the seafloor seismic network during the seafloor observation period which were larger than M0 and relocated the hypocenters using the double-difference method with the 3-D velocity structure created from Nakanishi et al. (2018). Their result indicated that hypocenters of offshore events shifted significantly landward after the relocation procedure, with a maximum shift of approximately 10 km.
Some of the seismic activity identified by Hirata et al. (2024, SSJ) overlapped with the aftershock region of the August 8, 2024, Hyuga-nada earthquake. Given that the same land-based seismic stations used in Hirata et al. (2024, SSJ) were operational during the 2024 earthquake, it is expected that the hypocenters of the aftershocks can be determined with high accuracy using the double-difference method. Preliminary analyses suggest that the southern aftershocks were distributed along the plate boundary. During the 2024 earthquake, the offshore system of the N-net (Nankai Trough Seafloor Observation Network for Earthquakes and Tsunamis) (Aoi et al., 2023) was already operational, and its data were recorded. In the future, we will incorporate P- and S-wave arrival time data of the ocean bottom seismometers to further improve the accuracy of hypocenter determination along the east-west direction in the offshore area. Furthermore, to compare seismic activity before and after the 2024 event with the M6.9 earthquake in October 1996 and the M6.7 earthquake in December 1996, we plan to pick P- and S-phase onsets additionally of university land-based seismic stations in operation since that time and relocate the hypocenters before and after these earthquakes using the double-difference method. By comparing the resulting hypocenter distribution with fault models based on previous studies, we aim to gain a better understanding of M7-class repeating earthquakes in the southern Hyuga-nada.
Acknowledgements
This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its “The Second Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research)” and “Research Project for Disaster Prevention on the great Earthquakes along the Nankai trough”. We thank the crews of the T/S Nagasakimaru (Faculty of Fisheries, Nagasaki University) and the R/V Kaiyomaru No. 3 (Kaiyo Engineering Co.) for their skillful work and these voyage participants from Kyoto University, Kyushu University, Kagoshima University, Tokyo University of Marine Science and Technology, and Tokyo University.