10:00 AM - 10:15 AM
[SGD03-05] Crustal deformation and fault model of the Hyuganada earthquakes (M7.1 and M6.6) occurred in August 2023 and January 2024
Keywords:interplate earthquake, Crustal deformation, GNSS, fault model
On August 8, 2024 and January 13, 2025, Mj7.1 and Mj6.6 earthquakes occurred in southern Hyuga-nada, respectively. The mechanism solution of these earthquakes indicates a low-angle reverse fault type and they are considered to be interplate earthquakes that occurred at the boundary between the subducting Philippine Sea Plate and the overriding continental plate. Distinct crustal deformations were observed at the GSI GEONET stations in the Hyuga-nada coastal area following these earthquakes, and fault models are being inferred.
The Miyazaki Observatory of the Disaster Prevention Research Institute, Kyoto University, had installed 13 GNSS continuous observation stations in the Hyuga-nada coastal area in 2017-18 as part of the KAKENHI "Slow Seismology" to strengthen monitoring crustal deformation. The 2024 and 2025 earthquakes occurred in the immediate vicinity of the observation network, Integrated analysis of the GNSS observation data from GSI at Kyoto University would enable a more detailed discussion of the epicenter area. Therefore, we report the coseismic displacement of the two earthquakes obtained from these observation networks and the results of the estimation of the source fault model.
GNSS daily positions were calculated using PPP-AR (Precise Point Positioning with ambiguity resolution) implemented in GipsyX 2.3 and the clock and orbit products by JPL (Jet Propulsion Laboratory), with the VMF1 mapping function, 2nd-order ionospheric correction, and tidal ocean loading deformation by the FES2014b model. The daily positions based on ITRF2020 were used to calculate the coordinates relative to the GEONET Misumi station (950388), and the coseismic displacement was calculated from the difference of average coordinates for the 10 days before and after the earthquake.
Comparing the seismic crustal deformations of the two earthquakes in 2024 and 2025, both observed eastward, subsiding coastal deformations, but the center of the deformation is shifted to the north in the 2025 earthquake. In addition, the maximum horizontal displacement is 15.2 cm in 2024, while it is 5.5 cm in 2025, less than half. Next, assuming a rectangular fault in an elastic half-space and constraining depth, strike, and dip to the plate boundary, a fault model for the 2025 earthquake was estimated using the method of Matsu'ura and Hasegawa (1987), immediately north of the 2024 earthquake The fault model of the 2025 earthquake was estimated just north of that of the 2024 earthquake. The estimated moment magnitudes were 7.00 and 6.85 with a rigidity of 40 GPa for 2024 and 2025, respectively. The fault models for the Mj6.9 October and Mj6.7 December 1996 Hyuga-nada earthquakes were estimated to the east and west within the 2025 fault model, and the sum of the seismic moments for the two earthquakes was 1.25 times that for the 2025 earthquake. Thus, the 2025 earthquake can be interpreted as having re-ruptured the source regions of the two 1996 earthquakes with smaller magnitude. Vertical deformation that may be related to the 1931 and 1961 Hyuga-nada earthquakes were also obtained by leveling survey, and the results of the analysis of the leveling survey will be presented.
Acknowledgement: We appreciate the Geospatial Authority of Japan and the Japan Meteorological Agency for providing GEONET and JMA unified data.
The Miyazaki Observatory of the Disaster Prevention Research Institute, Kyoto University, had installed 13 GNSS continuous observation stations in the Hyuga-nada coastal area in 2017-18 as part of the KAKENHI "Slow Seismology" to strengthen monitoring crustal deformation. The 2024 and 2025 earthquakes occurred in the immediate vicinity of the observation network, Integrated analysis of the GNSS observation data from GSI at Kyoto University would enable a more detailed discussion of the epicenter area. Therefore, we report the coseismic displacement of the two earthquakes obtained from these observation networks and the results of the estimation of the source fault model.
GNSS daily positions were calculated using PPP-AR (Precise Point Positioning with ambiguity resolution) implemented in GipsyX 2.3 and the clock and orbit products by JPL (Jet Propulsion Laboratory), with the VMF1 mapping function, 2nd-order ionospheric correction, and tidal ocean loading deformation by the FES2014b model. The daily positions based on ITRF2020 were used to calculate the coordinates relative to the GEONET Misumi station (950388), and the coseismic displacement was calculated from the difference of average coordinates for the 10 days before and after the earthquake.
Comparing the seismic crustal deformations of the two earthquakes in 2024 and 2025, both observed eastward, subsiding coastal deformations, but the center of the deformation is shifted to the north in the 2025 earthquake. In addition, the maximum horizontal displacement is 15.2 cm in 2024, while it is 5.5 cm in 2025, less than half. Next, assuming a rectangular fault in an elastic half-space and constraining depth, strike, and dip to the plate boundary, a fault model for the 2025 earthquake was estimated using the method of Matsu'ura and Hasegawa (1987), immediately north of the 2024 earthquake The fault model of the 2025 earthquake was estimated just north of that of the 2024 earthquake. The estimated moment magnitudes were 7.00 and 6.85 with a rigidity of 40 GPa for 2024 and 2025, respectively. The fault models for the Mj6.9 October and Mj6.7 December 1996 Hyuga-nada earthquakes were estimated to the east and west within the 2025 fault model, and the sum of the seismic moments for the two earthquakes was 1.25 times that for the 2025 earthquake. Thus, the 2025 earthquake can be interpreted as having re-ruptured the source regions of the two 1996 earthquakes with smaller magnitude. Vertical deformation that may be related to the 1931 and 1961 Hyuga-nada earthquakes were also obtained by leveling survey, and the results of the analysis of the leveling survey will be presented.
Acknowledgement: We appreciate the Geospatial Authority of Japan and the Japan Meteorological Agency for providing GEONET and JMA unified data.