5:15 PM - 7:15 PM
[SSS14-P02] Seismic activity and gravity-based fault structure associated with the 2024 M6.6 earthquake off western Ishikawa
Keywords:the Hakui-Oki West Fault, the Hakui-Oki East Fault, gravity anomaly, hypocenter relocation
An earthquake with a magnitude of 6.6 occurred off the western coast of Ishikawa Prefecture on November 26, 2024, adjacent to the source region of the 2024 Noto-Hanto Earthquake (M7.6), which struck on January 1, 2024. This study reports on the relocated hypocenters surrounding the M6.6 earthquake and the fault structure inferred from gravity anomaly analysis.
Off the western coast of Ishikawa Prefecture, active submarine faults, such as the Hakui-Oki West Fault and the Hakui-Oki East Fault, have been identified (Headquarters for Earthquake Research Promotion, 2024). The Coulomb failure stress changes induced by the 2024 Noto-Hanto Earthquake were positive on these faults. Notably, the number of earthquakes with magnitudes of 1.5 or greater in this region from January to October 2024 was approximately 10 to 100 times higher than in previous ten years, indicating a significant increase in seismic activity driven by these stress changes.
Hypocenter relocation using the Double-Difference (DD) method (Waldhauser and Ellsworth, 2000) revealed a predominantly westward-dipping distribution of hypocenters, while eastward-dipping distributions were also observed in the southern part of the aftershock area. Given that the fault plane exhibits a low-angle dip at depth and a steeper dip near the surface, the source fault of the M6.6 earthquake is likely located along the deeper extension of the Hakui-Oki West Fault. In the shallow portion of the M6.6 source area, seismic activity increased during several periods prior to the M6.6 event: from late January to early February, from late September to October, and in mid-November of 2024. The seismically active region during these periods appeared to expand at a rate of approximately 1 km per 10 days, suggesting the occurrence of small-scale slow slip.
Seismic reflection surveys in the offshore region of western Ishikawa Prefecture have provided detailed subsurface structural cross-sections (Katagawa et al., 2005; Okamura, 2007; Inoue et al., 2007). Sawada et al. (2022) used these cross-sections to develop a three-dimensional subsurface structural model comprising four layers.
In this study, gravity anomaly values were calculated using data from the Japan Gravity Database DVD (Geological Survey of Japan, AIST, 2013), assuming a density of 2300 kg/m³. To isolate gravity anomalies associated with the basement structure, corrections were applied to account for density differences between the upper three layers and the fourth layer in the model by Sawada et al. (2022). This allowed us to estimate gravity anomalies for a simplified two-layer model (hereafter referred to as the two-layer gravity anomaly). This two-layer gravity anomaly is expected to reflect gravity anomalies caused by the basement structure. In this study, horizontal and vertical first derivatives of the two-layer gravity anomaly were subsequently computed.
The horizontal first derivative exhibits high values along the western side of the fault traces of the Hakui-Oki West and Hakui-Oki East Faults, indicating vertical displacements in the basement. Conversely, the vertical first derivative shows high values on the eastern side of the fault traces. These observations suggest that the faults were formed through inversion tectonics, with vertical displacements in the basement only partially reversed. This implies that the basement retains its original normal fault structure. Furthermore, discontinuities in the horizontal and vertical first derivatives suggest that the Hakui-Oki West and Hakui-Oki East Faults are not structurally continuous with the Amamisaki-Oki East Fault or the Monzen Fault Zone to the north.
Acknowledgments: We used arrival time data provided by the Japan Meteorological Agency, seismic waveforms obtained from Hi-net by the National Research Institute for Earth Science and Disaster Resilience, stations of Kyoto University and the University of Tokyo. Gravity data from the Geological Survey of Japan, AIST, were also used.
Off the western coast of Ishikawa Prefecture, active submarine faults, such as the Hakui-Oki West Fault and the Hakui-Oki East Fault, have been identified (Headquarters for Earthquake Research Promotion, 2024). The Coulomb failure stress changes induced by the 2024 Noto-Hanto Earthquake were positive on these faults. Notably, the number of earthquakes with magnitudes of 1.5 or greater in this region from January to October 2024 was approximately 10 to 100 times higher than in previous ten years, indicating a significant increase in seismic activity driven by these stress changes.
Hypocenter relocation using the Double-Difference (DD) method (Waldhauser and Ellsworth, 2000) revealed a predominantly westward-dipping distribution of hypocenters, while eastward-dipping distributions were also observed in the southern part of the aftershock area. Given that the fault plane exhibits a low-angle dip at depth and a steeper dip near the surface, the source fault of the M6.6 earthquake is likely located along the deeper extension of the Hakui-Oki West Fault. In the shallow portion of the M6.6 source area, seismic activity increased during several periods prior to the M6.6 event: from late January to early February, from late September to October, and in mid-November of 2024. The seismically active region during these periods appeared to expand at a rate of approximately 1 km per 10 days, suggesting the occurrence of small-scale slow slip.
Seismic reflection surveys in the offshore region of western Ishikawa Prefecture have provided detailed subsurface structural cross-sections (Katagawa et al., 2005; Okamura, 2007; Inoue et al., 2007). Sawada et al. (2022) used these cross-sections to develop a three-dimensional subsurface structural model comprising four layers.
In this study, gravity anomaly values were calculated using data from the Japan Gravity Database DVD (Geological Survey of Japan, AIST, 2013), assuming a density of 2300 kg/m³. To isolate gravity anomalies associated with the basement structure, corrections were applied to account for density differences between the upper three layers and the fourth layer in the model by Sawada et al. (2022). This allowed us to estimate gravity anomalies for a simplified two-layer model (hereafter referred to as the two-layer gravity anomaly). This two-layer gravity anomaly is expected to reflect gravity anomalies caused by the basement structure. In this study, horizontal and vertical first derivatives of the two-layer gravity anomaly were subsequently computed.
The horizontal first derivative exhibits high values along the western side of the fault traces of the Hakui-Oki West and Hakui-Oki East Faults, indicating vertical displacements in the basement. Conversely, the vertical first derivative shows high values on the eastern side of the fault traces. These observations suggest that the faults were formed through inversion tectonics, with vertical displacements in the basement only partially reversed. This implies that the basement retains its original normal fault structure. Furthermore, discontinuities in the horizontal and vertical first derivatives suggest that the Hakui-Oki West and Hakui-Oki East Faults are not structurally continuous with the Amamisaki-Oki East Fault or the Monzen Fault Zone to the north.
Acknowledgments: We used arrival time data provided by the Japan Meteorological Agency, seismic waveforms obtained from Hi-net by the National Research Institute for Earth Science and Disaster Resilience, stations of Kyoto University and the University of Tokyo. Gravity data from the Geological Survey of Japan, AIST, were also used.