17:15 〜 19:15
[SCG45-P27] Gravity changes due to the 2023 M6.5 and 2024 M7.6 earthquakes in the Noto Peninsula
キーワード：能登半島、群発地震、流体、重力、地殻変動
In the northeastern Noto Peninsula, earthquake swarms have been active since the end of November 2020, followed by a M6.5 earthquake in May 2023 and a M7.6 earthquake in January 2024. Geophysical and geochemical observations suggest that crustal fluids originating from the upper mantle moved upward with seismic swarms and slow slip, causing these two major earthquakes. For example, a rapid up-dip migration of the aftershocks of the M6.5 event and the slow rupture initiation of the M7.6 event indicate that fluids were present in the source areas of the two events. GNSS data analysis shows that approximately 3 x 107 m3 of fluids had accumulated in the down dip of the source of the M6.5 event.
A more detailed information on fluid movement can be obtained by combining GNSS and microgravimetry. Observed gravity changes include contributions from surface uplift, which can be subtracted using GNSS-derived height changes. The remaining gravity change (i.e., gravity anomaly) reflects subsurface mass redistributions. A negative gravity anomaly indicates that the crust has been replaced by lighter density material. This method has been used to capture fluid movement in volcanic regions and a seismic swarm off the coast of Ito. In this study, we apply the same method to the above two events to obtain additional information on the fluid migration.
Our observations based on absolute and relative gravity measurements indicate that the M6.5 event caused a negative gravity anomaly that cannot be fully explained by dilatation due to the coseismic elastic deformation and environmental factors such as groundwater and sea level variations, at station SZHK on the northeastern coastal area of the peninsula. The gravity anomaly can be explained by a model that assumes that one-tenth of the fluid volume inferred by the GNSS data analysis rises through the source fault in the vicinity of the SZHK. Moreover, our observations indicate that the M7.6 event caused negative gravity anomalies at SZHK and Wajima tidal station. The amount of these anomalies are several times larger than the above anomaly for the M6.5 event. As before, these anomalies are difficult to explain by coseismic elastic deformation and environmental factors alone. A model of fluid movement consistent with SAR and GNSS data is under construction.
A more detailed information on fluid movement can be obtained by combining GNSS and microgravimetry. Observed gravity changes include contributions from surface uplift, which can be subtracted using GNSS-derived height changes. The remaining gravity change (i.e., gravity anomaly) reflects subsurface mass redistributions. A negative gravity anomaly indicates that the crust has been replaced by lighter density material. This method has been used to capture fluid movement in volcanic regions and a seismic swarm off the coast of Ito. In this study, we apply the same method to the above two events to obtain additional information on the fluid migration.
Our observations based on absolute and relative gravity measurements indicate that the M6.5 event caused a negative gravity anomaly that cannot be fully explained by dilatation due to the coseismic elastic deformation and environmental factors such as groundwater and sea level variations, at station SZHK on the northeastern coastal area of the peninsula. The gravity anomaly can be explained by a model that assumes that one-tenth of the fluid volume inferred by the GNSS data analysis rises through the source fault in the vicinity of the SZHK. Moreover, our observations indicate that the M7.6 event caused negative gravity anomalies at SZHK and Wajima tidal station. The amount of these anomalies are several times larger than the above anomaly for the M6.5 event. As before, these anomalies are difficult to explain by coseismic elastic deformation and environmental factors alone. A model of fluid movement consistent with SAR and GNSS data is under construction.