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[SVC26-13] Uplift of Iwo-jima island during 2007-2023 detected by InSAR and its physical interpretation: Effect of thermal stress
Keywords:Iwo-jima, InSAR, Thermal stress, FEM
Iwo-jima is a volcanic island located in the Ogasawara Islands that is still undergoing active uplift movements unparalleled in the world. This uplift movement has been mainly observed using GNSS and levelling surveys (e.g., Ueda et al., 2018), and the detailed spatiotemporal changes in uplift activity have not been revealed using InSAR. Furthermore, no physical model has yet been proposed to quantitatively explain the observed uplift. In this study, we perform InSAR time series analysis using SAR data acquired by ALOS (2006-2011) and ALOS-2 (2014-present) satellites to clarify the spatio-temporal evolution of crustal deformation of Iwo-jima Island from 2007 to 2023. We also present a physical model to quantitatively explain the crustal deformation.
InSAR analysis revealed a mortar-shaped uplift pattern centered on Motoyama in the center of the island (Fig. a). The east-west velocity component shows a contraction trend centered around Motoyama and an extension trend across by the Asodai Fault. This spatial pattern is consistent with the results reported by a previous study (Ozawa et al. 2010) for the period 2006-2007, but both the rate and amount of the uplift are much higher. Specifically, the maximum uplift rate from 2007 to 2011 was ~1 m/yr, and the cumulative uplift reached ~4 m. In addition, a maximum westward velocity of ~0.2 m/yr and a cumulative displacement of ~1 m were detected in the western part of the island. From 2014 to 2023, on the other hand, the uplift rate further increased to ~1.5 m/yr, and the cumulative uplift reached a maximum of ~12 m (Fig. a). The western part of the island was displaced westward at a maximum rate of ~1 m/yr, and the cumulative displacement reached ~5 m. This indicates that Iwo-jima has been undergoing rapid deformation for a long period of time, which is unprecedented in the world. The temporal variation of uplift in the central and eastern parts of the island was plotted from 2007 to 2023, and it was found that this “differential uplift” accelerated from the latter half of 2016 (Fig. d).
Next, we tried to quantitatively explain the surface displacement field detected by InSAR using the finite element method. First, we calculated the surface deformation caused by 10 MPa pressure increase of a cone-sheet magma reservoir based on the conceptual model previously proposed by NIED (2020), and found that, unlike the observed pattern, the central part of the island was the most significantly uplifted. Next, we assumed (1) a deep magmatic source of 35 MPa beneath Iwo-jima, and (2) a shallow magma pool that has been penetrated and isolated by past volcanic activity. The shape of the shallow magma pool is set as an ellipsoid, and the initial temperature is assumed to be 900 K. The heat conduction equation was solved to calculate the evolution of the temperature field of the entire medium, and the surface displacement field was calculated by the equation of motion taking the associated thermal stress into account. The observed values are generally explained by the calculated cumulative displacement during the ALOS-2 imaging period (Fig. b). Since the difference in uplift between the island center and the periphery increases rapidly with time, a large thermal expansion coefficient is required to explain this difference. The value used in this study is 7.5 × 10-5, but similar values have been obtained for Mt.Usu (Wang and Aoki, 2019) and Izu Oshima (Furuya, 2005). The discrepancy between observed and calculated values is more pronounced in the southwestern part of the volcano (Fig. c), which we attribute to local magmatic activity that caused the October 2023 eruption off Okinahama and to the movement of small active faults dense in this region.
InSAR analysis revealed a mortar-shaped uplift pattern centered on Motoyama in the center of the island (Fig. a). The east-west velocity component shows a contraction trend centered around Motoyama and an extension trend across by the Asodai Fault. This spatial pattern is consistent with the results reported by a previous study (Ozawa et al. 2010) for the period 2006-2007, but both the rate and amount of the uplift are much higher. Specifically, the maximum uplift rate from 2007 to 2011 was ~1 m/yr, and the cumulative uplift reached ~4 m. In addition, a maximum westward velocity of ~0.2 m/yr and a cumulative displacement of ~1 m were detected in the western part of the island. From 2014 to 2023, on the other hand, the uplift rate further increased to ~1.5 m/yr, and the cumulative uplift reached a maximum of ~12 m (Fig. a). The western part of the island was displaced westward at a maximum rate of ~1 m/yr, and the cumulative displacement reached ~5 m. This indicates that Iwo-jima has been undergoing rapid deformation for a long period of time, which is unprecedented in the world. The temporal variation of uplift in the central and eastern parts of the island was plotted from 2007 to 2023, and it was found that this “differential uplift” accelerated from the latter half of 2016 (Fig. d).
Next, we tried to quantitatively explain the surface displacement field detected by InSAR using the finite element method. First, we calculated the surface deformation caused by 10 MPa pressure increase of a cone-sheet magma reservoir based on the conceptual model previously proposed by NIED (2020), and found that, unlike the observed pattern, the central part of the island was the most significantly uplifted. Next, we assumed (1) a deep magmatic source of 35 MPa beneath Iwo-jima, and (2) a shallow magma pool that has been penetrated and isolated by past volcanic activity. The shape of the shallow magma pool is set as an ellipsoid, and the initial temperature is assumed to be 900 K. The heat conduction equation was solved to calculate the evolution of the temperature field of the entire medium, and the surface displacement field was calculated by the equation of motion taking the associated thermal stress into account. The observed values are generally explained by the calculated cumulative displacement during the ALOS-2 imaging period (Fig. b). Since the difference in uplift between the island center and the periphery increases rapidly with time, a large thermal expansion coefficient is required to explain this difference. The value used in this study is 7.5 × 10-5, but similar values have been obtained for Mt.Usu (Wang and Aoki, 2019) and Izu Oshima (Furuya, 2005). The discrepancy between observed and calculated values is more pronounced in the southwestern part of the volcano (Fig. c), which we attribute to local magmatic activity that caused the October 2023 eruption off Okinahama and to the movement of small active faults dense in this region.