[S09P-10] 伊豆・小笠原沈み込み帯におけるスロー地震分布と熱構造のモデル化
1. Introduction
Slow earthquakes have been identified by instrumental measurements in the Izu–Bonin arc. For instance, the continuous Global Navigation Satellite System (GNSS) stations on the Izu Islands were found to be moving horizontally eastward by up to ∼1 cm/year relative to the stable part of the Philippine Sea plate behind the northern part of the arc (Arisa and Heki, 2016). During 2007–2016, five slow slip events (SSEs) repeated quasi–periodically with similar displacement patterns and an average fault slip of ∼5 cm at the Bonin Islands (Arisa and Heki, 2017). However, the subduction thermal regime responsible for generating seismic and aseismic slip on the subducted Pacific Plate in the Izu‒Bonin arc remains enigmatic, which is fundamental to the transition from the cold forearc to the hot arc and the range and depth of the cold nose. Therefore, this study attempts to focus on the three–dimensional slab thermal state and hydraulic distribution in the Izu–Bonin arc and to investigate the distributions of observed fast–slow earthquakes on megathrusts.
2. Model and method
To characterize the Izu‒Bonin slow slip events and its geographically distributed hydrothermal state, we constructed a 3–D, time–evolving thermomechanical model developed from Stag3D code (Tackley and Xie, 2003) for the Izu‒Bonin subduction zone. An anelastic liquid approximation and the equations of conservation of mass, momentum, and energy are used in this study (e.g., Ji et al., 2016). The model region has dimensions of 1000×500×400 km3 (length×width×depth) and simulates the simultaneous penetration of the incoming plate over 15 Myr. The geometry of the subducted plate is well constrained by Slab2 (Hayes et al., 2018), and it is extrapolated within the modeled domain to ensure full subduction and comparatively steady slab thermal conditions. Based on thermomechanical models, we focus on the hydrothermal regime in the Izu‒Bonin arc and the distributions of fast and slow megathrust earthquakes.
3. Results and discussion
SSEs detected so far along the Izu‒Bonin arc are distributed along the Moho depth of the subduction interface at approximately 300 ℃, where large earthquakes have repeatedly occurred. Instrumental measurements are also based on stations on islands and thus are largely limited to the vicinity of islands, such as the northern Izu and northeastern Bonin Islands (Fig. 1). Fukao et al., (2021) deployed an array of ocean bottom pressure gauges and detected two large aseismic SSEs in September 2015 in the south-central Izu islands with a depth of 15∼30 km (Fig. 1). Therefore, according to our study, we suggest that the plate interface near the Moho depth (30 km) should host the source region of active SSEs. For the >50 km depth below the Izu‒Bonin arc, the subarc plate interface possibly accommodates certain SSEs where the Pacific slab releases a great amount of fluid from the eclogitization of subducted rocks, which influences the occurrence of fast to slow earthquakes.
Fig. 1 The modeled up‒to‒date geometry of the subducted Pacific Plate beneath Izu‒Bonin in our 3‒D thermal model. The isotherm contours (300∼700℃) are labeled. Colored spheres indicate M>3 earthquakes (IRIS, 2000∼2010). Yellow transparent zones indicate the SSEs detected so far.
Slow earthquakes have been identified by instrumental measurements in the Izu–Bonin arc. For instance, the continuous Global Navigation Satellite System (GNSS) stations on the Izu Islands were found to be moving horizontally eastward by up to ∼1 cm/year relative to the stable part of the Philippine Sea plate behind the northern part of the arc (Arisa and Heki, 2016). During 2007–2016, five slow slip events (SSEs) repeated quasi–periodically with similar displacement patterns and an average fault slip of ∼5 cm at the Bonin Islands (Arisa and Heki, 2017). However, the subduction thermal regime responsible for generating seismic and aseismic slip on the subducted Pacific Plate in the Izu‒Bonin arc remains enigmatic, which is fundamental to the transition from the cold forearc to the hot arc and the range and depth of the cold nose. Therefore, this study attempts to focus on the three–dimensional slab thermal state and hydraulic distribution in the Izu–Bonin arc and to investigate the distributions of observed fast–slow earthquakes on megathrusts.
2. Model and method
To characterize the Izu‒Bonin slow slip events and its geographically distributed hydrothermal state, we constructed a 3–D, time–evolving thermomechanical model developed from Stag3D code (Tackley and Xie, 2003) for the Izu‒Bonin subduction zone. An anelastic liquid approximation and the equations of conservation of mass, momentum, and energy are used in this study (e.g., Ji et al., 2016). The model region has dimensions of 1000×500×400 km3 (length×width×depth) and simulates the simultaneous penetration of the incoming plate over 15 Myr. The geometry of the subducted plate is well constrained by Slab2 (Hayes et al., 2018), and it is extrapolated within the modeled domain to ensure full subduction and comparatively steady slab thermal conditions. Based on thermomechanical models, we focus on the hydrothermal regime in the Izu‒Bonin arc and the distributions of fast and slow megathrust earthquakes.
3. Results and discussion
SSEs detected so far along the Izu‒Bonin arc are distributed along the Moho depth of the subduction interface at approximately 300 ℃, where large earthquakes have repeatedly occurred. Instrumental measurements are also based on stations on islands and thus are largely limited to the vicinity of islands, such as the northern Izu and northeastern Bonin Islands (Fig. 1). Fukao et al., (2021) deployed an array of ocean bottom pressure gauges and detected two large aseismic SSEs in September 2015 in the south-central Izu islands with a depth of 15∼30 km (Fig. 1). Therefore, according to our study, we suggest that the plate interface near the Moho depth (30 km) should host the source region of active SSEs. For the >50 km depth below the Izu‒Bonin arc, the subarc plate interface possibly accommodates certain SSEs where the Pacific slab releases a great amount of fluid from the eclogitization of subducted rocks, which influences the occurrence of fast to slow earthquakes.
Fig. 1 The modeled up‒to‒date geometry of the subducted Pacific Plate beneath Izu‒Bonin in our 3‒D thermal model. The isotherm contours (300∼700℃) are labeled. Colored spheres indicate M>3 earthquakes (IRIS, 2000∼2010). Yellow transparent zones indicate the SSEs detected so far.