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[SCG52-05] Geometrical dependence of seamount subduction on slip tendency: Does it promote or prohibit locking?
Keywords:Hyuga-Nada, slow earthquake, slip tendency
Nankai Trough is a convergent plate boundary where the Philippine Sea (PHS) plate subducts beneath the south-western Japan arc. Devastating earthquakes (M8 or larger) accompanied by tsunamis have repeatedly occur along the Nankai Trough with their interval ~90 years. Hyuga-Nada is located at a transition zone between the Nankai Trough to the east and the Ryukyu Trench to the west. The Kyushu-Palau Ridge (KPR, trending N30W) subducts beneath SW Japan at Hyuga-Nada. No M8-class earthquakes have been reported in Hyuga-Nada, whereas the low-frequency tremors (LFEs) and the very-low frequency events (VLFEs) frequently occur around the subducted KPR.
Subduction of seamounts or ridges has a significant influence on coupling/slip along the plate interface. Outcrop observations, numerical calculations, and experiments suggest that seamount subduction deforms and fractures the overlying strata, causing stress and strength heterogeneity (e.g. Dominguez 1998; Sun et al., 2020). Wang and Bilek (2014) propose that the complex structure and heterogeneous stresses of fracture network developed around the subducted seamount provide a favorable condition for aseismic creep and small earthquakes. High pore fluid pressure around seamounts may weaken interplate locking and promote creep (Mochizuki et al., 2008; Ellis et al., 2015; Takemura et al., 2022). According to the Coulomb failure theory, an earthquake or a slip along a fault can occur if the ratio of shear stress to effective normal stress exceeds the friction coefficient on the fault surface. In order to estimate this ratio (called the slip tendency, Ts) along the plate interface, we need to know its geometry as well as regional stress field.
To construct the geometry of subducting seamount, we compiled the seismic data in Hyuga-Nada to pick the basement. Although the seismic data is still sparse, interpolated image shows complicated geometry. A peak is located below the Toi seamount exposed on the seafloor. The basement sharply deepens westward to te west of subducted seamount. Assuming a uniform regional stress field around this area with sigma1 parallel to the plate conversion vector (WNW). We treated the amplitudes of horizontal principal stresses as parameters. Preliminary results indicate that Ts is generally high on and around the subducted seamount where the tremor frequence is large, too. In some frequent tremor areas, especially to the NNW of seamount, Ts is not high enough. This may indicate the needs for overpressures in that area. Further aspects might influence the seismicity pattern such as local stress variations induced by the subduction of the seamount, thermal stresses, differences in the frictional properties of the basement.
Subduction of seamounts or ridges has a significant influence on coupling/slip along the plate interface. Outcrop observations, numerical calculations, and experiments suggest that seamount subduction deforms and fractures the overlying strata, causing stress and strength heterogeneity (e.g. Dominguez 1998; Sun et al., 2020). Wang and Bilek (2014) propose that the complex structure and heterogeneous stresses of fracture network developed around the subducted seamount provide a favorable condition for aseismic creep and small earthquakes. High pore fluid pressure around seamounts may weaken interplate locking and promote creep (Mochizuki et al., 2008; Ellis et al., 2015; Takemura et al., 2022). According to the Coulomb failure theory, an earthquake or a slip along a fault can occur if the ratio of shear stress to effective normal stress exceeds the friction coefficient on the fault surface. In order to estimate this ratio (called the slip tendency, Ts) along the plate interface, we need to know its geometry as well as regional stress field.
To construct the geometry of subducting seamount, we compiled the seismic data in Hyuga-Nada to pick the basement. Although the seismic data is still sparse, interpolated image shows complicated geometry. A peak is located below the Toi seamount exposed on the seafloor. The basement sharply deepens westward to te west of subducted seamount. Assuming a uniform regional stress field around this area with sigma1 parallel to the plate conversion vector (WNW). We treated the amplitudes of horizontal principal stresses as parameters. Preliminary results indicate that Ts is generally high on and around the subducted seamount where the tremor frequence is large, too. In some frequent tremor areas, especially to the NNW of seamount, Ts is not high enough. This may indicate the needs for overpressures in that area. Further aspects might influence the seismicity pattern such as local stress variations induced by the subduction of the seamount, thermal stresses, differences in the frictional properties of the basement.