地震 第2輯
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
2001年芸予地震とプレートの曲げモーメント
佃 為成三浦 勝美
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ジャーナル フリー

2002 年 55 巻 2 号 p. 91-96

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A viscoelastic bending moment theory is applied to the subducting slab of the Philippine Sea plate. The slab is steeply bent at the northern end of the Kyushu-Ryukyu subduction zone compared with at the southern part of the arc. The plate boundary on the cross section of the slab beneath Aki-Nada, Iyo-Nada and Bungo Channel, sea regions between Kyushu and Shikoku Islands, represents an arc of a circle, as demonstrated by the microearthquake distribution during over 10 years. The elastic bending moment is evaluated based on this geometry, where the deformation due to viscosity is reduced applying a Maxwell viscoelastic theory. The bending is too large in strength to ascribe it only to the force of negative buoyancy of the slab. We here assume the horizontal pressure generated by the spreading mantle flow accompanying upwelling magmas at the volcanic area in northern Kyushu, where extensive ground movements have been detected by geodetic measurements. The exerted uniform pressure to bend the slab is estimated to amount to (σ=) 72 MPa, when we take the following model constants as standards : the thickness of the slab h = 30 km, Young modulus of the slab (E=) 1.8 x 105 MPa, Poisson's ratio (ν=) 0.27, an average elapsed time since the subduction of the slab initiated (t=) 1.9 × 106 y, the time constant of the viscoelastic slab (τ=) 3.0 × 105y, and the excess density of the slab compared with the surrounding athenosphere (Δρ=) 0.06 × 103 kg/m3. The region of the highest seismic activity is located at the vicinity of the upper locked portion of the plate, where destructive earthquakes have frequently occurred. The 2001 Geiyo earthquake of M6.4 was one of them and took place at the eastern end of this active seismic zone. The focal mechanism of the event shows normal faulting as similarly as those of the majority of small earthquakes in the seismic active region. The main cause of this large earthquake is considered to be the bending force of the plate.

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