2004年紀伊半島南東沖の地震の粘性緩和による変動

抄録

This study develops a three-dimensional viscoelastic model using the Finite Element Method to understand the postseismic deformation that followed the 2004 off the Kii peninsula earthquake. The questions how long the viscoelastic relaxation continues to the surface deformation and how much the viscoelastic relaxation affects the surface deformation are of particular importance. We first detected the long lived postseismic deformation up to end of 2016. This long lived postseismic deformation is explained by the viscoelastic relaxation caused by the main rupture. Our viscoelastic model consists of three viscoelastic media—the mantle wedge, oceanic asthenosphere and Lithosphere and Asthenosphere Boundary (LAB). The optimal viscosities for three viscoelastic media are the mantle wedge with 2×10¹⁸ Pa·s, the oceanic asthenosphere with 1×10¹⁹ Pa·s, and the LAB with 5×10¹⁸ Pa·s. The viscoelastic relaxation produces southward motion across the entire Chubu district, and continues for at least a few decades. The maximum horizontal displacement predicted by the viscoelastic relaxation reached 6.2 cm cumulative over 10 years at the tip of the Shima peninsula. The viscoelastic model explains most of the southward motion observed after the main rupture up to end of 2016. It is obvious that observed crustal deformation in the Chubu district contains the effect of viscoelastic relaxation caused by the 2004 off the Kii peninsula earthquake. Therefore we should consider these effects when we interpret the crustal deformation in this area. Otherwise we overestimate or underestimate the mechanism, such as the interplate coupling or the interplate fault slip.