Crustal Deformation in the Kanto District, Central Japan, Following the 2000 Seismo-volcanic Activity of the Izu Islands

Starting from June 26, 2000, an unprecedented seismic activity occurred around the Miyake-jima, Kohzu-shima, and Nii-jima Islands, in the northern Izu islands. This seismic swarm activity was initiated by the volcanic magma intrusion beneath the Miyake-jima volcano. An intrusion of massive (about 1km³) magma caused the seismic swarm activity and magnificent crustal deformation in the surrounding area within about 200km from the source region. After the seismic swarm activity calmed down, we detect a change in crustal displacement rates in the southern Kanto region from daily coordinate solutions of the continuous GPS network. Interestingly, the change appears mostly in the E-W components. Comparison of GPS velocity data for two time periods (1996-200 and 2001-2002) indicate that the westward displacement rate decreased by about 25% (from 23 mm/yr to 17 mm/yr) at Tateyama, the southern tip of the Boso Peninsula. On the other hand, we do not see significant changes in the N-S and vertical components. Continuous monitoring of crustal displacements with GPS has revealed that the post-swarm deformation is now coming back to the pre-swarm steady state. That is, the time series of E-W component show transient curves, converging into the original steady state. The transient curve can be equally well reproduced by an exponential decay or a logarithmic function. The relaxation time for the exponential curve is estimated as about 3 years. One possible explanation for this transient deformation is viscoelastic relaxation. Since the Izu Islands are situated on the oceanic Philippine Sea plate, the upper mantle with a low viscosity would response to the huge stress change cause by the magma intrusion. The other possibility is a change of frictional property on the plate interface between the Philippine Sea and the Pacific plate. Under the southern Kanto area, the subducted Philippine Sea slab leans on the subdcted Pacific slab. Interaction between these two oceanic plates is still not understood well. But the massive dyke intrusion strongly pushed the subducted Philippine Sea slab, changing the frictional status at the bottom of the Philippine Sea plate. Since the motion of the Pacific plate subduction is nearly westward, this idea can be a solution for the observation that only the E-W components are affected.