Long-lasting Dike Intrusion in the 2000 Eruption of Miyake-Jima Volcano: Creep or Chamber deflation?

The model for the 2000 intrusive event between Kozu-jima and Miyake-jima volcano, Japan, was reinvestigated. After the sudden earthquake swarm in Miyake-jima volcano, a dike intrusion of large volume lasting for two months was detected by the nation wide GPS network (Geonet). We extracted the step-wise displacement of the GPS station by fitting a function which represents a step, a linear trend and an annual sinusoidal variation. The displacements due to the dike intrusion were detected with stations in the area with a radius of about 200km. The displacement we obtained shows a distribution that is consistent with the idea that the source is located near Miyake-jima volcano. The displacement apparently indicates that a dyke of large volume was intruded between Miyake-jima and the neighboring Kozu-jima volcano. However, the displacement cannot be reproduce solely by the dike even with some earthquakes associated with the intrusive event. Therefore, Yamaoka et al. (2000) and Nishimura et al. (2001) introduced an aseismic creep near the dike tip opposite to Miyake-jima. The magnitude of the creep that is required corresponds to M7 which is anomalously large. As there is no evidence which support such a large aseismic creep, we tried to explain the deformation with another model. We, here, introduced a deflation model, which is well know as Mogi model, instead of creep source. We searched for the optimum values for the model with parameter search. We assumed a vertical dike with the length of 20km centerred at the middle of earthquake swarm region. We searched for the azimuth, the depth of the top, the depth extent of the dike and the tensile dislocation for the dike. For the deflation source we assumed that it locates beneath the dike between the depth of 15km and 40km. The optimum solution we obtained for the dike is 42 degree for the azimuth, 2.5 km for the top depth, 14km for the depth extent and 6m for the tensile dislocation. These values correspond to the volume expansion of 1.7km3. For the deflation point source, we obtained the deflation volume of 2.1km3 at the depth of 30km. This model also explains the regional and local pattern of displacement detected by GPS network.