A dike intrusion model in and around Miyakejima, Niijima and Kozushima in 2000

From June 26, 2000, an earthquake swarm started in Miyakejima, about 50 km south off Honshu, central Japan. Eruptions of Miyakejima and five large earthquakes with magnitudes 6.0 and above occurred for the following 2 months together with a large number of smaller earthquakes over 100,000. In this study, we focused on spatio-temporal crustal deformation observed at GPS stations in this area. In order to explain the seismicity and the crustal deformation, we considered a dike intrusion model placed between Miyakejima and Kozushima. Then, we attempted to obtain spatio-temporal distributions, the amount of the dike intrusion, deflation beneath Miyakejima, and fault slips of the seven large events by geodetic data inversion. For this purpose, we divided the time series of GPS data from June 12 to August 27 into 10 periods, which are related to significant events. In order to find location, depth, strike and dip of the dike plane and a depth of deflation beneath Miyakejima, we used the Monte Carlo method and tested 10,000 models for each period. As a result, we estimated that the total amount of the dike intrusion and the deflation beneath Miyakejima reached about 1.1 x 10(9) and 5.4 x 10(8) m(3), respectively. The maximum amount of the dike intrusion and the deflation beneath Miyakejima reached over 3.5 x 10(8), m(3) during the period from July 20 to July 28, 2000, and over 1.7 x 10(8) m(3) during the period from June 15 to June 28, 2000, respectively. Temporal change of the amount of the dike intrusion corresponds well to that of the deflation beneath Miyakejima until the middle of July. However, since large amount of the dike intrusion from July 20 to 28 did not correspond to that of the deflation beneath Miyakejima, we deduced that the magma source changed from Miyakejima to Kozushima and the magma might come from sub-crustal magma pockets from the middle of July. If we assume the open crack associated with the dike intrusion and the deflation beneath Miyakejima are filled with magma with a density of 2500 kg/m(3), the mass would be about 2.75 x 10(9) and 1.35 x 10(9) tons, respectively. We deduced that at least 1.4 x 109 tons of magma came from sub-crustal magma pockets. (C) 2002 Elsevier Science B.V. All rights reserved.