A marked magnetic field change exceeding 200 nT was detected by a three-component magnetometer at the beginning of the 2000 Miyakejima eruptive activity. The change in the magnetic field is correlated with an earthquake activity and crustal deformation, which indicate a dike intrusion very close to the observation site. A dike model based on the crustal deformation data shows that the primary cause of the magnetic change was the piezomagnetic effect associated with the dike emplacement, and that the other potential factors, including thermal and electrokinetic effects, were unlikely to be its principal cause. We obtained a new dike model by combining both the crustal deformation and the magnetic field change, which is sensitive to the strong stress changes near the edges of dike. The new dike model shows lateral and upward propagation of the dike during its intrusion, and suggests that shallow cracks induced by the dike intrusion near the ground surface contributed to the changes seen in the magnetic field.
