Investigation of Deformation Sources for the Miyakejima, Japan Intrusive Event

On June 26, 2000, a large seismic swarm began beneath the west coast of Miyakejima, Japan, an active volcanic island in the Izu arc. After a week-long propagation of seismicity to the northwest, seismic activity continued for approximately two months in the region. The swarm was accompanied by eruptions and caldera collapse on Miyakejima, as well as continuing crustal deformation measured by GEONET, the continuous GPS network of the Geographical Survey Institute of Japan. Based on observed deformation and locations of hypocenters, it has generally been assumed that dike propagation from a magma chamber beneath Miyakejima was responsible for the swarm. Although several models of varying complexity have been proposed to explain the deformation, there is little consensus on a geologically motivated model that explains both the long-lived deformation and seismicity. In addition, this event provides an interesting opportunity to study the relationship between dike propagation and earthquake nucleation, which, though long recognized, is not well understood. We tested several models of deformation based on observed displacements, hypocenter locations, overall tectonic setting, and pre-event data. Sources of deformation considered include various modes of dike propagation, magma chamber deflation, and faulting to the northwest of Miyakejima in a zone that was seismically active before the 2000 event. In order to test the viability of these mechanisms, we utilized both inverse and forward modeling of displacements during various stages of the event. Initial results show that a model consisting of only dike opening and Mogi deflation beneath Miyakejima can fit the total deformation well, but the best-fitting model does not coincide with the seismic swarm. In addition, there are substantial differences between the orientations and magnitudes of displacements observed during the first week (maximum 4.8 cm) and those for the entire event (maximum 48.4 cm). These preliminary results indicate that a simple model of dike propagation during the first week followed by steady opening does not adequately explain the observed deformation, and more complex source geometries and evolutions are considered.