Very-long-period seismic signals and caldera formation at Miyake Island, Japan.

Over a period of roughly 40 days, starting on 8 July 2000, a caldera structure 1.7 kilometers in diameter developed by means of gradual depression and expansion of the summit crater at Miyake Island, Japan. At the same time, very-long-period (VLP) seismic signals were observed once or twice a day. Source mechanism analyses of the VLP signals show that the moment tensor solutions are smooth step functions over a time scale of 50 seconds, with dominant volumetric change components. We developed a model to explain the caldera and the VLP signals, in which a vertical piston of solid materials in the conduit is intermittently sucked into the magma chamber by lateral magma outflow. This model offers potential for making quantitative estimations of the characteristic physical properties of magma systems.