Mechanisms of step‐like tilt changes and very long period seismic signals during the 2000 Miyakejima eruption: Insights from kinematic GPS

During the 2000 eruption of the Miyakejima volcano in Japan, step-like tilt changes (TC) generally accompanied by very long period (VLP) seismic signals with a pulse-like shape and widths of ∼50 s were repeatedly observed (TC/VLP events). Kinematic GPS time series for Miyakejima were investigated in order to detect displacements associated with these events. We found that the kinematic GPS time series could be interpreted as the superposition of the following features: (1) displacement associated with the TC/VLP events, the source of which possibly corresponded to a shallow magma chamber represented by an almost vertical ellipsoidal cavity elongated NE–SW at a depth of 2–3 km; (2) displacement following TC/VLP events that may have been caused by exponential-type volume decreases of the same magma chamber with a decay constant of approximately half a day; and (3) displacements that may be caused by continuous volume decreases of the same magma chamber. These features broadly support the piston model of VLP seismic signals, in which a vertical piston of solid conduit material intermittently sinks into a magma chamber located a few kilometer beneath the edifice following deflation caused by continuous outflux of magma. The volume increase of the magma chamber associated with the TC/VLP events was found to be much smaller than that of the collapsed caldera, suggesting that most of the mass in the conduit sank into the magma chamber without generating VLP seismic waves or step-like TC.