Intermittent inflations recorded by broadband seismometers prior to caldera formation at Miyake-jima volcano in 2000

Very-long-period (VLP) pulses with widths of 20 s on velocity seismograms were observed during volcanic activity at Miyake-jima Volcano, Japan in 2000. The VLP events occurred repeatedly during a few days prior to caldera formation and essentially vanished following the onset of caldera collapse. Waveform inversions of the pulse-like signals point to a source offset 3.5 km beneath and 1 km south of the summit. A candidate for the source mechanism is the inflation of an elliptical cylinder with axis tilted 20–30° from vertical and major axis of the elliptical cross section oriented northeast-southwest. The inferred mechanism appears consistent with a step-like pressurization of a magma reservoir impacted by a falling rock mass in response to gravitational instability. The repeated occurrences of the rock collapses lead to the caldera formation at Miyake-jima.

► We detected unusual seismic events during a few days before caldera formation.
► The seismic records show smoothed step-like ground displacements for 20 s.
► The mechanism is modelled by an inflation of an elliptical cylinder under the summit.
► We interpret that a falling rock block caused an inflation of a magma reservoir.
► The collapse of crustal rocks under the summit may led to the caldera formation.