Cyclic jerky opening of magma sheet and caldera formation during the 2000 Miyakejima volcano eruption

[1] NIED volcanic observation network at Miyakejima volcano, central Japan, repeatedly detected extraordinary step-like tilt changes during the 2000 Miyakejima eruption. Associated with these tilt steps, broadband seismometers recorded very long pulse waves with a duration of about 50-s. During this stage, the summit of the volcano collapsed and sank, forming a caldera with a depth of about 400 m and a diameter of about 1.6 km in total. Prior to the tilt steps, long-period earthquakes were acceleratingly generated in the shallow part of the volcano, and the ground tilt changes suggested inflation of the volcanic body. The analysis of these data from the tiltmeters and broadband seismometers suggests that the tilt steps and the 50-s pulse packets were caused by the opening of sill( sheet)-like tensile cracks oriented in a SE-NW direction beneath the volcano with a depth of about 6-8 km and a volumetric change of about 10(6)-10(7) m(3).