Step-like Tilt Change Occurred during the Caldera-forming Stage of the 2000 Miyakejima Volcanic Activity.

The National Research Institute for Earth Science and Disaster Prevention had been conducting continuous crustal tilt observations at five stations on Miyakejima, prior to the volcanic activity in the year 2000. During the caldera-forming stage, extraordinary step-like tilt signals were detected with the following time sequence.1) The initial step signal was detected coincidentally at the exact time of the explosive summit eruption on July 8, 18h41m.2) The 2nd step signal was detected on July 9, roughly 28 hours following the initial one. Since then, step signals have appeared intermittently, at frequencies of 1-3 steps within 1-2 days.3) The last step signal was detected at 18h09m of August 18, during the largest summit eruption.In total, 46 step signals were detected and all of the step signals occurred within a very short interval of less than two minutes. Tilting directions of the initial and the last step signals were in the sense of ground-up toward the summit. These were then followed by a reverse tilt. Similarly, tilting directions of the remaining 44 step signals were mostly in the sense of ground-up toward the southern part of the island. Again, they were followed by a reverse tilt. From these observational results we conjectured that the step signals were generated by an explosive source of volumetric expansion. Tilt movements after the steps seem to reflect a relaxation process. Since the initial step signal was synchronized with the eruption, which was considered to be a phreatic explosion, this must be the cause of the step. Although the extent of tilting of the last step was very small compared to the first one, their tilting patterns resembled each other. Accordingly, a similar small explosion may have occurred at the same time when the last step signal was detected. As for the remaining 44 step signals, tilt vectors at the northern three stations were aligned in their own directions toward the southern part of the island, while those at the southeast and southwest stations were rather widely spread. These features suggest that the pressure source is in the southern part of the island. A dike, which was considered to have intruded into this part of the island must be related to them.