Phreatic eruptions and deformation of Ioto Island (Iwo-jima), Japan, triggered by deep magma injection

• Published in: Earth, planets, and space Vol. 70; no. 1; pp. 1 - 15
• Main Authors: Ueda, Hideki, Nagai, Masashi, Tanada, Toshikazu
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 12-03-2018; Springer; Springer Nature B.V; SpringerOpen
• Subjects: 5. Volcanology; Altitude; Boreholes; Caldera; Crustal deformation; Deformation; Deformation mechanisms; Earth and Environmental Science; Earth Sciences; Earthquake; Earthquakes; Eruptions; Fault lines; Geodetic surveys; Geology; Geophysics/Geodesy; Geysers; Hydrostatic pressure; Hydrothermal systems; Lava; Magma; Natural history; Phreatic eruption; Phreatomagmatism; Precursor; Precursors; Reservoirs; Satellite observation; Seismic activity; Seismographs; Seismometers; Topography; Towards forecasting phreatic eruptions: Examples from Hakone volcano and some global equivalents; Transient deformation; Tuff; Uplift; Volcanoes; Japan; Crustal deformation; Earthquake; Transient deformation; Phreatic eruption; Precursor; Caldera
• ISSN: 1880-5981; 1343-8832; 1880-5981
• DOI: 10.1186/s40623-018-0811-y

On Ioto Island (Iwo-jima), 44 phreatic eruptions have been recorded since 1889, when people began to settle there. Four of these eruptions, after the beginning of continuous observation by seismometers in 1976, were accompanied by intense seismic activity and rapid crustal deformation beforehand. Other eruptions on Ioto were without obvious crustal activities. In this paper, we discuss the mechanisms of phreatic eruptions on Ioto. Regular geodetic surveys and continuous GNSS observations show that Ioto intermittently uplifts at an abnormally high rate. All of the four eruptions accompanied by the precursors took place during intermittent uplifts. The crustal deformation before and after one of these eruptions revealed that a sill-like deformation source in the shallow part of Motoyama rapidly inflated before and deflated after the beginning of the eruption. From the results of a seismic array and a borehole survey, it is estimated that there is a layer of lava at a depth of about 100–200 m, and there is a tuff layer about 200–500 m beneath it. The eruptions accompanied by the precursors probably occurred due to abrupt boiling of hot water in hydrothermal reservoirs in the tuff layer, sealed by the lava layer and triggered by intermittent uplift. For the eruptions without precursors, the hydrothermal systems are weakly sealed by clay or probably occurred on the same principle as a geyser because phreatic eruptions had occurred beforehand and hydrostatic pressure is applied to the hydrothermal reservoirs.