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

Abstract 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

MOESM3 of Phreatic eruptions and deformation of Ioto Island (Iwo-jima), Japan, triggered by deep magma injection

Additional file 3: Figure S3. A comparison of observed crustal deformation (red arrows) with that calculated (blue arrows) by the best-fit sill-like deformation source (gray rectangles) of a, b before and c, d after the beginning of the eruption of August 5, 2015. Red stars show eruption points. Displacement vectors are relative to the 0603 station of GSI (see Fig. 1a for location)

MOESM4 of Phreatic eruptions and deformation of Ioto Island (Iwo-jima), Japan, triggered by deep magma injection

Additional file 4: Figure S4. The layout of the seismic array. Red triangles indicate where the seismometers are installed. Contour intervals of topography are 5 m

MOESM2 of Phreatic eruptions and deformation of Ioto Island (Iwo-jima), Japan, triggered by deep magma injection

Additional file 2: Figure S2. A comparison of observed crustal deformation (red arrows) with that calculated (blue arrows) by the best-fit Mogi model (red circle) of (a, b) after the beginning of the eruption of April 29, 2012. Red stars show eruption points. Displacement vectors are relative to the 0603 station of GSI (see Fig. 1a for location)

MOESM1 of Phreatic eruptions and deformation of Ioto Island (Iwo-jima), Japan, triggered by deep magma injection

Additional file 1: Figure S1. The daily number of earthquakes and vertical movement at Motoyama before and following phreatic eruptions after 1976 were not accompanied by precursors. The eruption list is shown in Table 1. The green and blue curves denote vertical movement at GNSS observation stations. The white triangles show the occurrence times of the eruptions

Upper Crustal Structure under the Central Part of Japan : Miyota-Shikishima Profile

In October 1982, a seismic refraction experiment was conducted by the Research Group for Explosion Seismology in the Miyota-Shikishima Profile in Nagano and Yamanashi Prefectures, central Japan. Five shots were fired on the 60 km-long profile. The upper crustal structure derived from travel-time data is simple and has a high velocity gradient; 4.0km/s at the surface to 6.0km/s at about 2.5km depth. Based on the amplitude-distance pattern of the refraction phases, we confirmed the existence of the velocity gradient. A few explosive sources generated clear S waves with amplitudes large enough to be read. We obtained an average POISSON'S ratio of around 0.246 in the uppermost crust from S to P travel time ratios. The other later arrivals were interpreted as reflected phases from a deep interface, the Conrad discontinuity. We modeled the interface shape by the ray-tracing technique. The interface inclines teward the north by about 20°.地震予知計画による4度目の爆破地震の実験が, 1982年10月,日本中部の長野県から山梨県にかけての御代田-敷島測線上で行われた. 60kmの測線上の5個所で爆破が行われ,約60点の臨時観測点で観測された.この実験の概要と,得られた記録や走時データなどの基礎的な資料については,すでに報告されている(爆破地震動研究グループ, 1986).ここでは,これらのデータの解析結果について報告する

The 2002 Seismic Refraction/Reflection Surveys in the Kozu-Matsuda Fault Zone and Ashigara Valley

We conducted seismic surveys in western Kanagawa prefecture including the Kozu-Matsuda fault zone and Ashigara valley, in February and March 2002. The reflection surveys were carried out using vibrator sources along 2 receiver lines. Line A is 15-km long and extends east to west across the Kozu-Matsuda fault zone and Ashigara valley. Line B is 10-km long and extends north to south in the valley. 1151 receivers were deployed at intervals of 25m for 210 vibration points. 3-D refraction surveys were also conducted with 4-dynamite shots at both sides of the lines A and B and repeated vibroseis sweeps were made at 3 points. In addition to the 1151 reflection receivers, 108 receivers were deployed along the 5 lines between the shot points, and 128 independent receivers were located over a wide area in the Ashigara valley. We overview the surveys and present datasets of seismograms and travel times

日本中部下の上部地殻構造 : 御代野-敷島測線

In October 1982, a seismic refraction experiment was conducted by the Research Group for Explosion Seismology in the Miyota-Shikishima Profile in Nagano and Yamanashi Prefectures, central Japan. Five shots were fired on the 60 km-long profile. The upper crustal structure derived from travel-time data is simple and has a high velocity gradient; 4.0km/s at the surface to 6.0km/s at about 2.5km depth. Based on the amplitude-distance pattern of the refraction phases, we confirmed the existence of the velocity gradient. A few explosive sources generated clear S waves with amplitudes large enough to be read. We obtained an average POISSON\u27S ratio of around 0.246 in the uppermost crust from S to P travel time ratios. The other later arrivals were interpreted as reflected phases from a deep interface, the Conrad discontinuity. We modeled the interface shape by the ray-tracing technique. The interface inclines teward the north by about 20°.地震予知計画による4度目の爆破地震の実験が, 1982年10月,日本中部の長野県から山梨県にかけての御代田-敷島測線上で行われた. 60kmの測線上の5個所で爆破が行われ,約60点の臨時観測点で観測された.この実験の概要と,得られた記録や走時データなどの基礎的な資料については,すでに報告されている(爆破地震動研究グループ, 1986).ここでは,これらのデータの解析結果について報告する

国府津−松田断層帯及び足柄平野における2002年地下構造探査

We conducted seismic surveys in western Kanagawa prefecture including the Kozu-Matsuda fault zone and Ashigara valley, in February and March 2002. The reflection surveys were carried out using vibrator sources along 2 receiver lines. Line A is 15-km long and extends east to west across the Kozu-Matsuda fault zone and Ashigara valley. Line B is 10-km long and extends north to south in the valley. 1151 receivers were deployed at intervals of 25m for 210 vibration points. 3-D refraction surveys were also conducted with 4-dynamite shots at both sides of the lines A and B and repeated vibroseis sweeps were made at 3 points. In addition to the 1151 reflection receivers, 108 receivers were deployed along the 5 lines between the shot points, and 128 independent receivers were located over a wide area in the Ashigara valley. We overview the surveys and present datasets of seismograms and travel times

Focal Mechanism Solutions of Micro- and Small Earthquakes Occurred in the Western Kanagawa Area Situated in the Izu Collision Zone

Focal mechanism solutions of earthquakes in the western Kanagawa area are compiled and characteristics of fault types in the Izu collision zone are discussed. Reverse, strike-slip, and oblique fault mechanisms are seen in the Izu collision zone. Although only about 10 per cent of micro- and small earthquakes, are associated with strike-slip fault mechanisms, the largest and the second largest earthquakes (M 6.0 and M 5.6) are of that type. It was found that the direction of the P axis is somewhat different in the western and eastern Tanzawa areas : in western Tanzawa the average direction of the P axis is NW-SE, while it is NNW-SSE in eastern Tanzawa. We think this difference in the P axis direction reflects differences in tectonic conditions in those areas. In the Ashigara plain area, mechanism solutions are mostly reverse and oblique faults with the P axis direction between E-W and NNW-SSE