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 erup- tions 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 hydro- thermal reservoirs. Keywords: Phreatic eruption, Caldera, Earthquake, Crustal deformation, Precursor, Transient deformation the east side of Ioto is a resurgent dome formed at the Introduction central part of the caldera (Newhall and Dzurisin 1988). Ioto Island (Iwo-jima) is a volcanic island located about Suribachiyama, with an altitude of about 170 m, is a 1200 km south of Tokyo, Japan (Fig. 1). It is one of the monticule on the southwest of Ioto. Motoyama and Suri- volcanoes of the Izu-Bonin-Mariana island arc accom- bachiyama are connected by a wedge-shaped sandbar panying the subduction of the Pacific plate beneath the called Chidorigahara. Philippine Sea plate. The island is about 8 km × 4 km in Motoyama is covered with pyroclastic flow depos - size. Bathymetry shows that Ioto is the summit of a stra- its and lava flows from a large-scale magmatic eruption tovolcano with a height of about 2000 m from the ocean (the Motoyama eruption) dated to 2.7 cal kBP (Nagai floor and a width of about 40 km, and rock reefs at sea and Kobayashi 2015). A borehole survey (depth 150 m) showing ring-shaped topography suggest there is a cal- was conducted in the central part of Motoyama, and it dera rim with a diameter of about 10 km at the top of the provides the only result that has been published (Ossaka mountain (Fig. 1b). There are three main topographic et al. 1985). The drilling core shows alternating layers features on Ioto: Motoyama, Suribachiyama and Chido- consisting of lava flows and pyroclastic rocks. Lava lay - rigahara. Motoyama, with an altitude of about 110 m, on ers found on the east coast (Nagai and Kobayashi 2015) at depths of 24–97.9 and 106.6–130.5 m were presumed *Correspondence: firstname.lastname@example.org to be Motoyama lava and Hanareiwa lava, respectively. National Research Institute for Earth Science and Disaster Resilience, Hanareiwa lava was erupted before the Motoyama Tennôdai 3-1, Tsukuba-shi, Ibaraki-ken 305-0006, Japan © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Ueda et al. Earth, Planets and Space (2018) 70:38 Page 2 of 15 Motoyama Pacific Plate Kamaiwa 6 Philippine Sea Plate 200 km Chidorigahara 1 km Eruption Point NIED (Seismic, GNSS) NIED (Tide gauge) JMA (Seismic, Infrasound) GSI (GNSS) 5 km Suribachiyama Fig. 1 Location map. a Tectonic environment around Ioto Island. Lines show plate boundaries. Red triangles are active volcanoes in Japan. Ioto Island is a volcanic island belonging to the Izu-Bonin-Mariana volcanic arc. Square shows location of a reference GNSS station of GSI on Hahajima Island. b Map of Ioto Island. The broken circle is the estimated caldera rim of Ioto volcano using bathymetry. c Topography of Ioto Island and loca- tions of continuous observation stations and eruption points. Motoyama is a resurgent dome formed in the central part of the caldera (Newhall and Dzurisin 1988). Suribachiyama is a monticule in the southwest of Ioto. Circles with numbers are eruption points after 1922. The numbers correspond to the numbers in Table 1. Crosses and squares are observation stations. Orange lines are active faults (Kaizuka et al. 1983, 1985). Contour intervals of topography are 10 m eruption, but the eruption time is unknown. It was esti- a reef (Fig. 1c), but in 1969 it was connected to the main- mated that there was already a caldera rim during the land (Oyagi and Inokuchi 1985). According to surveys Motoyama eruption but that there had been a large-scale periodically conducted since 1976, Motoyama uplifted eruption forming a caldera even before 2.7 cal kBP. Since by about 0.5 m and Chidorigahara uplifted by about 4 m then, Suribachiyama has experienced at least three mag- from 1977 to 2002 (Ukawa et al. 2006). Ukawa et al. (2006) matic eruptions. Nagai and Kobayashi (2015) suggested showed that the crustal deformation is a superposition that the last eruption was earlier than 1779 as the current of the episodic uplift of the whole island and continuous shape of Suribachiyama resembles the shape of the 1779 deformation of Motoyama. This continuous deformation picture (King 1785). is due to the contraction of the central part of Motoyama It is known that a large-scale uplift has continued in Ioto and the uplift of the surrounding area. They interpreted for centuries. The C age of corals collected at the cen- the episodic uplift of the whole island and the continuous tral part of Motoyama at an altitude of about 110 m was deformation as a deep magma intrusion and a long-term estimated at 500–800 years, and the average uplift rate is movement of volcanic fluid, respectively. Active faults are presumed to be 15–20 cm per year (Kaizuka et al. 1983, predominantly distributed in Chidorigahara (orange lines 1985). The distribution of coastal terraces shows that in Fig. 1c) due to the severe uplift of Motoyama (Kaizuka intermittent uplift continues. Geodetic surveys and aerial et al. 1983, 1985). Seismic activity is also very pronounced photography from 1952 and 1968 indicate that Motoy- due to intense crustal movement. The earthquakes have ama uplifted by about 9 m during those 16 years (Tsuji a maximum magnitude of M3 with the majority of them et al. 1969). Kamaiwa, sticking out to the northwest, was occurring at depths shallower than 3 km BSL. Ueda et al. Earth, Planets and Space (2018) 70:38 Page 3 of 15 Ioto has very high geothermal activity throughout the Table 1 Eruption list island, with phreatic eruptions occurring frequently in No. Date Location Precursor References craters and fumarolic areas. Since settlement began in (Fig. 1c) 1889, 44 eruptions were recorded up until August 2017 1 1889 or 90 1? ? Ogawa (1912) (Table 1). The eruptions listed in Table 1 are those wit- 2 Jul. 1922 1 ? Toyoshima (1932) nessed by local residents, along with observed volcanic 3 1935 2 ? Morimoto et al. (1968) tremors by seismometers and confirmed eruptions by 4 Dec. 1944 3 ? Kumagai (1985) field surveys. In addition, there are instances where 5 Mar. 28, 1957 4 ? Corwin and Foster (1959) discolored water spotted on the sea surface led to the 6 Dec. 23, 1967 1 ? Morimoto et al. (1968) assumption of eruptions. In an eruption that occurred 7 Jun. 20, 1968 5 ? Morimoto et al. (1968) in Chidorigahara on March 28, 1957 (No. 4 in Fig. 1c), 8 Jan. 12, 1969 1 ? Kumagai (1985) a plume of about 60–90 m in height rose and a crater 9 Nov. or Dec. 1969 6 ? Kumagai (1985) with a diameter of 30 m was formed (Corwin and Fos- 10 Nov. 1975 3 ? Kumagai (1985) ter 1959). This is a typical phreatic eruption, blowing 11 Jan. 1976 7 ? Kumagai (1985) out steam and sand. At least two eruptions occurred 12 Dec. 11, 1978 7 No Kumagai (1985) at the Asodai sinkhole from November 28–29, 1982 13 Mar. 13, 1980 3 No Kumagai (1985) (No. 7 in Fig. 1c), and volcanic earthquakes observed 14 Mar. 9–10, 1982 8 No Kumagai (1985) from August 25–30 reached 1492 in number, and 15 Nov. 28–29, 1982 7 Yes Kumagai (1985) many fault movements was detected (Kumagai et al. 16 Dec. 1982 7 No Kumagai 1985 1985). Recently, eruptions have taken place at Million 17 Aug. 22, 1994 9 No Ukawa et al. (2002) dollar hole during 2012–2013 (No. 1 in Fig. 1c) and 18 Sep. 10, 1999 7 No JMA (2013a) Idogahama (No. 8 in Fig. 1c) and Kitanohana (No. 3 19 Sep. 21–22, 2001 10 Yes Ukawa et al. (2002) in Fig. 1c) in 2015 (Japan Metrological Agency 2012, 20 Oct. 19–23, 2001 8 No Ukawa et al (2002) 2013a, b, c, d, 2015a, b, c). Juvenile material was not 21 Oct. 7, 2002 7 No JMA (2013a) detected in either eruption; they were phreatic erup- 22 Nov. 8, 2002 7 No JMA (2013a) tions. The eruption points 9, 10, 12, 14 and 15 in Fig. 1c 23 Apr. 28, 2004 9 No JMA (2013a) are located in the ocean area. In 1994, white smoke and 24 Jun. 6–8, 2004 7 No JMA (2013a) muddy water erupted and a crater was formed at a rock 25 Dec. 19–20, 2007 7 No JMA (2013a) reef by eruption point 9. Also, several tens of meters of 26 Feb. 2012 1 No JMA (2013a) plume had been observed at 10, and a volcanic tremor 27 Mar. 7, 2012 1 No JMA (2013a) was observed when an eruption took place at 12. 28 Apr. 5–6, 2012 1 No JMA (2013a) They were definitely phreatic eruptions. However, at 29 Apr. 28, 2012 11 Yes JMA (2013a) points 14 and 15 in 2013, only discolored waters were 30 Apr. 29–30, 2012 12 No JMA (2013a) confirmed. 31 Jul. 9, 2012 1 No JMA (2013a) Here, we aim to investigate the mechanisms of the 32 Sep. 7 2012 13 No JMA (2013a) phreatic eruptions of Ioto. Some phreatic eruptions have 33 Dec. 1, 2012 1 No JMA (2012) been accompanied by clear precursors. However, erup- 34 Feb. 17–18, 2013 1 No JMA (2013b) tions without precursors have also been reported. Phre- 35 Mar. 5–6, 2013 1 No JMA (2013c) atic eruptions and related activities such as earthquakes, 36 Apr. 11, 2013 1 No JMA (2013d) deformation and fumaroles on the ground are considered 37 Aug. 21, 2013 14 No JMA (2013a) to be related with hydrothermal reservoirs of shallow 38 Aug 28–30, 2013 15 No JMA (2013e) depths (e.g., Kaneshima et al. 1996). We will clarify the 39 Dec. 16, 2014 16 No JMA (2014) relationship between phreatic eruptions and hydrother- 40 May 22–24, 2015 8 No JMA (2015a) mal reservoirs in Ioto with seismic and crustal defor- 41 Jun. 20, 2015 8 No JMA (2015b) mation data. Although the scale of phreatic eruptions is 42 Aug. 7, 2015 3 Yes JMA (2015c) small in Ioto, they are frequent, occurring everywhere on 43 Aug. 25–26, 2015 1 No JMA (2015c) the island. Elucidation of the regularity and mechanisms 44 Sep. 1, 2016 7 No JMA (2015d) of phreatic eruptions helps to reduce the risk to residents on the island. a Only discolored waters were confirmed Ueda et al. Earth, Planets and Space (2018) 70:38 Page 4 of 15 80 60000 No Seismic Data 40 30000 No Seismic Uncounted Data 0 0 1980 1985 1990 1995 2000 2005 2010 2015 IJTV IJMV -1 1980 1985 1990 1995 2000 2005 2010 2015 Year Fig. 2 A comparison between earthquake activity and vertical movement of Ioto after 1977. a Monthly average of daily number of earthquakes counted at Motoyama. The numbers are counted at seismic stations of Japan Ministry of Defense around 0604 station of GSI before 1986 and at IJMV after 1992. The red curve shows the cumulative number of earthquakes. The red triangles and white triangles denote occurrence times of phreatic eruptions with and without precursors, respectively. b Vertical movement by campaign leveling and GPS surveys after 1977 at survey points near observation points where GNSS continuous observation is carried out. In the leveling survey up until 1995, average sea level as meas- ured by the tide level gauge was taken as the reference height Observations one point since 2011. Geospatial Information Authority Volcanic unrest of Ioto is characterized by intense seis- of Japan (GSI) has been conducting continuous observa- mic activity and rapid uplift. In order to investigate the tion of GNSS at two points since 1997. Japan Ministry of relationship between the occurrence of phreatic eruption, Defense had been conducting continuous observation of seismic activity and uplift, we first looked at observations a seismometer from March 1976 around 0604 station of of the earthquakes and crustal deformation of Ioto. Fig- GSI, but it was discontinued in the 1990s. NIED also con- ure 1c shows the distribution of continuous observation ducts periodic surveys every 2 years, as will be described stations used in this study. National Research Institute later in this paper. for Earth Science and Disaster Resilience (NIED) has Figure 2a shows the monthly average number of daily conducted continuous observation of seismometers and earthquakes occurring on the island observed at Motoy- GNSS at three observation points. Earthquakes have ama. From March 1976 to June 1985, it was the obser- been observed since 1982, and continuous observation vation point of Japan Ministry of Defense, and after by GNSS started in 2003. Japan Meteorological Agency that, the number of earthquakes counted came from (JMA) has been observing earthquakes and infrasound at records at IJMV. This number of earthquakes was chosen Monthly Average of Daily Number of Eq. Vertical Displacement(m) Cumulative Number Ueda et al. Earth, Planets and Space (2018) 70:38 Page 5 of 15 160 60000 No Seismic No Seismic Data Data 80 30000 0 0 2000 2005 2010 2015 IJTV IJMV -1 2000 2005 2010 2015 Year Fig. 3 A comparison between earthquake activity and vertical movement of Ioto after 1997. a Daily number of earthquakes counted at IJMV. The red curve shows the cumulative number of earthquakes. The red triangles and white triangles denote occurrence times of phreatic eruptions with and without precursors, respectively. b Vertical movement by daily solutions of continuous GNSS observation after 1997 at the continuous stations by counting what is supposed to have occurred inside used as the reference height for the levelling surveys. the island. Since 2003, we have determined the hypo- Since we do not have tide level data for 1976, the survey center and that the depth of 95% of the earthquakes that result is not included in Fig. 2b. occurred inside the island is less than 3 km. Although it After 1977, the three observation points of Motoyama was not counted from June 1985 until March 1991, it was showed definite periods of uplift approximately every reported that earthquake activity was low (NIED 1992). 10 years (1982–1984, 1991–1993, 2000–2002, 2006– We will compare this seismic activity with the crustal 2016), and subsided in the intervening periods. This deformation after 1976 (Ukawa et al. 2006). NIED con- intermittent uplift is a common crustal deformation of ducted leveling and trilateration 11 times every other caldera volcanoes, as exemplified at Campi Flegrei (Bel - year from 1976 to 1995. Furthermore, GPS surveys lucci et al. 2006), Yellowstone (Chang et al. 2007), and were conducted 10 times every 2 years from 1996 to Rabaul (Robertson and Kilburn 2016) calderas. The inter - 2016. Results until 2002 are summarized in Ukawa et al. mittent uplift of Ioto is interpreted as a consequence of (2006). Figure 2b shows vertical movement obtained by magma injections into a deep magma reservoir (Ukawa leveling and GPS from 1977 at the survey point within et al. 2006). A comparison of the number of earthquakes 10 m of the observation point where GNSS continuous and vertical movement in Fig. 2a shows that the number observation was carried out. In the leveling survey until of earthquakes is also relatively large during intermittent 1995, the average sea level measured by the tide level uplifts. Figure 3 shows a more detailed view of the rela- gauge was taken as the reference height (the location of tionship between earthquakes and uplift by GNSS, which tide level gauge is shown by a star in Fig. 1c). Temporary allows for a higher time resolution. We compared the tide level observation from 1977 to 1980 and a tidal level vertical displacement after the start of GNSS continuous gauge of continuous observation from 1980 to 1995 were observation from 1997 with the number of daily earth- employed to monitor the tide level, and the results were quakes. Using GNSS data of NIED and GSI observation Daily Number of Eq. Vertical Displacement(m) Cumulative Number Ueda et al. Earth, Planets and Space (2018) 70:38 Page 6 of 15 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 01 02 03 04 05 06 07 08 09 200 0.5 0.4 0.3 0.2 0.1 0 0.0 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 0.5 0.4 0.3 0.2 0.1 0.0 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 01 02 03 04 05 06 07 08 09 10 11 12 0.5 0.4 0.3 0.2 0.1 0.0 21 22 23 24 25 26 27 28 29 30 31 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 Fig. 4 The daily number of earthquakes and vertical movement at Motoyama before and after phreatic eruptions accompanied by precursors. a The eruption on November, 28–29, 1982; b the eruption on September 21, 2001; c the eruption on April 29, 2012; d the eruption on August 6, 2015. The green and blue curves denote vertical movement at GNSS observation stations. The red triangles denote the occurrence times of the eruptions, but the occurrence time of the eruption on November, 28–29, 1982 is not clear stations sampled every 30 s, the coordinates of each day 1982, September 2001, April 2012, and August 2015 were were estimated by static analysis using GAMIT/GLOBK shown to be accompanied by seismic activity and crus- software. Motoyama uplifted during 2001–2003 and after tal deformation (Fig. 4). From November 28–29, 1982, 2006. The uplift was accelerated in 2011–2013. When the two phreatic eruptions took place at Asodai sinkhole. uplift rate is high, the number of earthquakes tends to The times of these occurrences were not clear. Dur - be large; and earthquake activity is almost synchronized ing this period, the largest earthquake swarm occurred with the intermittent uplifts. since the beginning of continuous observation in 1976, After the beginning of continuous observation of earth- and 1492 earthquakes were observed from November quakes in Ioto in March 1976, the eruptions of November 25–30 (Fig. 4a). Although continuous observation of the Daily Number of Eq. Daily Number of Eq. Daily Number of Eq. Daily Number of Eq. Vertical Displacement(m) Ueda et al. Earth, Planets and Space (2018) 70:38 Page 7 of 15 crustal deformation has not yet begun, Kumagai (1985) Motoyama GNSS observation stations (Fig. 4c). From reported that many faults moved in the southern area, 4:30 on April 29, continuous volcanic tremors due to the from Motoyama to Suribachiyama. eruption were observed. Since the eruption took place From around 20:00 on September 20 (Fig. 4b), about at nighttime, plume was not confirmed, but the explo - 14 h before the eruption on September 21, 2001, the sion sounded. No evidence of magmatic eruption has number of earthquakes increased. At around 10:15 ( Japan been found. To see this in more detail, the seismometer Standard Time) on September 21, 2001, white turbid dis- amplitude is shown in Fig. 5a. This is a 10-min average colored waters with a length of 300–400 m were spotted of the root mean square amplitude of the vertical com- off the south of Ioto (No. 10 in Fig. 1c); sometimes, the ponent seismometer at IJMV, and bandpass filters of sea water blows up by dozens of meters and white smoke 0.1–2 Hz are applied. Discolored water was confirmed off rises to 100–300 m. After the beginning of the eruption, the northeast of Ioto from April 29–30 (Japan Metrologi- continuous volcanic tremors were observed. Earthquake cal Agency 2013a). In a field survey conducted on May activity became quiet after vapor with a height of 100 m 24–25, we confirmed that the cliff on the coast of Tame - was spotted on the morning of September 22 (Japan Met- hachi collapsed and an upwelling of discolored water rological Agency 2001). GNSS data from 1 week before off the coast of Tamehachi (No. 11 in Fig. 1c) occurred. and 1 week after the eruption show that station 0604 Earthquake activity weakened after May 6. After the uplifted 10.6 ± 0.5 cm. eruption of April 29, Motoyama had subsided by about The number of earthquakes increased from April 40 cm as of July 2012. Vertical displacement is superior 27–28, 2012, and uplift of about 10 cm was observed at to horizontal displacement, as shown in the next section. VT earthquake -4 swarm a Low-frequency continuous tremor -5 -6 -7 24 25 26 27 28 29 30 01 02 03 Apr. 2012 -4 -5 Low-frequency continuous tremor -6 -7 14 15 16 17 18 19 20 Feb. 2013 -4 VT earthquake -5 10 Low-frequency swarm continuous tremor -6 -7 03 04 05 06 07 08 09 Aug. 2015 Fig. 5 The 10-min average of the root mean square amplitude of the seismometer at IJMV before and after the phreatic eruptions of a April 29, 2012; b February 17, 2013; and August 6, 2015. The seismometer data are vertical components and applied bandpass filters of 0.1–2 Hz Amplitude(m/s) Amplitude(m/s) Amplitude(m/s) Ueda et al. Earth, Planets and Space (2018) 70:38 Page 8 of 15 Earthquake activity temporarily increased around 10:30 any activation of seismic activity or crustal deformation on August 6, 2015, and continuous volcanic tremors were (Fig. 6b). A phreatic eruption that occurred in Febru- observed from around 16:00 (Fig. 4d). To see this in more ary 2013 at the Million Dollar hole ejected lapilli up to detail, the seismometer amplitude is shown in Fig. 5c. Con- 220 m, but we did not also observe any activation of seis- tinuous volcanic tremors were not observed on August 8. mic activity or crustal deformation (Fig. 6c). To see this According to Japan Maritime Self-Defense Force, small in greater detail, the seismometer amplitude is shown in eruptions intermittently occurred from around 2:43 on Fig. 5b. Several phreatic eruptions occurred at Million August 7 at Kitanohana (No. 3 in Fig. 1c). The height of Dollar hole from 2012 to 2013, but none of them were the plume was about 100 m. An uplift and a subsidence of accompanied by precursors. Seismic activity and crus- about 2 cm were observed before the eruption. tal deformation related to other eruptions after 1976 are On the other hand, there are eruptions without earth- shown in Additional file 1: Figure S1 as an electronic sup- quake activity and crustal deformation. In an eruption plement. In this paper, the precursor refers to the sharp that occurred on a beach at Idogahama (No. 8 in Fig. 1c) increase in seismic activity and crustal deformation that in March 1982, no increase in the earthquake count was are seen just before an eruption. In addition, one precur- observed despite ejecting lapilli up to 300 m (Fig. 6a). In sor corresponds to one eruption, and after the eruption an eruption that occurred at Idogahama in October 2001, its seismic activity and crustal deformation decrease. a crater with a depth of about 50 m and a diameter of Intermittent uplift has occurred almost every 10 years, several tens of meters was formed. According to Japan but the uplift does not correspond to each eruption and Maritime Self-Defense Force, a lapillus of about 5 cm continues even after an eruption, so we do not call it a in diameter reached about 250 m. We did not observe precursor here. 20 21 22 23 24 25 26 27 28 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 Feb.-Mar. 1982 200 0.5 0.4 0.3 0.2 0.1 0 0.0 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Oct. 2001 50 0.5 0.4 0.3 0.2 10 0.1 0 0.0 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 01 02 Feb. 2013 Fig. 6 The daily number of earthquakes and vertical movement at Motoyama before and after phreatic eruptions were not accompanied by precursors. a The eruption on March 9, 1982; b the eruption on October 19, 2001; c the eruption on February 17, 2013. The green and blue curves denote vertical movement at GNSS observation stations. The white triangles show the occurrence times of the eruptions Daily Number of Eq. Daily Number of Eq. Daily Number of Eq. Vertical Displacement(m) Ueda et al. Earth, Planets and Space (2018) 70:38 Page 9 of 15 Figures 2 and 3 show the occurrence times of four observed by GNSS observation. Figure 7 shows the hori- eruptions accompanied by precursors with red triangles, zontal and vertical displacement vectors before and after and the others with white triangles. All four eruptions the beginning of the eruption with red arrows. The dis - accompanied by precursors occurred at a time when placement before the eruption was calculated from the intermittent uplift occurred. Intermittent uplift and difference between April 27–28 and March 26–April 26 phreatic eruptions with precursors may have relevance, levels, and the displacement after the eruption was cal- which will be discussed later. culated from the difference between April 27–28 and July 1–31 levels. These displacements were referenced to Data analysis 0603 station on Hahajima Island (see Fig. 1a). The hori - Modeling of geodetic data zontal vectors after the eruption point to the center of To elucidate the mechanisms of the occurrence of phre- Motoyama. The vertical displacement was shown to be atic eruptions, we investigated the deformation source of greater than the horizontal displacement. In addition, crustal deformation observed before and after an erup- the displacement of Motoyama was greater than that of tion on the northern coast on April 29, 2012, which Suribachiyama. The deformation suggests that the defor - was the most obvious example of crustal deformation mation source is shallow and sill-like. Apr. 1-26 ~ Apr. 27-28, 2012 Apr. 27-28 ~ Jul. 1-31, 2012 NIED GSI OBS. 2 cm OBS. 10 cm CAL. 2 cm CAL. 10 cm OBS. CAL. 10 cm OBS. CAL. 10 cm Fig. 7 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 April 29, 2012. Red stars show eruption points. Displacement vectors are relative to the 0603 station of GSI (see Fig. 1a for location) Horizontal D a isplacement c Horizontal Displacement b Vertical Displacement d Vertical Displacement Ueda et al. Earth, Planets and Space (2018) 70:38 Page 10 of 15 Assuming that the deformation source is a sill (Okada 1992), we estimated the best-fit parameter of the loca - tion of the center of the sill, size, and opening/closing amounts by using a method based on a genetic algorithm (Ueda et al. 2005) for the crustal deformation after the eruption where larger movements are observed. Further- more, assuming that the depth and size of the source do not change before and after the beginning of the erup- tion, we estimated the location of the center of the sill and the opening/closing amounts for the crustal defor- mation before the eruption. Gray rectangles in Fig. 7 show the estimated sill source. Our results show that the observed crustal deforma- tion can be explained by best-fit parameters. Blue arrows in Fig. 7 show the calculated displacements by the best fit sill source. The best-fit parameters of sill source for after the eruption are depth 800 m, size 3.0 km × 3.9 km, 01 23 45 and closing amount 40 cm. The 95% confidence inter - Depth of sill-like source(km) vals of the parameters are 500–1300 m, 8.7–17 km , and Fig. 8 The relationship between the optimum value of the depth 36–49 cm, respectively. The 95% confidence intervals and the residual sum of squares for crustal deformation after the were estimated by adding observation noise to the obser- beginning of the eruption on April 29, 2012. The star shows the vation data and inverting repeatedly. There is a tradeoff optimum value of a depth of 800 m of the sill-like source. The curve between depth and closing amount. The estimated open - shows the residual sum of squares at each depth ing amount before eruption is 11 cm. The confidence interval is the range of the variation of the optimum value due to the observation noise. Motoyama is covered with Seismic array analysis porous pyroclastic flow deposits and is probably satu - The analysis of the crustal deformation data that was a rated with water as it is close to the sea. Therefore, the precursor of the eruption showed that there is a sill-like deformation may be affected by a poroelastic effect. In pressure source at a shallow depth. However, the depth addition, there is a possibility that it may be affected by resolution is limited. In order to investigate the defor- an inelastic effect, such as for active faults, as the dis - mation source, we investigate the one-dimensional seis- tance between the observation station and the deforma- mic velocity structure from the analysis of microtremors tion source is very close. Figure 8 shows the relationship using a seismic array. between the optimum value of the depth and the residual NIED conducted a seismic array observation with sum of squares. Although we assume elastic bodies and it seven short-period seismometers at 300 m northwest of is difficult to estimate the inelastic effect, we reckon that IJMV. The seismic array has been installed to monitor it is shallower than 2 km. On the other hand, the result of the incoming direction of volcanic tremors and to esti- assuming the Mogi model (Mogi 1958) as the deforma- mate the seismic velocity structure of shallow depths. tion source after the beginning of an eruption is shown in The layout of the array is shown in Additional file 4: Fig- Additional file 2: Figure S2 as an electronic supplement. ure S4 as an electronic supplement. The seismometers In the case of the Mogi model, the source is located at the were installed at the vertex of an equilateral triangle with center of Motoyama 2.2 km deep and cannot explain the a side length of 170 m, the midpoint of three sides, and outstanding vertical displacement compared to horizon- the center. The sensors are vertical component seismom - tal displacement. Analysis results of crustal deformation eters with the natural period of 1 Hz (Sercel L-4), set at before and after the eruption in August 2015 are shown a depth of 50 cm. We conducted continuous observation in Additional file 3: Figure S3 as an electronic supple- with a sampling frequency of 200 Hz, and AD conver- ment. The analysis method is the same as was used for sion is performed at 27 bits with the clock calibrated by the April 2012 eruption, but the size of the sill before and GPS. Although it was installed as planned from Decem- after the eruption is not assumed to be the same. For the ber 2014 for 1 year to monitor volcanic tremors, the nat- August 2015 eruption, crustal deformation with greater ural period changed in 1 week because of high ground vertical displacement than horizontal displacement was temperatures, and some seismometers failed within half observed and can be explained by a sill source of 860 m a month of installation. To estimate the underground in depth. structure, we used nighttime data and analyzed each of Residual sum of squares (cm ) Ueda et al. Earth, Planets and Space (2018) 70:38 Page 11 of 15 the 2 days to see the dispersion of the results in order to a high-velocity layer at 100–180 m, a low-velocity layer avoid artificial noise. at 180–480 m, and a high-velocity layer at deeper layers. Figure 9 shows the one-dimensional velocity structure of the S wave estimated using microtremor observation Discussion for 3 h from 0:00 to 3:00 on December 6 and 12, 2014, In this section, we discuss the mechanisms of phreatic and their average. A band-pass filter with a frequency eruptions on Ioto by using observed crustal deforma- of 0.1–20 Hz was applied, and the sampling frequency tion and seismic activity, the results of seismic array was resampled to 100 Hz. We divided the record of 3 h analysis, and other information. As a result of investigat- every 163.84 s and calculated the spatial autocorrelation ing the sources of crustal deformation before and after function. When we estimated the spatial autocorrelation the eruption on the northern coast on April 29, 2012, function, a Parzen window with a bandwidth of 0.05 Hz it was found that it could be explained by an expansion was applied to the cross spectrum. From the spatial auto- and contraction of the shallow horizontal planar body correlation function, the phase velocity was estimated beneath Motoyama. Because this planar body rapidly by employing the spatial autocorrelation method. The expanded before the phreatic eruption and contracted S wave velocity structure was estimated from the phase with the onset of the eruption, we consider here the pos- velocity. Inverse analysis of the S wave velocity structure sibility that the deformation is caused by inflation and used a genetic algorithm employed by Yamanaka and deflation of an aquifer filled with hot water. Our results Ishida (1995). Common to the results for 2 days, we can are consistent with Ukawa et al. (2006). They showed that see a low-velocity layer at a depth shallower than 100 m, the crustal deformation of Ioto from the survey results up to 2002 is due to a superposition of the episodic uplift of the whole island and continuous deformation of Motoy- ama. This continuous deformation is due to the contrac - tion of the central part of Motoyama and the uplift of Vs(km/s) the surrounding area. The contraction of Motoyama is 0.00.5 1.01.5 2.0 0.0 explained by a plate-like contraction source at a depth of 0.1–2.4 km in the central part of Motoyama. They inter - 0.1 preted it as a long-term movement of volcanic fluid. Our analytical results show a short-term fluctuation of the 0.2 volcanic fluid at the shallow depth. The results of the seismic array deployed on Motoyama 0.3 show that there is a low-velocity surface layer up to a depth of 100 m, a high-velocity layer of 1.2–1.3 km/s at a depth of 0.4 100–180 m, a low-velocity layer of 1.1 km/s at 180–480 m and a high-velocity layer of 1.7 km/s at a depth of 480 m 0.5 or more. In the borehole survey (depth 150 m) carried 0.6 out near the northeast 900 m of the seismic array, alter- nating layers consisting of lava flows and pyroclastic rocks 0.7 were found (Ossaka et al. 1985). The lava layers at depths of 24–97.9 and 106.6–130.5 m are presumed to be the 0.8 Motoyama lava and Hanareiwa lava, respectively, which can be seen on the coast (Nagai and Kobayashi 2015). 0.9 The area deeper than 135.2 m is also a lava layer (Ossaka et al. 1985). Both the location of the seismic array and the 1.0 borehole are covered with the same Motoyama tuff on the Fig. 9 The one-dimensional velocity structure of the S wave ground (Nagai and Kobayashi 2015), which suggests that estimated using microtremor observation at the seismic array for there is no large difference in the underground structure. 3 h from 0:00 to 3:00 on December 6 (dotted line) and 12 (broken Probably, the high-velocity layer found by the seismic array line), 2014, and their average (solid line). The results of the analysis show that there is a low-velocity surface layer up to a depth of at a depth of 100–180 m is a layer of lava. The low-velocity 100 m, a high-velocity layer of 1.2–1.3 km/s at a depth of 100–180 m, layer beneath the high-velocity layer is thought to consist a low-velocity layer of 1.1 km/s at a depth of 180–480 m, and a of thick tuff rocks due to an unknown previous caldera- high-velocity layer of 1.7 km at a depth of 480 m or more. The dark forming eruption. The high-velocity layer beneath the low- gray and light gray layers show lava and tuff layers obtained by the velocity layer may correspond to high-density intruding borehole survey of a depth of 150 m. (Reproduced with permission from Ossaka et al. 1985) Depth(km) Ueda et al. Earth, Planets and Space (2018) 70:38 Page 12 of 15 rocks suggested by a high gravity anomaly at the center of uplift, and seismic activity is the most probable mecha- Motoyama by Ehara (1985). nism of the abrupt boiling because large deformation The low-velocity layer found at a depth of about 200– and severe earthquake activity occur in the island during 500 m is presumed to be a comparatively high perme- the intermittent uplift and the phreatic eruptions with ability tuff. Although it is suggested that the depth of the intense precursors are observed only when intermittent contraction source from the crustal deformation is less uplift is occurring. The intermittent uplift is interpreted than 2 km, there is limited resolution that can be judged as a consequence of magma injections into a deep magma as coinciding with the low-velocity layer (depth 200– reservoir (Ukawa et al. 2006). The pressure increase in 500 m) suggested from the seismic array. In addition, the the deep magma reservoir may supply high temperature depth of the borehole is 150 m; there is no information fluid to shallow hydrothermal reservoirs. Therefore, a of the deeper strata. However, as the low seismic veloc- partial boiling point exceeded by the supply of high tem- ity layer suggests the presence of low density substances perature fluid from the deep magma reservoir is another such as tuff and water, it is likely that hydrothermal res - probable mechanism of the abrupt boiling. An expan- ervoirs will be present at this depth. Therefore, an aquifer sion of the horizontal planar hydrothermal reservoir filled with hot water, as suggested by the crustal deforma - will cause stress concentration at the tip of it, resulting tion data, is presumed to be present in this tuff layer. This in horizontal extension of the planar reservoir. This leads aquifer was covered with a layer of lava with low perme- to further pressure reduction and boiling. After that, if ability. A hydrothermal reservoir sealed in this manner is the tip of the planar reservoir reaches an active fault or a subject to lithostatic pressure and stably exists, even at crack, hot water is blown out from it and a phreatic erup- relatively high temperatures and high pressure. tion occurs. It is also consistent with phreatic eruptions This result suggests that phreatic eruptions with pre - occurring relatively often in places where there are many cursors occur with the following mechanism (Fig. 10). active faults in the surrounding area of Motoyama, and The hydrothermal reservoir rapidly expanded due to not in it covered with lava. With a phreatic eruption, the boiling before the eruption. The rapid expansion and an planar reservoir rapidly contracts. The hydrothermal res - increase in seismic activity before the eruption indicate ervoir already connected to the ground surface boils at a that the existing stable high temperature hydrothermal lower temperature than when sealed as only hydrostatic fluid confined in the reservoir became unstable before pressure is applied. Thus, further boiling is promoted. boiling and expanding (Morgan et al. 2009). Partial pres- The planar reservoir expanded about 10 cm before the sure decreases due to crustal deformation by intermittent eruption on April 28, 2012, but shrank 40 cm after the a b Uplift by magma Iwojima Caldera injections into a deep magma reservoir Pyroclastic deposit lava earthquakes crack Hydrothermal reservoirs high temperature fluid from deep magma Rapid uplift by inflation Rapid subsidence by deflation of hydrothermal of hydrothermal resovoir Active fault resovoir Eruption Fig. 10 A schematic illustration of an inferred mechanism of phreatic eruptions accompanied by precursors in Ioto. a Before the eruption, hydro- thermal reservoirs were sealed beneath the lava layer and subjected to lithostatic pressure, and existing in a stable manner even at a relatively high temperature and high pressure. b Intermittent uplift and seismic activity cause a partial pressure decrease and a partial boiling point exceeded by supply of high temperature fluid from the deeper section due to a pressure increase in a magma chamber. c The horizontal planar hydrothermal reservoir inflates and leads to further pressure reduction and boiling. After that, if the tip of the planar reservoir hits an active fault or a crack, hot water is ejected and a phreatic eruption occurs. d After the beginning of the phreatic eruption, the planar hydrothermal reservoir rapidly contracts Ueda et al. Earth, Planets and Space (2018) 70:38 Page 13 of 15 eruption (as of July 2012). If the pressure before and after narrow crack, the hot water does not circulate, and the the eruption of the reservoir is the same, the amount of water in the hydrothermal reservoir can suppress boiling inflation just before the eruption should be the same as by hydrostatic pressure. When the water begins to boil, the amount of deflation after the eruption. The larger hydrostatic pressure decreases due to extrusion of water amount of deflation after the eruption indicates that the near the ground surface suddenly boiling and extruding pressure after the eruption is greatly lower than before the hot water filling the conduit (Rojstaczer et al. 2003). the eruption. This indicates that the pressure inside the In both cases, it is presumed that there is no precursor reservoir was lithostatic pressure before the eruption, but phenomenon, such as crustal deformation or seismic it decreased to hydrostatic pressure after the eruption. In activity, because hot water filling the conduit near the the eruption of September 2001, subsidence of 4 ± 0.4 cm ground is pushed rather than the planar reservoir being was also observed until the middle of the following Octo- pushed by a rise in pressure. In such craters, eruptions ber (Fig. 6b). However, during this period, large-scale repeatedly occur as long as there is water and a heat uplift occurred whereby subsidence is underestimated. source, and eruption may occur at any time regardless of Phreatic eruptions that occur in Ioto mostly come an occurrence of an intermittent uplift. without severe earthquake activity and crustal deforma- A clear precursor before a phreatic eruption is a very tion. Such eruptions occurred at Idogahama and Mil- useful signal to avoid damage. However, since hydrother- lion dollar hole, where eruptions took place many times mal reservoirs are distributed under Motoyama, it is dif- in the past (Table 1). In the vicinity of the eruption of ficult to estimate in advance where an eruption will occur April 29, 2012, the occurrences of past eruptions were even if we could observe precursors. Since the central not recorded. In addition, the Asodai sinkhole, where an area of Motoyama is covered with lava, the possibility of eruption occurred on November 28, 1982, was formed phreatic eruptions seems to be small. On the other hand, around 1971; and only small eruptions occurred in since the fracture is developed in the southwestern part 1976 and 1978. There were no occurrences of eruptions of Ioto, where there are many active faults, phreatic erup- accompanied by precursors after the 1982 eruption at the tion tends to occur. In order to avoid damage from this Asodai sinkhole. Therefore, the hydrothermal reservoirs type of eruption, it is advisable not to approach active related to eruptions without a precursor are probably faults or coastal areas and retreat indoors if possible. On weakly sealed compared to those related to eruptions the other hand, phreatic eruptions without clear precur- with a precursor. In the latter case, the hydrothermal res- sors are difficult to predict. However, since it is thought ervoirs are firmly sealed by the lava layer. On the other that this type of eruption is already connected with a hand, in the former case, the hydrothermal reservoirs hydrothermal reservoir and crater, it is better not to are connected to shallow depths, such as a sinkhole on approach fumaroles or hot water pools. Also, it should the ground surface by previous eruptions. The plumb - be noted that even if it seems safe to approach, there is a ing connecting hydrothermal reservoirs and the shallow high possibility that water near the surface of the ground depths may be weakly sealed by clay generated through will spring up just before an eruption. In such cases, you hydrothermal alteration of lava and tuff. If they are sealed should evacuate immediately. with clay, the pressure of hydrothermal reservoirs does not become large, so it is presumed that no precursor Summary seismic activity or crustal deformation will occur. Fur- In Ioto, phreatic eruptions occur with intense seismic thermore, in geothermal areas such as the Asodai sink- activity and crustal deformation beforehand as well hole, which has already been made fumarolic, it is not as with no obvious precursors. After the beginning of sealed. If the ground surface and the hydrothermal reser- continuous seismic observation in March 1976, four voir are connected by thin plumbing, there is a possibility eruptions were shown to have precursors and 29 were that phreatic eruptions may occur with the same mecha- shown to have none. The former are observed only dur - nism as a geyser. A geyser is a hot spring characterized ing periods of repeated intermittent uplifts. In addition, by intermittent discharge of water. The common prereq - crustal deformation before and after the four erup- uisite for geysers to exist is the availability of water and tions accompanied by precursors indicates that a sill- a supply of heat (e.g., Hurwitz and Manga 2017). Ioto, shaped deformation source beneath the shallow part of an island with high geothermal activity, fits this descrip - Motoyama rapidly expands before an eruption and con- tion. In such cases, a hollow in which steam is trapped tracts with an eruption. An eruption accompanied by a is formed in a section of the hydrothermal reservoir. The precursor is thought to occur due to abrupt boiling of pressure rises and extruded hot water filling the conduit water in a hydrothermal reservoir in the tuff layer due causes an eruption (e.g., Belousov et al. 2013). In addi- to intermittent uplift. For the eruptions without precur- tion, when connected with the conduit through a very sors, the hydrothermal systems are weakly sealed by clay Ueda et al. Earth, Planets and Space (2018) 70:38 Page 14 of 15 References or probably occur on the same principle as a geyser, as Bellucci F, Woo J, Kilburn C, Rolandi G (2006) Ground deformation at Campi phreatic eruptions occurred in the past and a crater on Flegrei, Italy: implications for hazard assessment. In: Troise C, De Natale the ground surface and a hydrothermal reservoir were G, Kilburn C (eds) Mechanisms of activity and unrest at large calderas, Special Publications, vol 269. Geol. 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A comparison of observed crustal deforma- Japanese) tion (red arrows) with that calculated (blue arrows) by the best-fit Mogi Japan Meteorological Agency (2012) Monthly Volcanic Activity Report model (red circle) of (a, b) after the beginning of the eruption of April 29, (December 2012) http://www.data.jma.go.jp/svd/vois/data/tokyo/eng/ 2012. Red stars show eruption points. Displacement vectors are relative to volcano_activity/2012/2012_12_monthly.pdf. Accessed 23 Aug 2017 (in the 0603 station of GSI (see Fig. 1a for location). Japanese) Additional file 3: Figure S3. A comparison of observed crustal deforma- Japan Metrological Agency (2013a) National Catalogue of the Active Volca- tion (red arrows) with that calculated (blue arrows) by the best-fit sill-like noes in Japan (fourth edition, English version) deformation source (gray rectangles) of a, b before and c, d after the Japan Meteorological Agency (2013b) Monthly Volcanic Activity Report beginning of the eruption of August 5, 2015. Red stars show eruption (February 2013) http://www.data.jma.go.jp/svd/vois/data/tokyo/eng/ points. Displacement vectors are relative to the 0603 station of GSI (see volcano_activity/2013/2013_02_monthly.pdf. Accessed 23 Aug 2017 (in Fig. 1a for location). Japanese) Additional file 4: Figure S4. The layout of the seismic array. Red trian- Japan Meteorological Agency (2013c) Monthly Volcanic Activity Report gles indicate where the seismometers are installed. 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Accessed 23 Aug 2017 (in Japanese) Japan Meteorological Agency (2014) Monthly Volcanic Activity Report Acknowledgements (December 2014) http://www.data.jma.go.jp/svd/vois/data/tokyo/eng/ We would like to thank the JSDF, which supported the research at Ioto for an volcano_activity/2014/2014_12_monthly.pdf. Accessed 23 Aug 2017 (in extended period. Furthermore, we appreciated review comments from Dr. Japanese) Mike Poland and Dr. Halldór Geirsson, which greatly improved this manuscript. Japan Meteorological Agency (2015a) Monthly Volcanic Activity Report We also thank Geospatial Information Authority of Japan for providing us with (May 2015) http://www.data.jma.go.jp/svd/vois/data/tokyo/eng/vol- GNSS data from GEONET and DEM data for the Ioto volcano. cano_activity/2015/2015_05_monthly.pdf. Accessed 23 Aug 2017 (in Japanese) Competing interests Japan Meteorological Agency (2015b) Monthly Volcanic Activity Report The authors declare that they have no competing interests. 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