Volcanic unrest of Ioto is characterized by intense seismic activity and rapid uplift. In order to investigate the relationship between the occurrence of phreatic eruption, seismic activity and uplift, we first looked at observations of the earthquakes and crustal deformation of Ioto. Figure 1c shows the distribution of continuous observation stations used in this study. National Research Institute for Earth Science and Disaster Resilience (NIED) has conducted continuous observation of seismometers and GNSS at three observation points. Earthquakes have been observed since 1982, and continuous observation by GNSS started in 2003. Japan Meteorological Agency (JMA) has been observing earthquakes and infrasound at one point since 2011. Geospatial Information Authority of Japan (GSI) has been conducting continuous observation of GNSS at two points since 1997. Japan Ministry of Defense had been conducting continuous observation of a seismometer from March 1976 around 0604 station of GSI, but it was discontinued in the 1990s. NIED also conducts periodic surveys every 2 years, as will be described later in this paper.
Figure 2a shows the monthly average number of daily earthquakes occurring on the island observed at Motoyama. From March 1976 to June 1985, it was the observation point of Japan Ministry of Defense, and after that, the number of earthquakes counted came from records at IJMV. This number of earthquakes was chosen by counting what is supposed to have occurred inside the island. Since 2003, we have determined the hypocenter and that the depth of 95% of the earthquakes that occurred inside the island is less than 3 km. Although it was not counted from June 1985 until March 1991, it was reported that earthquake activity was low (NIED 1992).
We will compare this seismic activity with the crustal deformation after 1976 (Ukawa et al. 2006). NIED conducted leveling and trilateration 11 times every other year from 1976 to 1995. Furthermore, GPS surveys were conducted 10 times every 2 years from 1996 to 2016. Results until 2002 are summarized in Ukawa et al. (2006). Figure 2b shows vertical movement obtained by leveling and GPS from 1977 at the survey point within 10 m of the observation point where GNSS continuous observation was carried out. In the leveling survey until 1995, the average sea level measured by the tide level gauge was taken as the reference height (the location of tide level gauge is shown by a star in Fig. 1c). Temporary tide level observation from 1977 to 1980 and a tidal level gauge of continuous observation from 1980 to 1995 were employed to monitor the tide level, and the results were used as the reference height for the levelling surveys. Since we do not have tide level data for 1976, the survey result is not included in Fig. 2b.
After 1977, the three observation points of Motoyama showed definite periods of uplift approximately every 10 years (1982–1984, 1991–1993, 2000–2002, 2006–2016), and subsided in the intervening periods. This intermittent uplift is a common crustal deformation of caldera volcanoes, as exemplified at Campi Flegrei (Bellucci et al. 2006), Yellowstone (Chang et al. 2007), and Rabaul (Robertson and Kilburn 2016) calderas. The intermittent uplift of Ioto is interpreted as a consequence of magma injections into a deep magma reservoir (Ukawa et al. 2006). A comparison of the number of earthquakes and vertical movement in Fig. 2a shows that the number of earthquakes is also relatively large during intermittent uplifts. Figure 3 shows a more detailed view of the relationship between earthquakes and uplift by GNSS, which allows for a higher time resolution. We compared the vertical displacement after the start of GNSS continuous observation from 1997 with the number of daily earthquakes. Using GNSS data of NIED and GSI observation stations sampled every 30 s, the coordinates of each day were estimated by static analysis using GAMIT/GLOBK software. Motoyama uplifted during 2001–2003 and after 2006. The uplift was accelerated in 2011–2013. When the uplift rate is high, the number of earthquakes tends to be large; and earthquake activity is almost synchronized with the intermittent uplifts.
After the beginning of continuous observation of earthquakes in Ioto in March 1976, the eruptions of November 1982, September 2001, April 2012, and August 2015 were shown to be accompanied by seismic activity and crustal deformation (Fig. 4). From November 28–29, 1982, two phreatic eruptions took place at Asodai sinkhole. The times of these occurrences were not clear. During this period, the largest earthquake swarm occurred since the beginning of continuous observation in 1976, and 1492 earthquakes were observed from November 25–30 (Fig. 4a). Although continuous observation of the crustal deformation has not yet begun, Kumagai (1985) reported that many faults moved in the southern area, from Motoyama to Suribachiyama.
From around 20:00 on September 20 (Fig. 4b), about 14 h before the eruption on September 21, 2001, the number of earthquakes increased. At around 10:15 (Japan Standard Time) on September 21, 2001, white turbid discolored waters with a length of 300–400 m were spotted off the south of Ioto (No. 10 in Fig. 1c); sometimes, the sea water blows up by dozens of meters and white smoke rises to 100–300 m. After the beginning of the eruption, continuous volcanic tremors were observed. Earthquake activity became quiet after vapor with a height of 100 m was spotted on the morning of September 22 (Japan Metrological Agency 2001). GNSS data from 1 week before and 1 week after the eruption show that station 0604 uplifted 10.6 ± 0.5 cm.
The number of earthquakes increased from April 27–28, 2012, and uplift of about 10 cm was observed at Motoyama GNSS observation stations (Fig. 4c). From 4:30 on April 29, continuous volcanic tremors due to the eruption were observed. Since the eruption took place at nighttime, plume was not confirmed, but the explosion sounded. No evidence of magmatic eruption has been found. To see this in more detail, the seismometer amplitude is shown in Fig. 5a. This is a 10-min average of the root mean square amplitude of the vertical component seismometer at IJMV, and bandpass filters of 0.1–2 Hz are applied. Discolored water was confirmed off the northeast of Ioto from April 29–30 (Japan Metrological Agency 2013a). In a field survey conducted on May 24–25, we confirmed that the cliff on the coast of Tamehachi collapsed and an upwelling of discolored water off the coast of Tamehachi (No. 11 in Fig. 1c) occurred. Earthquake activity weakened after May 6. After the eruption of April 29, Motoyama had subsided by about 40 cm as of July 2012. Vertical displacement is superior to horizontal displacement, as shown in the next section.
Earthquake activity temporarily increased around 10:30 on August 6, 2015, and continuous volcanic tremors were observed from around 16:00 (Fig. 4d). To see this in more detail, the seismometer amplitude is shown in Fig. 5c. Continuous volcanic tremors were not observed on August 8. According to Japan Maritime Self-Defense Force, small eruptions intermittently occurred from around 2:43 on August 7 at Kitanohana (No. 3 in Fig. 1c). The height of the plume was about 100 m. An uplift and a subsidence of about 2 cm were observed before the eruption.
On the other hand, there are eruptions without earthquake activity and crustal deformation. In an eruption that occurred on a beach at Idogahama (No. 8 in Fig. 1c) in March 1982, no increase in the earthquake count was observed despite ejecting lapilli up to 300 m (Fig. 6a). In an eruption that occurred at Idogahama in October 2001, a crater with a depth of about 50 m and a diameter of several tens of meters was formed. According to Japan Maritime Self-Defense Force, a lapillus of about 5 cm in diameter reached about 250 m. We did not observe any activation of seismic activity or crustal deformation (Fig. 6b). A phreatic eruption that occurred in February 2013 at the Million Dollar hole ejected lapilli up to 220 m, but we did not also observe any activation of seismic activity or crustal deformation (Fig. 6c). To see this in greater detail, the seismometer amplitude is shown in Fig. 5b. Several phreatic eruptions occurred at Million Dollar hole from 2012 to 2013, but none of them were accompanied by precursors. Seismic activity and crustal deformation related to other eruptions after 1976 are shown in Additional file 1: Figure S1 as an electronic supplement. In this paper, the precursor refers to the sharp increase in seismic activity and crustal deformation that are seen just before an eruption. In addition, one precursor corresponds to one eruption, and after the eruption its seismic activity and crustal deformation decrease. Intermittent uplift has occurred almost every 10 years, but the uplift does not correspond to each eruption and continues even after an eruption, so we do not call it a precursor here.
Figures 2 and 3 show the occurrence times of four eruptions accompanied by precursors with red triangles, and the others with white triangles. All four eruptions accompanied by precursors occurred at a time when intermittent uplift occurred. Intermittent uplift and phreatic eruptions with precursors may have relevance, which will be discussed later.