Volume change of the magma reservoir relating to the 2011 Kirishima Shinmoe-dake eruption—Charging, discharging and recharging process inferred from GPS measurements
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB 2013
Received: 20 November 2012
Accepted: 29 May 2013
Published: 8 July 2013
Using GPS data, we evaluate the volume change of the magma reservoir associated with the eruption of Kirishima Shinmoe-dake volcano, southern Kyushu, Japan, in 2011. Because ground deformation around Shinmoe-dake volcano is strongly affected not only by regional tectonic movement but also by inflation of Sakurajima volcano located approximately 30–40 km to the southwest, we first eliminate these unwanted contributions from the observed data to extract the signals from Shinmoe-dake volcano. Then, we estimate the source locations and volume change before, during, and after the highest eruptive activity occurring between January 26 and 31. Our model shows that the magma began to accumulate about one year prior to the sub-Plinian eruption, with approximately 65% of the accumulated magma being discharged during the peak of the eruptive activity, and that magma accumulation continued until the end of November 2011. An error analysis shows that the sources during the three periods indicated above are located in almost the same position: 5 km to the northwest of the summit at a depth of 8 km. The 95% confidence interval of the estimated source depth is from 7.5 to 13.7 km.
Shinmoe-dake volcano is a part of the Kirishima volcano group located in southern Kyushu, Japan, and is one of the most active volcanoes in the group. Prior to the 2011 eruption, phreatic explosions occurred in August 2008, and March, April, May, June, and July, 2010. A series of mag-matic eruptions started on January 19, 2011, with a phreato-magmatic eruption with a trace of juvenile magma (Suzuki et al., 2013b). This was the first major eruption since the 1716–1717 eruption that evolved from a phreatic to a mag-matic eruption (Imura and Kobayashi, 1991). A sub-Plinian eruption started in the afternoon of January 26, 2011 (local time), and two other sub-Plinian eruptions occurred, one at midnight and one in the afternoon of January 27. After the eruptions, lava started to be extruded through the conduit, and accumulated inside the summit crater between January 29 and January 31. Tiltmeters installed around the volcano showed that significant ground deflation started on January 26 and ended on 31 (Japan Meteorological Agency: JMA, 2011). The activity was most explosive during these six days, and we define this activity as the “climax event” in this paper. Vulcanian eruptions followed the climax event and lasted until September 2011. A detailed chronology of the eruptions is given by Nakada et al. (2013).
The volcano edifice started to inflate about a year prior to the climax event (GSI, 2011). At the same time, seis-micity around the Kirishima volcano group increased and was maintained at an elevated level (JMA, 2011). To monitor the magma accumulation process, we installed GPS sites before the climax event, and added some sites after the event. In this paper, we infer the volume change of the magma reservoir associated with the 2011 Kirishima Shinmoe-dake eruption from GPS data. We processed all available dual-frequency GPS data around Shinmoe-dake volcano and detected the inflation and deflation processes before, during and after the climax event. Details of the data processing are shown in Section 2. To extract precisely the volcanic deformation which originated from Shinmoe-dake, we needed to pay close attention to the ground deformation of tectonic origin and that which originated from a nearby volcano, details of which will be described in Section 3. In Section 4, we estimate the source location and the volume change as well as their uncertainties before, during, and after the climax event using the GPS data corrected using the method described in Section 3. Then we compare the model estimated in this study with geological insights and petrological analyses of melt inclusion of the lava and ash emitted during the climax event.
2. Observations and Data Analysis
GPS sites used in this study.
Location (latitude, longitude, altitude)
after February 3, 2011
after April 9, 2010
after April 10, 2010
after June 21. 2007
after October 6, 2010
after March 2, 2007
after April 20, 2007
after August 15, 2010
after January 31, 2011
after February 8, 2011
after January 29, 2011
after January 31, 2011
We combined all available dual-frequency GPS data surrounding Shinmoe-dake volcano, and estimated the daily positions of each station using Bernese GPS Software Ver. 5.0 with the Bernese Processing Engine (Dach et al., 2007). We used the International GNSS Service for Geodynamics (IGS) precise ephemerides and the International Earth Rotation and Reference Systems Service (IERS) Earth rotation parameters (Altamimi et al., 2011). The coordinates of the GPS sites were estimated with respect to ITRF2008 (Altamimi et al., 2011). We also estimated tropospheric delays every hour and their horizontal gradients every six hours to improve the accuracies of the station coordinates. The correction of tropospheric delay is important because rainfalls are abundant and humidity is high in the area from June to the first half of August. We used the mapping function by Boehm et al. (2006), which is based on a numerical weather model.
3. Removing the Effects of Tectonic Ground Deformation and the Influence of Sakurajima Volcano around Shinmoe-dake Volcano from GPS Measurements
The mechanisms of the regional crustal deformation in southern Kyushu are controversial. Nishimura et al. (2004) proposed that block rotation of the Ryukyu arc is the main factor. Takayama and Yoshida (2007) suggested that temporal variations in the inter-plate coupling affect the deformation rate. Wallace et al. (2009) tried to explain the deformation field assuming an east-west striking shear zone in southern Kyushu. All of these studies mentioned two common features: (1) Ground deformation in southern Kyushu is strongly affected by the subduction of the Philippine Sea Plate. However, the deformation rate is almost uniform in the southernmost Kyushu regions, but is non-uniform to the north of 32.0N, possibly reflecting the non-uniform interplate coupling. (2) Dilatational deformation is dominant around Sakurajima volcano, a very active volcano. Because the scope of this paper is to estimate ground inflation and deflation caused by magma emplacement associated with the 2011 Shinmoe-dake eruption, we simply remove these components and do not discuss deformation of a tectonic origin.
Assumed volume sources associated with the deformation of the Sakurajima volcano.
Location (latitude, longitude, depth)
(beneath Minami-dake, Sakurajima)
Isotropic volume source
(beneath the Aira caldera)
Isotropic volume source
4. Volume Change of the Magma Reservoir beneath Shinmoe-dake Volcano
From the temporal changes of the corrected baseline shown in Fig. 2(b), we can infer the entire process of accumulation and discharge of magma related to the 2011 Kirishima Shinmoe-dake eruption. Here, we use the GPS data corrected using the model mentioned in the previous section. The volcano inflated slowly before the climax event, it deflated rapidly during the climax event, and then inflated again after the event. In this study, we divide the period of activity into three parts: (1) inflation period prior to the climax event, from the beginning of inflation (the end of December 2009) to just before the sub-Plinian eruption (January 25, 2011); (2) the deflation period during the climax event, from the beginning of the sub-Plinian eruption (January 26, 2011) to the time when magma extrusion to the summit crater terminated (January 31, 2011); and (3) the secondary inflation period after the climax event, from February 1 to the end of November when the inflation stopped. We estimated the source location and volume change during the three periods, independently.
We model the deformation field assuming a single isotropic source, or the Mogi source (Mogi, 1958), embedded in an elastic, isotropic, and homogenous half space. These assumptions might be too simple and unrealistic. For example, the subsurface structure is layered, and the magma reservoir is not a point. Nevertheless, considering the limited number of stations, we did not introduce a complex source model and realistic underground structure.
Optimum location with 95% confidence intervals of the pressure source for the period before, during and after the climax event.
Volume (106 m3)
(Inflation process before the climax event)
2010 Aug. 20-
2011 Jan. 25
(Deflation process during the climax event)
2011 Jan. 15-
2011 Feb. 11
(Inflation process after the climax event)
2011 Feb. 25-
2011 Nov. 25
Estimated total volume and accumulation or discharging rate of magma for the period before, during and after the climax event and their 95% confidence intervals.
Time interval (days)
Total volume (106 m3)
Accumulation rate in average (m3/s)
(Inflation process before the climax event)
From Dec. 25, 2009
Till Jan. 25, 2011
(Deflation process during the climax event)
From Jan. 26, 2011
Till Jan. 31, 2011
(Inflation process after the climax event)
From Feb. 2, 2011
Till Nov. 25, 2011
Several phreatic explosions were observed prior to the sub-Plinian eruption in January 2011, the largest of which occurred on August 22, 2008 (Geshi et al., 2010). Figure 2(b) indicates that the magma was not stored in the reservoir estimated here prior to the 2008 eruption. On the other hand, Takagi et al. (2011) found an inflation source localized near the summit of Shinmoe-dake between 2006 and 2008 from the data of three single-frequency continuous GPS stations (installed by JMA) within 2 km from the summit of Shinmoe-dake volcano and four campaign GPS sites on the rim of the summit crater. The source location was estimated as being several hundred meters beneath the summit, indicating that the pressure source at a shallow depth may also have been inflated during the 2011 eruption. However, the GPS network used in this study is sparse and cannot detect the inflation of magma sources in shallow depths close to the summit crater. Our results do not rule out the presence of shallow and small magma sources during the 2011 eruption. Joint analysis of regional GPS sites used in this study and those installed close to the summit crater is required to reveal the more detailed process of the eruption to address the behavior of the shallow source.
In this study, we assumed that the source is an isotropic point source embedded in a homogenous and elastic halfspace. The real magma reservoir is not an infinitesimal sphere, and the crust has heterogeneous and layered structures. Many previous studies have tried to model ground deformation by using a complex source geometry (e.g., Hashimoto and Tada, 1988; Segall et al., 2001; Ueda et al., 2005), or heterogeneous crustal structures (e.g., De Natale and Pingue, 1993; Fernandez and Rundle, 1994; Trasatti et al., 2003). We acknowledge that the models used in this study are too simple to represent real phenomena, the limited station distribution does not allow us to introduce a complex source model or heterogeneous crustal structure. Here, we just show how our results will change by assuming a heterogeneous crustal structure. Hautmann et al. (2010) estimated the effect of a realistic underground structure using a finite-element method. Their model involves a single discontinuity and a Young’s modulus increasing with depth, which is more realistic than the homogeneous half-space assumed in this study. Hautmann et al. (2010) showed that the source depth tends to be underestimated by incorrectly assuming a uniform half-space. This reflects an enhancement of the horizontal component of surface displacement relative to the vertical component caused by the presence of a low-rigidity layer near the ground surface. However, this effect becomes minor if the source depth is greater than around 10 km. Therefore, our study may have underestimated the source depth to some extent, i.e., the source may be deeper than 8 km and volume changes may be larger. However, such differences are not serious, because the estimated source depth is deep enough.
Nakada et al. (2013) estimated the total amount of magma erupted during the climax eruption from the ejected tephra and the volume of lava accumulated in the summit crater. They concluded that the total amount was 21–27 × 106 m3 DRE (dense rock equivalent). Our analysis shows that the reservoir deflated 13.4 × 106 m3 (with the 95% confidence interval between 10.4 and 17.7 × 106 m3), and this is smaller than the magma volume extruded to the surface during the climax event. The ratio of the extruded magma volume to the deflation volume (Rv) is 1.8 ± 0.5. The value is affected not only by the magma compressibility (Rivelta and Segall, 2008) but also by the shape of the magma reservoir (Amoruso and Crescentini, 2009). Rivalta and Segall (2008) pointed out that the ratio should be greater than one if the magma within the reservoir is compressible. They presented observed facts that R v = 3.8 ± 0.8 for the 1997 intrusion/eruption at Kilauea volcano, and R v = 5 ± 1 for the 2005 dike intrusion at Afar. The value estimated here is smaller than the values mentioned above, and it may reflect the fact that the magma within the reservoir beneath Shinmoe-dake is less compressible than the above cases.
Next, we again discuss the magma source location prior to the climax event. The source depth of 9.2 km prior to the climax event is slightly deeper that those during (8.3 km) and after (8.2 km) this event. Considering the confidence intervals, it cannot be proved that the source depth became shallower after the climax event. Nevertheless, we point out the possibility that the source actually became shallower after the climax event. From petrological analyses of the melt inclusions in lava and ash ejected during the climax eruption, Suzuki et al. (2013a) asserted that the eruption involved two separate magma reservoirs: a silicic andesitic magma reservoir as deep as 5 km or so (125 MPa), and a basalt andesitic reservoir as deep as 10 km or so (250 MPa). They demonstrated that a small amount of basalt andesite magma at a high temperature migrated upward and partly melted the silicic andesite magma reservoir prior to the sub-Plinian eruption. This process led to a reduction in viscosity of the silicic andesite magma and gave rise to a sub-Plinian eruption. According to their model, magma accumulated mainly in the deeper part of the reservoir prior to the climax eruption. If the magma recharged in the region where both silicic magma and basalt andesite magma remained after the climax event, the source would have become shallower than the depth prior to the climax event.
To understand the relation between magma accumulation and volcanic eruption in the 2011 Kirishima Shinmoe-dake eruption, we analyzed GPS data and evaluated ground inflation and deflation quantitatively. Because the ground deformation near Shinmoe-dake volcano was strongly affected by regional tectonic deformation as well as dilatational deformation generated by Sakurajima volcano, we subtracted these effects from the GPS measurements and reduced the effects from observed GPS data to extract the signals caused by magma emplacement at Shinmoe-dake volcano.
From long-term deformation data, we saw that magma began to accumulate during a one-year period, and was then discharged rapidly over 6 days during the climax event, and was recharged during the post-eruption period of around 10 months. The edifice began to inflate at the end of November 2009 and continued until just before the sub-Plinian eruption. The charging rate was 0.6 m3/s on average and the total amount of magma reached 21 × 106 m3. The estimated location of the reservoir was about 5 km to the northwest of the summit crater of Shinmoe-dake volcano at a depth of 8–9 km. During the climax event, the magma reservoir shrank by 13×106 m3 in volume, which is smaller than the amount of erupted magma volume estimated from geological surveys (24 × 106 m3 DRE). This mismatch would reflect the compressible property of magma. The magma was stored again at the same place just after the climax event and the recharging process continued until the end of November. The total amount of the recharged magma was 11×106 m3, with an average recharging rate of 0.4 m3/s. Using the bootstrap method, the source location for the three periods is presented with confidence intervals.
The authors express great thanks to the anonymous reviewers and the editor whose suggestions were very helpful in improving this manuscript. They pointed out the incompleteness of our analysis. We are also greatly indebted to Prof. Kosuke Heki for improving the manuscript. We are grateful to Dr. Kazuhiko Goto, Mr. Shuichiro Hirano, Mr. Yasuhiro Hirata, and Mr. Masayoshi Ichiyanagi for helping to construct GPS stations after the sub-Plinian eruption. We are also grateful to Kirishima Primary School, Nation Livestock Breeding Center Miyzaki Station, Kita-Kirishima Cosmo Dome, Takachiho Primary School, Manzen Primary School, Kurino Primary School, and Kirishima Open-air Museum for providing us with the GPS observation sites. We express great thanks to the Geospatial Information Authority of Japan and the National Research Institute for Earth Science and Disaster Prevention for providing GPS data. We also wish to express our gratitude to Prof. S. Nakada for letting us join the research group of the 2011 Kirishima Sinmoe-dake eruption. This research was supported by MEXT/JSPS KAKENHI grant 22900001.
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