Subtle changes in strain prior to sub-Plinian eruptions recorded by vault-housed extensometers during the 2011 activity at Shinmoe-dake, Kirishima volcano, Japan
© 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: 11 March 2013
Accepted: 7 September 2013
Published: 6 December 2013
This study focuses on strain change observations with a precision of 10−9 associated with the 2011 Shinmoe-dake eruptions in Japan, using vault-housed extensometers installed approximately 18 km northwest of the Shinmoe-dake crater. The extensometers recorded major strain changes of 10−7 during three sub-Plinian eruptions and subsequent magma effusion. Our analysis indicates that these extensometer records provide a reasonable estimation of the parameters of an isotropic point source that can explain eruption-related ground deformation. The extensometers also recorded subtle strain changes of 10−9 prior to the three sub-Plinian eruptions. Time series data indicate that changes in strain at these rates are generally only observed immediately before explosive eruptions, suggesting that these strain changes are precursors to sub-Plinian eruptions. The source of these subtle strain changes is likely to be shallower than the magma chamber associated with these eruptions. The precursory strain changes might have been caused by a pressure increase and a subsequent pressure decrease under the volcano. One possible scenario that can explain these pressure changes is the accumulation of volcanic gases at depth, causing an increase in pressure that was eventually released during gas emissions from the crater prior to the explosive eruptions.
Some important views on the dynamics of volcanic eruptions owe their origins to precise observations of eruption-related crustal deformations of the order of 10−9 in strain and tilt. Previous research has demonstrated that subtle ground deformation occurs prior to the eruptions of some volcanoes, as has been observed using vault-housed extensometers at Sakurajima in Japan (e.g., Kamo and Ishihara, 1986; Ishihara, 1990; Iguchi et al., 2008) and Piton de la Fournaise in the Réunion Islands (Peltier et al., 2007), and by high-precision tiltmeters and dilatometers at the Soufrière Hills volcano on Montserrat (Voight et al., 1998, 1999, 2010; Druitt et al., 2002; Chardot et al., 2010; Linde et al., 2010). Measuring and studying such deformations are clearly important if we are to understand the eruption processes of volcanoes.
However, previously reported examples of eruption-related crustal deformation have not documented all possible types of eruptive activity, and continuous crustal deformation observations, especially high-precision strain observations, have only been undertaken on a few active volcanoes (e.g., Dvorak and Dzurisin, 1997; Peltier et al., 2007). The majority of routine observations have been conducted on volcanoes that frequently erupt (e.g., Linde et al., 1993; Peltier et al., 2007; Iguchi et al., 2008). As a consequence, less frequent events, such as Plinian or sub-Plinian eruptions, have rarely been investigated using continuous crustal deformation data with a precision of 10−9. This situation contrasts sharply with Vulcanian eruptions that have been intensively investigated (e.g., Chardot et al., 2010; Linde et al., 2010; Voight et al., 2010).
This paper presents extensional ground strain data with a precision of 10−9, or better, obtained during the most recent eruptions of Shinmoe-dake. In particular, we demonstrate precursory ground deformations of a type that has not previously been recognized.
2. Vault-housed Strain Observations near the Shinmoe-dake Cone and Their Resolution
The precision of the strain measurement data at ISA is 1 × 10−9 in the E1 and E3 directions. The precision of the E2 component, obtained during the 2011 eruption of Shinmoe-dake, is somewhat poorer than the precision of the E1 and E3 components, at around 2–3 × 10−9, primarily because the electrical system in this part of the vault was damaged during a lightning strike one month before the eruption.
The large distance between the Shinmoe-dake crater and ISA observatory means that the strain data obtained at ISA may be less sensitive to volumetric changes of subsurface pressurized magma than at facilities elsewhere that are closer to the volcanoes being observed. Vault-housed extensometers for monitoring specific volcanoes are usually installed at distances less than several kilometers, as exemplified by the vault-housed extensometers at the Harutayama observatory near the Minamidake crater of Sakurajima volcano. These extensometers have similar geometries (i.e., three components arranged in a triangular shape) and lengths (~30 m) to the extensometers at ISA, but the distance between the crater and the observatory is <3 km (e.g., Kamo and Ishihara, 1986).
Nevertheless, we can confirm that the somewhat distal extensometers at ISA are more sensitive to some types of subsurface volume changes than are GPS-based measurements. Assuming that the resolution of displacement using GPS is up to 1 mm, the detection limit for subsurface volume changes using extensometers located 20 km away from the source epicenter is much better than when using densely deployed GPS, except in the case of an extremely shallow source (~1 km; see Appendix for detail).
3. Strain Changes Associated with Major Eruptive Events
To clarify the nature of the strain changes generated during the eruptions, we estimated eight major strain time-series tidal components and removed them from the original time series. This was done using the algorithm proposed by Tamura et al. (1991) based on Akaike Bayesian Information Criteria (ABIC; Akaike, 1980). Although the underground recording of strain data by extensometers is distorted significantly by precipitation (e.g., Kasahara et al., 1983), the data acquired during the 2011 Shinmoe-dake eruptions were only slightly distorted because the eruptions occurred during a period of low precipitation.
Figure 3 shows that three rapid strain changes occurred at 15:30–18:50 (local time) on 26 January, and 01:30–05:00 and 16:30–17:50 on 27 January. These three intervals correspond to the duration of the three sub-Plinian eruptions that were recorded by other means, including seismograms, infrasonic monitoring, and eyewitness observations (e.g., Earthquake Research Institute, University of Tokyo, 2011). The duration of these three strain variations, all of which were longer than 1 hour, justifies the classification of this event as a sub-Plinian eruption. A significant and gradual variation in strain was also recorded between 28 and 31 January, associated with magma effusion within the summit crater. It is reasonable to consider that these strain variations were generated by decreases in volume of pressurized magma that had been stored underground before its effusion to the surface.
The location of the magma chamber associated with the eruptions between 26 January and 1 February and the associated volume changes have already been estimated from the analysis of GPS data. By assuming the deformation was generated by an isotropic point source, the Geospatial Information Authority of Japan (GSI, 2011) estimated that the source was located at 31.945°N, 130.827°E, with a depth of 6.2 km, and that the total change in volume was 1.0 × 107 m3. The estimated horizontal location of this point source is 11.6 km from the ISA observatory towards S55.75°E, and 6.9 km away from the Shinmoe-dake crater towards N54.58°W (Fig. 1).
We have attempted to estimate the parameters of the source of these major events using extensometer-derived data, including the determination of separate source parameters for each event (i.e., the three sub-Plinian eruptions and the magma effusion event). The extensometer records on their own do not allow us to determine unique deformation source locations or volume changes in stored magma. However, we can estimate source depths and volume changes by assuming an isotropic point source and a given horizontal distance from the ISA observatory to the ground deformation epicenter.
Observed changes in strain (unit: 10−9) during sub-Plinian eruptions (SP1–3) and estimated isotropic point source parameters, together with estimation errors due to possible uncertainties in the observed values*1.
Observed strain change
Estimated source parameters and errors
Volume change (× 106 m3)
SP1 (15:30–18:50, Jan 26)
SP2 (01:50–05:00, Jan 27)
SP3 (16:30–17:50, Jan 27)
Magma effusion (Jan 28–30)
The estimated source parameters are listed in Table 1, together with possible errors. The source depth for the magma effusion stage is estimated to have been slightly deeper than the source for the sub-Plinian eruptions, although these estimates are within error. These differences are likely to relate to a contribution from the volcanic conduit, where explosive (sub-Plinian) eruptions could have involved a shallow part of the magma chamber plus the conduit, yielding a shallower source depth. Note that the total volume change during the magma effusion event is larger than the sum of the volume changes during the three sub-Plinian eruptions.
In general, estimating the location and size of deformation sources using data obtained from a single observatory may introduce significant errors, as the data obtained at this observatory may be distorted, for example by variations in local elastic properties around the site. An approach using model idealization that ignores possible contributions from a finite-length conduit may also introduce errors. As documented above, estimates based on strain data obtained from the ISA observatory differ from estimates based on data from multiple stations. However, the differences are rather small.
The volumes of lava and tephra produced between 26 and 31 January, 2011, were also estimated by other means. Field measurements suggest that 24 × 106 kg of tephra was produced during the three sub-Plinian eruptions (National Institute of Advanced Industrial Science and Technology (AIST) and Asia Air Survey Co., Ltd., 2011), and assuming a density of 2500 kg/m3 yields a dense rock equivalent volume of 9.6 × 106 m3. Additional estimates used a photograph taken on 31 January, 2011, which suggests the volume of lava accumulated in the Shinmoe-dake summit crater was 14 × 106 m3 (Earthquake Research Institute et al., 2011). The magma extruded as lava is approximately 1.4 times larger than that ejected as tephra. The volume change estimated from extensometer records is 2.56 × 106 m3 and 3.07 × 106 m3, respectively, for the three sub-Plinian eruptions and magma effusion (Table 1) and the ratio of magma effusion to sub-Plinian eruptions is 1.2. Both field and photograph-based measurements are larger than the extensometer-based volume estimates presented here, but the ratio approximately coincides with each other. This indicates that the strain data obtained at the ISA observatory can detect more detail volcanic process and may provide approximate but reasonable depth and volume change estimates for other volcanic events.
4. Subtle Changes in Strain Prior to the Sub-Plinian Eruptions
The subtle inflation followed by rapid deflation recorded during each event seems to be associated with a causal process associated with each sub-Plinian eruption. However, it is unclear whether this hypothesis is plausible, as similar-sized strain variations that are not associated with volcanic processes may have also been measured. Further research is needed to determine whether similar strain changes are only recorded before the three sub-Plinian eruptions that form the focus of this study. We investigated this possibility by analyzing the rate of variation in the E1 component strain. To obtain the strain rate values, the derivative of the smoothed time series trend was calculated. The smoothed time series trend was obtained following Tamura et al. (1991), with ABIC used to determine the optimum parameter that controls the smoothness of the trend. Because of the smoothing process, phenomena with time scales less than 30 minutes are ignored; however, it does not affect the following discussion.
The above discussion further indicates that if the horizontal distance from the ISA site to the source epicenter was the same for both the precursory deformation and the sub-Plinian eruptions, the source depth of the precursory deformation would have been shallower than the source depth of the magma chamber, at least for the first subtle strain event that was recorded at around 6:00–7:40 on 26 January, 2011. Even if this horizontal distance to the source is not fixed, the source depth is still likely to be shallow unless the horizontal distance is very large (Fig. 7).
Other geophysical acquisition techniques (e.g., Kato and Fujiwara, 2012; Maehara et al., 2012), including eyewitness reports, provided several other records of phenomena prior to the sub-Plinian eruptions. In the case of the first sub-Plinian eruption, for example, an increase in seismic amplitudes was observed at about 7:15 (Fig. 5(a)), coincident with the onset of a rapid extension recorded in the E1 direction (Fig. 5(a)). A minor ash eruption with increased amounts of volcanic smoke was observed at this same time, and was followed by a further increase in the amount of volcanic smoke venting at around 7:50. At about 14:50, seismic amplitudes increased again (Fig. 5(a)), which corresponds to the subtle deformation just before the first sub-Plinian eruption. At this time, steam emissions were also observed (Maehara et al., 2012). Increases in seismic amplitudes were again observed in association with the subtle deformations before the second and third sub-Plinian eruptions, although the exact timings of the onset of these two sets of phenomena are not exactly the same (Fig. 5(b and c)).
The precursory deformation associated with each of the sub-Plinian eruptions documented in this study can be interpreted as follows: a gradual E1 component contraction corresponds to the accumulation of volcanic gases at depth, and a rapid extension of the E1 component corresponds to decreasing pressure during the release of volcanic gases within the crater. The shallow source depth estimates for these precursory deformation events is consistent with this interpretation. If this interpretation is correct, temporal changes in strain will be important factors in any further investigation of these processes.
Similarities between the subtle strain changes (indicated by variations in strain rates from negative to positive) prior to each of the sub-Plinian eruptions are also evident in the strain rate graph shown in Fig. 6(b). The shapes of subtle strain changes for each of the sub-Plinian eruptions are similar, suggesting that the precursory phenomena corresponding to each sub-Plinian eruption involve similar processes.
Figure 6(b) also shows, however, that the relationships between subtle changes in strain and the eruptions are not simple. For example, the first eruption on 26 January was associated with two subtle changes in strain: one approximately 8 hours before the eruption and a second at 15:00–15:30, immediately prior to the eruption. The second eruption occurred around 3 hours after the subtle strain change at 21:20–22:20 on 26 January. The last sub-Plinian eruption was almost simultaneous with the ending of this period of subtle strain change. In summary, the time interval between the beginning of the subtle strain change and each sub-Plinian eruption decreased over time. It may also be noted that variations are also apparent in the speeds of deflation during sub-Plinian eruptions
The variations in the timing relationships between subtle strain changes, increases in seismic amplitudes, and the onset of sub-Plinian eruptions, together with the increasing rate of deflation during each eruption may be related to changes either in the magma properties or in the magma supply system (e.g., magma viscosity, expansion of a magma conduit as a result of repeated eruptions, or gradual magma ascent). Additional discussion requires at least a quantitative estimation of the location and magnitude of subtle strain changes.
Further investigation requires other geophysical datasets. Because the direct measurement of strain of the order of 10−9 was performed at only one site, some other approach is required to investigate the precursory deformation more fully. The use of broadband seismograms may be a promising way to deduce tilt changes at particular points (e.g., Aoyama and Oshima, 2008). In fact, tilt changes prior to the sub-Plinian eruptions of Shinmoe-dake were indicated by data from a broadband seismogram positioned about 1 km from the crater of Shinmoe-dake (Maehara et al., 2012). Combining such a result with direct measurements of strain would make it possible to estimate the sizes and locations of the sources of precursory crustal deformation more accurately, which, in turn, would improve our understanding of the processes involved in the sub-Plinian eruptions.
Subtle (order 10−9) strain changes were recorded prior to sub-Plinian eruptions during the activities of Shinmoe-dake in January 2011 by using vault-housed extensometers installed approximately 18 km away from the summit crater of the volcano. Each subtle strain change can be accounted for by an inflation followed by a deflation at depth. The depth of the source is estimated to be shallower than the magma chamber that produced most of the effusive material during the eruptions of Shinmoe-dake. This may indicate that the movement of magma or volcanic gas from the main magma chamber towards the ground surface occurred not only during the sub-Plinian eruptions but also before these eruptions. The observed subtle changes in strain provide significant constraints on the possible mechanisms of the sub-Plinian eruptions of Shinmoe-dake, although further studies using other geophysical datasets are required if we are to investigate the processes more fully.
Observations at Isa and Yoshimatsu were supported by staff of the DPRI, Kyoto University, particularly Yasumi Sonoda and Tetsuro Takayama. The program Generic Mapping Tools (Wessel and Smith, 1998) was used to prepare some of the figures presented here. Comments on an earlier version of the manuscript by Barry Voight and anonymous reviewers helped us to significantly improve both the content and the presentation of this manuscript.
- Akaike, H., Likelihood and Bayes Procedure, in Bayesian Statistics, edited by Bernardo, J. M., M. H. DeGroot, D. V., Lindley, D. V. and A. F. M. Smith, 143–166, University Press, Valencia, 1980.Google Scholar
- Aoyama, H. and H. Oshima, Tilt change recorded by broadband seismometer prior to small phreatic explosion of Meakan-dake volcano, Hokkaido, Japan, Geophys. Res. Lett., 35, L06307, 2008.Google Scholar
- Chardot, L., B. Voight, R. Stewart, S. I. Sacks, A. Linde, D. Hidayat, A. Clarke, D. Elsworth, R. Foroozan, J.-C. Komorowski, G. S. Mattioli, and S. Sparks, Explosion dynamics from strainmeter and microbarometer observations, Soufrière Hills Volcano, Montserrat: 2008–2009, Geophys. Res. Lett., 37, L00E24, 2010.View ArticleGoogle Scholar
- Druitt, T. H., S. R. Young, T. Baptie, C. Bonadonna, E. S. Calder, A. B. Clarke, P. D. Cole, C. L. Harford, R. A. Herd, R. Luckett, G. Ryan, and B. Voight, Episodes of cyclic Vulcanian explosive activity with fountain collapse at Soufrière Hills Volcano, Montserrat, in The Eruption of Soufrière Hills Volcano, Montserrat, from 1995 to 1999, edited by T. H. Druitt and B. P. Kokelaar, Geological Society Memoir No. 21, 281–306, 2002.Google Scholar
- Dvorak, J. J. and D. Dzurisin, Volcano geodesy: the search for magma reservoirs and the formation of eruptive vents, Rev. Geophys., 35, 343–384, 1997.View ArticleGoogle Scholar
- Earthquake Research Institute, University of Tokyo, Eruption of Shinmoe-dake (Kirishima volcano group), Japan, 2011. (URL: http://outreach.eri.u-tokyo.ac.jp/eqvolc/201101_shinmoe/eng/; last updated on 22 Feb 2011; last accessed, 5 Jul 2013).
- Earthquake Research Institute, the University of Tokyo, Kagoshima University, Hokkaido University, and Kokusai Kogyo, Co. Ltd., Volume of lava in the crater of Shinmoe-dake on Jan 31, 2011, material presented at the 118th meeting of Coordinating Committee for Prediction of Volcanic Eruption, 15 February 2011. (in Japanese; URL: http://www.eri.u-tokyo.ac.jp/imoto/ERIkirishima%20geologyrev.pdf; last access, 18 Jun 2013).
- Geospatial Information Authority of Japan, Crustal deformation around Kirishima Volcano, Rep. Coord. Comm. Predict. Volcan. Erupt., 108, 197–220, 2011 (in Japanese).Google Scholar
- Iguchi, M., H. Yakiwara, T. Tameguri, M. Hendrasto, and J. Hirabayashi, Mechanism of explosive eruption revealed by geophysical observations at the Sakurajima, Suwanosejima and Semeru volcanoes, J. Volcanol. Geotherm. Res., 178, 1–9, 2008.View ArticleGoogle Scholar
- Ishihara, K., Pressure sources and induced ground deformation associated with explosive eruptions at an andesitic volcano: Sakurajima volcano, Japan, in Magma Transport and Storage, edited by Ryan, M., 335–356, Wiley, New York, 1990.Google Scholar
- Japan Meteorological Agency, Catalog of Active Volcanoes in Japan (English version), 3rd edition, 2005.Google Scholar
- Kamo, K. and K. Ishihara, Precursor of summit eruption observed by water-tube tiltmeters and extensometers, Annuals Disas. Prev. Res. Inst., Kyoto Univ., 29B-1, 1–12, 1986 (in Japanese with English abstract).Google Scholar
- Kasahara, M., R. Shichi, and Y. Okada, On the cause of long-period crustal movement, Tectonophysics, 97, 327–336, 1983.View ArticleGoogle Scholar
- Kato, K. and Y. Fujiwara, The tilt change preceding magmatic eruption of Shinmoedake Volcano, paper presented at Annual meeting of Volcanological Society of Japan, Miyoda, Nagano, Japan, 14–16 October, 2012 (in Japanese).Google Scholar
- Linde, A. T., K. Agustsson, I. S. Sacks, and R. Stefansson, Mechanism of the 1991 eruption of Hekla from continuous borehole strain monitoring, Nature, 365, 737–740, 1993.View ArticleGoogle Scholar
- Linde, A., S. I. Sacks, D. Hidayat, B. Voight, A. Clarke, D. Elsworth, G. S. Mattioli, P. Malin, E. Shalev, S. Sparks, and C. Widiwijayanti, The Vulcanian explosion at Soufrière Hills Volcano, Montserrat on March 2004 as revealed by strain data, Geophys. Res. Lett., 37, L00E07, 2010.View ArticleGoogle Scholar
- Maehara, Y., M. Takeo, T. Ohminato, M. Ichihara, and J. Oikawa, Tilt motions associated with sub-Plinian, Vulcanian eruptions, and an effusive stage in the 2011 Shinmoe-dake eruption, Paper presented at Japan Geoscience Union Meeting 2012, Chiba, Japan, 20–25 May, 2012.Google Scholar
- Nakao, S., Y. Morita, H. Yakiwara, J. Oikawa, H. Ueda, H. Takahashi, Y. Ohta, T. Matsushima, and M. Iguchi, Volume change of the magma reservoir relating to the 2011 Kirishima Shinmoe-dake eruption-Charging, discharging and recharging process inferred from GPS measurements, Earth Planets Space, 65, 505–515, 2013.View ArticleGoogle Scholar
- National Institute of Advanced Industrial Science and Technology (AIST) and Asia Air Survey co., Ltd., Amount of tephra effused from Shinmoe-dake after Jan 26, 2011, material presented at 120th meeting of Coordinating Committee for Prediction of Volcanic Eruption, 7 June 2011 (in Japanese; URL: https://www.gsj.jp/hazards/volcano/kirishima2011/works-history.html; last access, 18 Jun 2013).
- Peltier, A., T. Staudacher, P. Catherine, L.-P. Ricard, P. Kowalski, and P. Bachelery, Subtle precursors of volcanic eruptions at Piton de la Fournaise detected by extensometers, Geophys. Res. Lett., 33, L06315, 2007.Google Scholar
- Tamura, Y., T. Sato, M. Ooe, and M. Ishiguro, A procedure for tidal analysis with a Bayesian information criterion, Geophys. J. Int., 104, 507–516, 1991.View ArticleGoogle Scholar
- Ueda, H., T. Kozono, E. Fujita, Y. Kohno, M. Nagai, Y. Miyagi, and T. Tanada, Crustal deformation associated with the 2011 Shinmoe-dake eruption as observed by tiltmeters and GPS, Earth Planets Space, 65, 517–525, 2013.View ArticleGoogle Scholar
- Voight, B., R. P. Hoblitt, A. B. Clarke, A. B. Lockhart, A. D. Miller, L. Lynch, and J. McMahon, Remarkable cyclic ground deformation monitored in real time on Montserrat and its use in eruption forecasting, Geophys. Res. Lett., 25, 3405–3408, 1998.View ArticleGoogle Scholar
- Voight, B., R. S. J. Sparks, A. D. Miller, R. C. Stewart, R. P. Hoblitt, A. Clarke, J. Ewart, W. P. Aspinall, B. Baptie, E. S. Calder, P. Cole, T. H. Druitt, C. Hartford, R. A. Herd, P. Jackson, A. M. Lejeune, A. B. Lockhart, S. C. Loughlin, R. Luckett, L. Lynch, G. E. Norton, R. Robertson, I. M. Watson, R. Watts, and S. R. Young, Magma flow instability and cyclic activity at Soufrie`re Hills Volcano, Montserrat, British West Indies, Science, 283, 1138–1142, 1999.View ArticleGoogle Scholar
- Voight, B., D. Hidayat, S. I. Sacks, A. Linde, L. Chardot, A. Clarke, D. Elsworth, R. Foroozan, G. S. Mattioli, N. McWhorter, S. Sparks, and C. Widiwijayanti, Unique strainmeter observations of Vulcanian explosions, Soufrière Hills Volcano, Montserrat, July 2003, Geophys. Res. Lett., Special Section on Montserrat, 37, L00E18, 2010.Google Scholar
- Wessel, P. and W. H. F. Smith, New, improved version of Generic Mapping Tools released, Eos Trans. AGU, 79, 579, 1998.View ArticleGoogle Scholar
- Yamakawa, N., On the strain produced in a semi-infinite elastic solid by an interior source of stress, Zishin 2nd series (J. Seismol. Soc. Jpn.), 8, 84–98, 1955.Google Scholar
- Yamazaki, K., M. Teraishi, S. Komatsu, Y. Sonoda, and Y. Kano, On the possibility of the 2011 Tohoku-oki earthquake reactivating Shinmoe-dake volcano, southwest Japan: insights from strain data measured in vaults, Nat. Haz. Earth Syst. Sci., 11, 2655–2661, 2011.View ArticleGoogle Scholar