Gravity variation around Shinmoe-dake volcano from February 2011 through March 2012—Results of continuous absolute gravity observation and repeated hybrid gravity 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: 27 October 2012
Accepted: 22 March 2013
Published: 8 July 2013
We report here on continuous absolute gravity measurements made between February 2011 and March 2012 and repeated relative gravity measurements in the vicinity of Shinmoe-dake volcano, which commenced erupting in late January 2011. We find that 20 of 24 eruptive events are associated with precursory short-term gravity decreases occurring over 5–6 hours followed by quick recoveries lasting 1–2 hours. Also evident are significant long-term gravity changes arising principally from hydrological processes around the volcano, where annual precipitation exceeds 5,000 mm. To isolate the gravity signal associated with volcanic processes, we compared gravity measurements made at 15 sites in March 2011 and again in March 2012. The gravity changes and crustal deformation observed during the one year period are well explained by 6×106 m3 inflation of a magma reservoir at a depth of 9 km and intrusion at shallower depths of a dike with dimensions of 10 km × 0.5 km × 0.5 m.
Eruptions of Mt. Shinmoe-dake, a member of the Kirishima volcano group, are documented to have occurred in 1716–1717, 1771, 1822, 1959, 1991, 2008, and 2010 before the most recent magmatic eruptions in January 2011. Subplinian eruptions on January 26 and 27, 2011 were followed by formation of a lava dome and frequent vulcanian eruptions in February 2011 (Nakada et al., 2013; Suzuki et al., 2013a, b). Although crustal deformation data revealed deflation of a magma source in the early stage of the eruption, subsequent observations until November 2011 suggested that magma accumulation was continuing (Nakao et al., 2011; Geospatial Information Authority of Japan et al., 2012).
We carried out two types of gravity observations from early February 2011 to investigate mass transport processes related to the volcanic activity. They are continuous absolute gravity measurements at a fixed station and repeated gravity observations at sites around the Kirishima volcanoes. These two types of measurement are complementary in the sense that the former has higher temporal resolution and poorer spatial resolution than the latter, and vice versa. This study presents the results of integrating the two types of observations to describe the overall picture of mass movement beneath the Kirishima volcanoes.
2. Absolute Gravity Measurement
Overview of absolute gravity measurements made between February 2011 and March 2012. Mean gravity values are computed for each set of 50 drops.
(Operation halted due to the Tohoku Earthquake)
2.2 Location of the absolute gravity station
Geographical coordinates of KVO (see also Fig. 1).
(Latitude, Longitude, Height)
Distance to Shinmoe-dake
(31.9474°, 130.8394°, 1,200 m)
2.3 Short-term gravity signal
2.3.1 Time series of absolute gravity at KVO
Since both precursory and co-eruptive ground tilt changes are recorded in association with eruptions VE#2 and VE#3 (Fig. 3), tilt events T#1–T#19 are likely to represent either aborted or otherwise unobserved eruptions, that release overpressure from the crater without ejecting significant amount of materials (Japan Meteorological Agency et al., 2011; Kato and Yamasato, 2013). This interpretation is supported by the fact that event T#9, which occurred at midnight local time on February 22, 2011, is most likely an unobserved eruption since a small amount of fresh volcanic ash was identified on the morning of February 23, 2011 (personal communication from Dr. S. Nakada).
The data shown in Fig. 2 raise the possibility that eruption hysteresis after the end of March, 2011 could be examined. However, the coarser gravity measurement sampling after March 30, 2011 does not allow us to derive reliable correlations between tilt and gravity changes (Table 1).
2.3.2 Statistical test on the significance
F-test results for linear regression models applied to gravity changes before eruptions and impulsive tilt events in February and March 2011. T#3 and T#17 are not tested due to the limited number of data with suitably small errors.
Event ID in Fig. 2
Date and time (UT)
Number of data N
Gravity change rate (μgal/hour)
F(1, N − 2)
Critical F for 1% confidence (5%)
11:00–13:00, Feb. 11
02:36, Feb. 11
05:00, Feb. 13
20:07, Feb. 13
05:00, Feb. 15
19:00, Feb. 15
09:00, Feb. 16
09:16, Feb. 18
18:30, Feb. 19
08:25, Feb. 20
07:45, Feb. 22
15:00–16:00, Feb. 22
10:50, Feb. 23
14:00–18:37, Feb. 23
08:00, Feb. 28
12:00, Feb. 28
20:30, Mar. 25
09:30, Mar. 26
04:30, Mar. 28
18:33, Mar. 28
2.3.3 Modeling the pre-eruption gravity change
Source parameters for the model illustrated in Fig. 7 derived from inversion of gravity change (March 2011–March 2012) and displacement data (February 2011–February 2012).
ΔV = 6 × 106 m3
D = 9.0 km
ΔM = 1.25 #x00D7; 1011 kg (upperbound setto 1.25 × 1011 kg)
(Tensile dislocation on a rectangle)
Location (SE corner)
L = 10 km
W = 0.5 km
Depth to the top edge
dTop = 0.5 km
Strike angle from north
U = 0.5 m
2.4 Long-term gravity changes
The 13 month record shows gravity variations of as much as 25 μgal (Fig. 5(a)). The gravity changes cannot be ascribed solely to volcanic processes as gravity is also sensitive to variations in the water table and soil moisture (Torge, 1989). It is thus crucially important to eliminate hydrolog-ical disturbances from the observed gravity data set so that we may isolate the volcanic gravity signals of interest.
Gravity relaxation observed after points A′ and B′ in Fig. 5(a) indicates that the relaxation time JΔt = 30 days and a j = [1 − (j/J)]. After trial and errors, we find C = 0.016 μgal/mm, 40% of the theoretical value of 0.042 μgal/mm for a flat distribution of groundwater (Torge, 1989), accounts for the transient gravity changes well. This value is reasonable since groundwater above the absolute gravimeter partly cancels out the attraction of water beneath it, and KVO is located on a hill slope of 15 degrees.
Once the data have been corrected for rainfall, few abrupt changes remain in the record of Δg(t) = g(t) − δg(t) (Fig. 5). However, the long-term gravity changes Δg(t) do still exhibit seasonal variations of ±5 μgal (high in summer and low in winter), emphasizing the need to perform repeated relative gravity measurements in the same seasons to avoid hydrological effects.
3. Repeated Hybrid Gravity Measurement
3.1 Instrument and setting
Precise relative gravity measurements were made at 15 points around Shinmoe-dake volcano in March and August 2011 and in March 2012 using four LaCoste & Romberg gravimeters (Fig. 1(b)). From now on, we restrict our attention to the one-year period between March 2011 and March 2012; the data in August 2011 indicates significant groundwater disturbance to the observed gravity (Ueki et al., 2011) as discussed in Subsection 2.4.
3.2 Spatial gravity changes in the year following the 2011 eruption
The gravity measurements are tied to the absolute gravity data at KVO so that we may determine absolute gravity value at each points. Figure 6(a) shows the gravity change Ag in the year following the 2011 eruption.
4. Modeling the Volcanic Activity
The optimum value of the objective function L defined by Eq. (11) is L =51.1, which is comparable with the value of expected for a variable drawn from a χz -squared distribution with (n + 3m − p) degrees of freedom (Menke, 1989), where p stands for the number of source parameters (n = 15, m = 13 and p = 11 in our case). The inferred depth to the deep inflation/deflation source is 9 km, a value consistent with Nakao et al. (2011), while the dike intrusion parameters are in reasonable agreement with values obtained by the Geospatial Information Authority of Japan et al. (2012).
A final comment on possible topographic effects on the deformation is appropriate. Segall (2010) showed that when the ratio of the topographic height-to-length scales H/L is less than 0.05, the perturbation to the vertical displacement for a flat earth is less than 15% when an inflation source is buried at a depth 5 km. Since H ≈ 1,000 m (Fig. 1) and L ≈ 20 km (Fig. 8) in our case, we may safely ignore topographic effects.
5. Conclusions and Discussion
We carried out absolute gravity measurements at Kirisima Volcano Observatory over 13 months from February 2011, two weeks after the onset of the 2011 Shinmoe-dake eruption, until March 2012. The data reveal precursory gravity decreases lasting 5–6 hours followed by abrupt gravity increases lasting 1–2 hours during the first 2 months when the Shinmoe-dake volcano was erupting frequently. Most such changes are associated with and preceded eruptions or aborted eruptions inferred from ground tilt changes. Local gravity around Shinmoe-dake volcano exhibited increase of several to several tens μgal during the one-year period following the 2011 Shinmoe-dake eruption. We have derived a physical model that provides a quantitative explanation of the long-term local gravity changes and displacements measured with GPS. This model is consistent with a conceptual model of the volcano’s magma plumbing system based on the short-term absolute gravity changes observed prior to vulcanian eruptions.
We express our sincere thanks to the Japan Meteorological Agency for providing us with ground tilt data at Takachihogawara. Special thanks are given to the Geospatial Information Authority of Japan, the Japan Meteorological Agency and National Research Institute for Earth Science and the Disaster Prevention for allowing us to use the displacement data derived from the GNSS. Drs. J. Oikawa, A. Watanabe and K. Aizawa are acknowledged for their technical support in acquiring the absolute gravity data. The critical comments of anonymous reviewers were helpful in improving this manuscript. This work was supported in part by Grant-in-Aid for Scientific Research (22900001 and 23244092). Some of the figures were prepared using the GMT program (Wessel and Smith, 1998).
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