Rapid dike intrusion into Sakurajima volcano on August 15, 2015, as detected by multi-parameter ground deformation observations
© Hotta et al. 2016
Received: 5 December 2015
Accepted: 18 April 2016
Published: 27 April 2016
We present observations of ground deformation at Sakurajima in August 2015 and model the deformation using a combination of GNSS, tilt and strain data in order to interpret a rapid deformation event on August 15, 2015. The pattern of horizontal displacement during the period from August 14 to 16, 2015, shows a WNW–ESE extension, which suggests the opening of a dike. Using a genetic algorithm, we obtained the position, dip, strike length, width and opening of a dislocation source based on the combined data. A nearly vertical dike with a NNE–SSW strike was found at a depth of 1.0 km below sea level beneath the Showa crater. The length and width are 2.3 and 0.6 km, respectively, and a dike opening of 1.97 m yields a volume increase of 2.7 × 106 m3. 887 volcano-tectonic (VT) earthquakes beside the dike suggest that the rapid opening of the dike caused an accumulation of strain in the surrounding rocks, and the VT earthquakes were generated to release this strain. Half of the total amount of deformation was concentrated between 10:27 and 11:54 on August 15. It is estimated that the magma intrusion rate was 1 × 106 m3/h during this period. This is 200 times larger than the magma intrusion rate prior to one of the biggest eruptions at the summit crater of Minami-dake on July 24, 2012, and 2200 times larger than the average magma intrusion rate during the period from October 2011 to March 2012. The previous Mogi-type ground deformation is considered to be a process of magma accumulation in preexisting spherical reservoirs. Conversely, the August 2015 event was a dike intrusion and occurred in a different location to the preexisting reservoirs. The direction of the opening of the dike coincides with the T-axes and direction of faults creating a graben structure.
In this study, we present observations of rapid ground deformation in August 2015 and model the deformation using GNSS, tilt and strain data in order to interpret the event.
Observation and data
We used 20 GNSS stations: 16 GNSS stations from the Sakurajima Volcano Research Center (SVRC) of Kyoto University and 4 GNSS Earth Observation Network (GEONET) stations (Fig. 1b). We used the L1 and L2 GPS frequency bands. Daily positions of the stations were calculated using Leica GNSS Spider Version 5.2 (automatic) and Leica Geo Office 8.2 (manual). Sampling intervals of 1 s were used. In addition to the GNSS data, we also used data obtained from 2 sets of water-tube tiltmeters, 7 borehole tiltmeters and 2 sets of linear strainmeters (Fig. 1b). The tiltmeters and strainmeters operated by the SVRC were positioned radially and tangentially to the Minami-dake crater (the radial components were positive toward the Minami-dake crater, and the tangential components were positive in the direction rotated 90° clockwise from the radial components), and 3 borehole tiltmeters operated by the JMA were installed in the north and east directions. The water-tube tiltmeters and linear strainmeters were installed in the 400-m-long Arimura underground tunnel (AVOT) operated by the Ministry of Land, Infrastructure, Transport and Tourism of Japan (MLIT) and the 250-m-long Harutayama underground tunnel (HVOT). The span of each component was 28 m. Sensitivities were 0.056 V/μrad for the water-tube tiltmeters and 0.056 V/μstrain for the linear strainmeters. The sensitivity of borehole tiltmeters was 0.02 V/μrad. Borehole tiltmeters were installed at depths of 85–350 m. The analog-to-digital resolution for all tiltmeters and strainmeters was 24 bit.
Ground deformation and seismicity during the deformation event
Along with the second increase in the deformation rate at 8:54, the seismicity of VT earthquakes also increased. During the period with the highest deformation rate from 10:27 to 11:54, the seismicity drastically increased, including the largest earthquake at 10:47 (M2.3). After 11:54, the seismicity decreased except for the felt earthquakes at 14:39, 14:46 and 14:52, after which, only 73 earthquakes occurred on August 16.
Analysis and results
Search space, step size, parameter estimates and their 95 % confidence intervals obtained from the GNSS, tilt and strain data
−20.0 to 20.0 km
0.8 ± 0.1 km
−20.0 to 20.0 km
−0.3 ± 0.2 km
0.0 to 20.0 km
1.0 ± 0.1 km
0° to 90°
74° ± 4°
0° to 359°
34° ± 4°
0.0 to 20.0 km
2.3 ± 0.2 km
0.0 to 20.0 km
0.6 ± 0.1 km
−20.0 to 20.0 m
1.97 ± 0.27 m
Discussion and conclusion
The most rapid deformation was observed from 10:27 to 11:54 on August 15. Half of the total amount of deformation was concentrated within this 1.5-h period. It is estimated that the magma intrusion rate was 1 × 106 m3/h during this period. The magma intrusion volume beneath Minami-dake prior to the vulcanian eruption on July 24, 2012, one of the biggest eruptions at the Minami-dake crater, was 1 × 105 m3 (Iguchi 2013). The deformation that accompanied this intrusion continued without interruption for 21 h, and an intrusion rate of 5 × 103 m3/h was obtained. The intrusion rate for the event on August 15, 2015, was 200 times larger than that on July 24, 2012. The ground deformation during the period from October 2011 to March 2012 was one of the largest long-term deformation events after the resumption of eruptive activity at the Showa crater on June 2006. Hotta et al. (2016) found that the magma intrusion volume toward Sakurajima beneath the Kita-dake crater during this six-month period was 2 × 106 m3, which yields an intrusion rate of 460 m3/h. The intrusion rate for the event on August 15, 2015, was 2200 times larger than that for the long-term average intrusion during the period from October 2011 to March 2012.
Previously, a combination of spherical sources in an elastic and homogeneous half-space (Mogi 1958) was applied to the observed deformation in and around Sakurajima (e.g., Iguchi et al. 2013; Yamamoto et al. 2013; Hotta et al. 2016). The ground deformation during these observation periods had characteristics suggestive of isotropic sources. The deformation pattern for the August 2015 event, however, is not suitable for applying the Mogi model. The horizontal displacements do not indicate a radial expansion, but a WNW–ESE extension, which is approximated by a nearly vertical dike with a NNE–SSW strike. The previous Mogi-type ground deformation is considered to be a process of magma accumulation in preexisting spherical reservoirs beneath the Minami-dake craters. Conversely, the August 2015 intrusion event was a dike intrusion in a different location to the preexisting reservoirs.
The tensile direction caused by tectonic ground deformation around this area is ESE–WNW (Watanabe and Tabei 2004). The earthquakes at the southwestern part of Sakurajima have normal fault mechanisms with T-axes of ESE–WNW and are considered to be affected by tectonic stress in southern Kyushu (Hidayati et al. 2007). The direction of the opening of the dike coincides with the T-axes and the direction of the faults in the Aira caldera creating the structures of the Kagoshima graben (Hidayati et al. 2007), and influencing the opening direction of the dike that intruded on August 15.
KH participated in the study conception and design, acquisition of data, analysis and interpretation of data and drafted the manuscript. MI participated in the study conception and design, acquisition of data, analysis of data and helped to draft the manuscript with critical revisions. TT participated in the acquisition of data, analysis of data and helped to draft the manuscript with critical revisions. All authors read and approved the final manuscript.
We used GNSS data from Tohoku University managed by Dr. Ohta; GEONET and JPGIS data from the Geospatial Information Authority of Japan; tilt, strain and seismic data from the AVOT station from the Osumi Office of River and National Highway, Kyushu Regional Development Bureau, MLIT; GNSS, tilt and seismic data from the JMA; generic mapping tools (Wessel and Smith 1995); and the program code of Dr. Carroll’s FORTRAN genetic algorithm driver (Carroll 1999). We also thank Drs. Yosuke Aoki and James Hickey and an anonymous reviewer for their helpful comments.
The present study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research Program.
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