Crustal deformation associated with the 2011 Shinmoe-dake eruption as observed by tiltmeters and GPS
© 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: 12 September 2012
Accepted: 2 March 2013
Published: 8 July 2013
The National Research Institute for Earth Science and Disaster Prevention (NIED) developed volcano observation stations at the Kirishima volcanic group in 2010. The stations observed remarkable crustal deformation and seismic tremors associated with the Shinmoe-dake eruption in 2011. The major eruptive activity began with sub-Plinian eruptions (January 26) before changing to explosive eruptions and continuous lava effusion into the summit crater (from January 28). The observation data combined with GEONET data of GSI indicated a magma chamber located about 7 km to the northwest of Shinmoe-dake at about 10 km depth. The tiltmeter data also quantified detailed temporal volumetric changes of the magma chamber due to the continuous eruptions. The synchronized tilt changes with the eruptions clearly show that the erupted magma was supplied from the magma chamber; nevertheless, the stations did not detect clear precursory tilt changes and earthquakes showing ascent of magma from the magma chamber just before the major eruptions. The lack of clear precursors suggests that magma had been stored in a conduit connecting the crater and the magma chamber prior to the beginning of the sub-Plinian eruptions.
The volcano observation stations of the National Research Institute for Earth Science and Disaster Prevention (NIED) initiated their operation at the Kirishima volcanic group in 2010. The stations clearly observed seismic activity and crustal deformation related to magma accumulation before the eruption and discharge during the eruption. Following the major eruptions in January and February 2011, which caused damage at and around the volcano (mainly owing to volcanic ash and lapilli), explosive eruptions continued until September 2011; activity has decreased markedly throughout 2012.
Over the last 300 years, major volcanic eruptions of Shinmoe-dake have occurred in 1717–18, 1771–72, and 1822 (Imura and Kobayashi, 1991). The eruption style and composition of the 2011 eruption have been reported to be similar to those of the 1717–18 eruption (e.g., Geshi et al., 2011; Nakada et al., 2011), in which the largest eruption occurred at the end of the activity, which lasted for one and half years and included intervals of several months of continuous eruption. It is therefore important to monitor the Kirishima group carefully using real-time precise observations. For the efficient monitoring of eruptions and forecasting of future activity based on monitoring data, it is essential to investigate the magma supply system and eruption mechanisms by analyzing observational data from the major eruptions at the beginning of the 2011 activity. The volcano observation stations of NIED succeeded in measuring crustal deformation and earthquakes associated with the sub-Plinian eruptions and continuous lava effusion into the summit crater. Here, we describe the geodetic data of the NIED networks and estimate the magma supply system related to the 2011 Shinmoe-dake eruption by using both NIED data and GEONET data from the Geospatial Information Authority of Japan (GSI), focusing on the major eruptive activity from January 26 to February 2.
2. The Volcano Observation Networkof NIED (V-net)
We began seismic and geodetic observation at the Kirishima volcanic group using two seismic/deformation stations (KRMV and KRHV) in April 2010 (see Fig. 1(c) for location); both stations are part of the volcano observation network (V-net) that NIED began to develop in 2009 to research active volcanoes such as Usu, Iwate, Asama, Kusatsu-Shirane, and Aso. The stations consist of short-period seismometers and pendulum-type tiltmeters (Mitutoyo ABS-143) at the bottom of a borehole 200 m deep, in addition to broadband seismometers (Nanomet-rics Trillium-240) and GPS receivers (JAVAD Delta-G3T and RingAnt-DM) on the ground. The dual frequency GPS data are collected with a sampling interval of 30 seconds and downloaded to NIED every hour. We process the coordinates of the GPS stations with the reference frame of ITRF2005 by GAMIT/GLOBK software every day together with the dual frequency GPS data of nearby GEONET stations. The seismometer and tiltmeter data are digitized with sampling frequencies of 100 Hz and 20 Hz, respectively. The data are transmitted to NIED in real time via an IP-VPN network and are also transmitted to the Japan Metrologi-cal Agency (JMA) for volcano monitoring purposes. The real-time observational data from the Shinmoe-dake eruption contributed to volcano warnings and alerts for evacuation of local residents, and the data collection system was commonly used as part of the High Sensitivity Seismograph Network, Japan (Hi-net) operated by NIED. In this paper, we also used data from Hi-net stations SUKH and MJNH (Fig. 1(c)) near the Kirishima volcanic group; these stations are installed with the same types of short-period seismometers and pendulum-type tiltmeters as the other sites, but the instrumentation is located at the bottoms of boreholes 100 m deep.
3. Crustal Deformation
The ratio between KRMV and KRHV exhibited small fluctuations during the step-like tilt changes on January 26 and 27 (see arrows in Fig. 4). Although the ratio of total tilt changes between stations was stable, relatively large steplike tilt changes and subsequent reverse tilt changes were observed at KRMV compared to KRHV. The time series of tilt change for KRMV in Fig. 2 seemed to overshoot during the steps; such tilt changes were seen at only KRMV. No similar deformation is seen at other stations (e.g., Japan Meteorological Agency, 2011). Since KRMV is the closest station to the modeled deformation source (discussed below), the tilt change of KRMV may contain a local crustal deformation due to the volcanic activity. It is difficult to pursue a physical mechanism of the tilt change, however, because it is observed at only one station (KRMV).
The model parameters of the estimated models.
Volumetric Change (106 m3)
Figure 8(b) shows the deflation rate of the model magma chamber estimated by differentiation of the temporal volume changes. The three distinct peaks in deflation rate on January 26 and 27 correspond to the three sub-Plinian eruptions. We also note two less distinct peaks during January 28 to 29, which suggest the occurrence of eruptions. During the period from January 29 to 30, the deflation continued at a low and almost constant rate. The average modeled deflation rate was 96 × 103 m3/h from 12:00 on January 29 to 12:00 on January 31. The sharp peaks for January 26 and 27 are about ten times larger than the baseline deflation rate. Although the deflation volume associated with each eruption was different, the heights of the peaks are similar. The existence of an upper limit to the deflation rate suggests that the heights of the peaks (i.e., the maximum rate of discharge of magma from the magma chamber) are probably controlled by factors such as magma properties, magma flow rate and vent geometry.
The modeled deflation volume and rate associated with each sub-Plinian eruption act as good indicators of eruption scale and effusion rate. Since the data were collected in real time, rapid estimations of the deflation volume can be used as immediate constraints on the volume and scale of an eruption, which will aid hazard assessment and disaster response. We note that the deflation volume and rate are not equal to erupted volume and rate of discharge of lava from the vent, respectively; such relations depend on the elastic properties and bubble content of magma. Precise estimation of erupted volume and discharge rate of each eruption was conducted by Kozono et al. (2013). They combined the temporal volumetric change of the magma chamber estimated by the geodetic data during the eruption with that of accumulating lava in the crater measured by Synthetic Aperture Radar (SAR) images during the continuous lava effusion. Based on the comparison of these estimated temporal changes of volume, they estimated dense-rock equivalent volumes and discharge rates of magma during the three sub-Plinian eruptions considering magma compressibility and volume variation of a bubble-bearing magma.
The seismicity observed during the eruptions included volcanic tremor, earthquakes associated with explosive eruptions, and harmonic tremor. Figure 8(c) gives 10-min averages of seismometer amplitudes. Although clear seismic activity was observed during each sub-Plinian eruption, we could not detect precursory earthquake activity before the major eruptions beginning on January 26. It is clear that increases in seismic amplitudes due to seismic tremor occurred during the three sub-Plinian eruptions on January 26–27. The seismic amplitude exhibits good agreement with temporal changes in volume and the deflation rate. The seismic tremors associated with the eruptions coincided with the deflation of the magma chamber. Increases in amplitude during January 28–29 also corresponded to increases in the deflation rate, suggesting the occurrence of additional small eruptions. The good agreement indicates that volcanic tremor was caused by eruptive activity, such as explosions at the summit and friction between the magma and the conduit wall during magma flow.
Although we have not uncovered any evidence to suggest a conduit connecting the magma chamber to the summit, it is reasonable to assume that the conduit extends linearly from the magma chamber toward the southeast. Imakiire and Oowaki (2011) showed using GPS the existence of a shallow magma chamber at a depth of 3.4 km beneath Shinmoe-dake crater. They used data of GPS stations deployed near the crater by JMA in addition to GPS data of NIED and GEONET of GSI. The GPS data near the crater provide a more detailed view of a shallow region beneath the crater compared to our model. Their result shows that the deep magma chamber to the northwest of Shinmoe-dake and the shallow chamber were inflated 13.8 × 106 m3 and 1.2 × 106 m3, respectively, during the period from December 2009 to January 2011. Therefore, the magma had been stored in these chambers before the beginning of the 2011 eruption, and flowed in a conduit that connects the chambers and the crater during the eruption.
During the eruption, the erupted magma was supplied from the deep magma chamber which is about 7 km northwest from the Shinmoe-dake crater. Some major craters at Kirishima, such as Karakuni-dake, Ohachi, and Ohnami-ike, are situated in the area to the southeast of the modeled magma chamber (Fig. 1(c)). It is not likely that each crater is supplied by a different magma chamber; therefore, the conduit probably branches toward these craters from a single zone of magma accumulation. Since the summit crater of Shinmoe-dake has been capped by lava and the deep magma chamber has reinflated since the eruption, it is important to monitor activity not only at Shinmoe-dake but also at the other craters that probably share its magma supply system.
The lack of clear precursory tilt changes and earthquakes just before the beginning of the sub-Plinian eruptions suggests that the erupted magma had been gradually ascending from the deep magma chamber toward the crater in the conduit for a long period of time. The GPS observation near the Shinmoe-dake crater shows that the magma accumulated a shallow chamber at the depth of 3.4 km beneath the Shinmoe-dake crater during the period from December 2009 to January 2011 (Imakiire and Oowaki, 2011). Petro-logic investigations of the erupted material related to the small eruption on January 19 indicates that it had the same magma source as the sub-Plinian eruptions (e.g., Geshi et al., 2011), showing the magma had already ascended before the beginning of the sub-Plinian eruptions. Therefore, we probably would not detect precursory tilt changes and earthquakes because the ascending rate of magma was very low as compared with the ascending rates at the beginning of the Izu-Oshima fissure eruption, and the Izu-Tobu and Miyakejima eruptions. The difference between the 2011 Shinmoe-dake eruption without precursory tilt changes and earthquakes and the Izu-Oshima fissure eruption, and the Izu-Tobu, and Miyakejima eruptions are probably due to whether a conduit exists or not. At the beginning of the latter eruptions, the magma ascended abruptly by dike intrusions during the periods of several hours or days (Yamamoto et al., 1988; Okada and Yamamoto, 1991; Ueda et al., 2005). In the case of the 2011 Shinmoe-dake eruption, the magma ascended gradually up the conduit over a period of about 2 years (Imakiire and Oowaki, 2011), and was stored in the conduit and the shallow magma chamber until the beginning of the sub-Plinian eruptions.
The NIED V-net stations at the Kirishima volcanic group precisely observed crustal deformation and seismic activity associated with the Shinmoe-dake eruption in 2011. These data, and GEONET data of GSI, indicate the presence of a magma reservoir located about 7 km to the northwest of Shinmoe-dake and which fed magma to the surface during the sub-Plinian eruptions and the subsequent lava effusion at the summit crater. The observation stations did not exhibit clear precursory signals prior to the major eruptions, suggesting that the erupted magma was stored in a shallow part of an existing conduit that was connected to the deep magma chamber before the beginning of the eruption.
Deformation data indicate the presence of a magma chamber; however, the precise characteristics of the conduit that connect the chamber to the surface are still unknown. For monitoring the eruptions of this volcano and forecasting its future activity, it is necessary to investigate the geometry, location, and properties of the conduit and develop techniques for monitoring anomalous changes using observational data from V-net. In addition, both detection of precursory signals and immediate estimations of the location, scale, and type of eruptions, using precise real-time observational data, could contribute to timely hazard assessment and disaster response.
We thank Dr. Mike Poland and an anonymous reviewer for their useful comments and language editing which have greatly improved the manuscript. We thank the Geospatial Information Authority of Japan for providing us with GPS data from GEONET and DEM data for the Kirishima volcano.
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