Groundwater pressure changes and crustal deformation before and after the 2007 and 2014 eruptions of Mt. Ontake
© Koizumi et al. 2016
Received: 27 November 2015
Accepted: 26 February 2016
Published: 17 March 2016
Volcanic activity generally causes crustal deformation, which sometimes induces groundwater changes, and both of these phenomena are sometimes detected before volcanic eruptions. Therefore, investigations of crustal deformation and groundwater changes can be useful for predicting volcanic eruptions. The Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, has been observing groundwater pressure at Ohtaki observatory (GOT) since 1998. GOT is about 10 km southeast of the summit of Mt. Ontake. During this observation period, Mt. Ontake has erupted twice, in 2007 and in 2014. Before the 2007 eruption, the groundwater pressure at GOT clearly dropped, but it did not change before or after the 2014 eruption. These observations are consistent with the crustal deformation observed by Global Navigation Satellite System stations of the Geospatial Information Authority of Japan. The difference between the 2007 and 2014 eruptions can be explained if a relatively large magma intrusion occurred before the 2007 eruption but no or a small magma intrusion before the 2014 eruption.
Volcanic activity generally causes crustal deformation, which sometimes induces groundwater changes, and both the crustal deformation and the accompanying groundwater changes are sometimes detected before a volcanic eruption or volcano-related seismic activity (e.g., Okada et al. 2000; Sparks 2003; Koizumi et al. 2004). Therefore, investigations of crustal deformation and groundwater changes can be useful for predicting volcanic eruptions.
In this study, we examined groundwater changes and crustal deformation associated with the 2007 and 2014 eruptions of Mt. Ontake. Those associated with the 2007 eruption were clearly different from those associated with the 2014 eruption. We investigated the reason for this difference and its relationship with the mechanisms of these two eruptions of Mt. Ontake.
To examine crustal deformation, we used daily positional information from four Global Navigation Satellite System (GNSS) stations operated by the Geospatial Information Authority of Japan (GSI), TKN, MTK, OTK, and HGW and calculated the baseline distance between pairs of stations. The daily positional data were downloaded from the homepage of GSI (Geospatial Information Authority 2015). We also used tilt data from a tilt station (TNH) operated by the Japan Meteorological Agency.
The original groundwater pressure data measured at GOT includes diurnal and semidiurnal oscillations caused mainly by earth tides (tidal volumetric strain changes). Atmospheric pressure and precipitation also affect the groundwater pressure. These tidal changes and the effects of atmospheric pressure were estimated and subtracted from the groundwater pressure by using the BAYTAP-G tidal analysis program (Tamura et al. 1991), and the residual values were used as the corrected groundwater pressure (Fig. 2).
Discussion and conclusions
To summarize, the Geological Survey of Japan, AIST, has been observing groundwater pressure at GOT since 1998. The groundwater pressure at GOT clearly dropped before the 2007 eruption, but it did not change before or after the 2014 eruption. These changes are consistent with crustal deformation observed by nearby GNSS stations. The difference between the 2007 and 2014 eruptions can be explained if a relatively large magma intruded before the 2007 eruption but no or a small magma intrusion occurred before the 2014 eruption.
NK carried out the data processing and analysis of the groundwater data and drafted the manuscript. TS carried out the groundwater observations and helped draft the manuscript. YK carried out the tidal analysis of the groundwater pressure and helped draft the manuscript. TO analyzed the GNSS data and helped draft the manuscript. All authors read and approved the final manuscript.
We thank the residents of Otaki village for their cooperation in this research. We are grateful to an anonymous reviewer for reviewing our manuscript and valuable comments.
The authors declare that they have no competing interests.
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