Abstract
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.
Highlights
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)
The original groundwater pressure data measured at groundwater pressure at Ohtaki obser‐ vatory (GOT) includes diurnal and semidiurnal oscillations caused mainly by earth tides
Atmospheric pressure and precipitation 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)
Summary
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). Investigations of crustal deformation and groundwater changes can be useful for predicting volcanic eruptions. Mt. Ontake is a volcano situated on the border between Nagano and Gifu prefectures in central Japan (Fig. 1). The first recorded phreatic eruption of Mt. Ontake occurred in 1979, and small phreatic eruptions occurred again in 1991 and 2007 (e.g., Nakamichi et al 2009; Japan Meteorological Agency 2014). On September 27, 2014, Mt. Ontake produced another, larger phreatic eruption
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