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Additional file 1: Fig. S1 Sounding curves of the apparent resistivity and phase for four components (Zxx, Zxy, Zyx, and Zyy), and the real (upper panel) and imaginary (lower panel) parts of tipper vector, respectively, at three sites (402, 403, and 405) around Naka-dake crater. Black dots with error bars and open red circles indicate observed and synthetic data, respectively. Blue crosses indicate responses calculated from the model of Hata et al. 2018. Fig. S2 Sounding curves of the apparent resistivity and phase for four components (Zxx, Zxy, Zyx, and Zyy), and the real (upper panel) and imaginary (lower panel) parts of tipper vector, respectively, at three sites (407, 408, and 410) around Naka-dake crater. Black dots with error bars and open red circles are observed and synthetic data, respectively. Blue crosses are responses calculated from the model of Hata et al. 2018. Fig. S3 Results of sensitivity test model 1. The uppermost panel shows a NNE–SSW-oriented cross-section passing through Naka-dake crater for the test model. The three lower panels show a comparison of the sounding curves for observed data (black dots), final model (red circles), and test model (blue triangles) at sites 022, 409, and 410. Fig. S4 Results of sensitivity test model 2. The uppermost panel shows a NNE–SSW-oriented cross-section passing through Naka-dake crater for the test model. The three lower panels show a comparison of the sounding curves for observed data (black dots), final model (red circles), and test model (blue triangles) at sites 022, 409, and 410. Fig. S5 Results of sensitivity test model 3. The uppermost panel shows a NNE–SSW-oriented cross-section passing through Naka-dake crater for the test model. The three lower panels show a comparison of the sounding curves for observed data (black dots), final model (red circles), and test model (blue triangles) at sites 022, 409, and 410. Fig. S6 Results of sensitivity test 4. The upper panels show a horizontal section for the test model at 4.25–5.00 km BSL and the locations of the sites (028, 032, and 035) selected for this test (left figure), and a E–W-oriented cross-section passing through Naka-dake crater for the test model (right figure). The three lower panels show a comparison of the sounding curves for observed data (black dots), final model (red circles), and test model (blue triangles) at sites 028, 032 and 035.

As a part of the joint geophysical and geochemical observations of Usu volcano in 2006, the airborne infrared thermal survey was conducted at Usu volcano, Noboribetsu geothermal area and Tarumai volcano. The observation was operated by the helicopter which has a hole on the floor for aerial photo of vertical direction. The portable infrared thermal camera (AVIO TVS-620) was fixed on the hole to face the ground perpendicularly using the stand designed for this survey. The position and horizontal direction of the camera were measured by GPS. The vertical angle of the camera was determined by two crossing electronic clinometers placed on the stand. The thermal images (one second interval) ware transferred directly from the camera to computer through the USB cable. The number of the pixel of each frame is 320 for horizontal direction and 236 for vertical direction. The instant view of one pixel is 1.4 mrad. The altitude of the flight was 2300m so that the temperature at a pixel represents the mean temperature of about 3 m2 and the each frame corresponds to the area of 1030 m × 730 m. The detective wavelength of the camera is 8 -14 ìm and the signal attenuation by the atmosphere was ignored in the analysis of the surface temperature. The helicopter kept the constant height during the measurement. The thermally active area was covered by 6, 2 and 3 measurement lines in Usu, Noboribetsu and Tarumai, respectively. The selected thermal images with suitable condition were combined (about 90 frames in Usu volcano) considering the horizontal orientation, vertical angle and the position. The resultant image represents the overall surface temperature distribution at respective field. The surface temperature distribution of Usu volcano shows that the thermally active area is mainly composed from four parts: Nishi-yama crater, Kompira-yama crater, summit area and Showa-shinzan. Nishiyama and Kompira-yama craters were newly formed craters at 2000 eruption activity. The surface temperature distribution of Noboribetsu shows the fumarole activity at Ohyu-numa and Jigoku-dani thermal area and that of Tarumai shows the fumarole activity at A and B craters, respectively. The detailed thermal structure at respective field is cleared by this observation. It is confirmed that this convenient observation system is satisfied for the practical use.

Aerial IR surveys were carried out in September 2010 over geothermal areas at Usu, Noboribetsu and Hokkaido-Komagatake volcanoes, Japan. The heat discharge rate from steaming grounds that formed during the 2000 eruption of Usu volcano is estimated to be 5.6 MW, which is half the rate measured in September 2006. Integration of the heat discharge rate from April 2000 to September 2010 yields an accumulated discharge of approximately 2.4 × 1016 J. This amount of heat corresponds to several percent of the total intruded magma heat estimated from analyses of ground deformation associated with the 2000 eruption. Areas of steaming ground associated with the 2000 eruption showed rapid growth compared with those of the 1977 eruption but discharged less heat. We suspect that differences in the hydrological environments of the two eruptions, led to contrasting patterns of propagation of the hydrothermal systems around the intruded magmas. The heat discharge rate from the summit areas of Usu volcano, Showa-shinzan and Noboribetsu volcano in 2010 are estimated to be 40, 6.2 and 22 MW, respectively, similar to the rates measured in September 2006. No high-temperature grounds were detected at the vents and fissure formed during the 1942 eruption of Hokkaido-Komagatake volcano. We observed no geothermal activities at vents associated with the 1929, 1996, 1998 and 2000 eruptions, due to the absorption effect of atmospheric cloud.

We estimate heat-discharge rates for a geothermal area formed during the 2000 eruption of Usu volcano, Japan. Field observations carried out in September 2006 reveal that heat-discharge rates from fumaroles, areas of steaming grounds and crater lakes are 8, 9.2, and 2 MW, respectively. The total heat-discharge rate measured in September 2006 represents just 1% of the rate immediately following the eruption. Integration of the heat-discharge rate from April 2000 to September 2006 yields an accumulated discharge of approximately 2.3 × 10^[16]J, equating to the cooling of 8 × 10^6 m^3 of magma. This amount of magma corresponds to several percent of the total intruded magma volume estimated from analyses of ground deformation associated with the 2000 eruption. Compared with the 1977 eruption, the 2000 eruption involved the discharge of large amounts of heat from fumaroles. Fumaroles that developed during the 2000 eruption showed a decline in activity in short time. Areas of steaming ground associated with the 2000 eruption showed more rapid growth compared with those of the 1977 eruption but discharged less heat. We suspect that differences in the hydrological environments of the two eruptions (e.g., permeability around the intruded magmas) led to contrasting patterns of propagation of the hydrothermal systems around the intruded magmas.

The thermal activity of Iwodake volcano is characterized by the predominant volcanic gas ejection at the summit crater and the heat discharge from high ground temperature area which is distributed widely from the summit crater to hillside of the mountain. The volcanic gas at the summit crater is of magmatic origin and its temperature is observed to be 880°C in maximum. The total amount of the volcanic gas discharged from the summit area is estimated to be 150kg /sec from the SO2 measurement and the chemical composition of the volcanic gas. The heat discharge rate form the high ground temperature area at the hillside, which is distributed around the fumaroles is estimated to be 50MW from the surface temperature measurement. These thermal activities of Iwodake volcano is thought to be continued for more than 800 years. A plausible model to explain the long degassing of large amount of volcanic gas is magma circulation in a conduit which extended from the deep-seated magma reservoir. The continuous active degassing probably causes the hydrothermal system within the volcano because the volcanic gas ascending the conduit from the top of the magma to the surface is diffused to the surrounding formation. The development of such hydrothermal system is studied using the mathematical simulation. The result shows that the overall thermal activity of Iwodake volcano such as the volcanic gas ejection at the summit crater, widely distributed ground temperature anomaly at hillside and hot springs along the coast can be caused by the degassing activity. The important factors in order to induce the wide-ranging hydrothermal system are permeability of the volcanic edifice and the depth of the degassing. The simulation indicate that the permeability of 10-13m2 and the degassing at sea level are suitable condition for the Iwodake thermal activity.