Numerical simulation is a useful method for studying the magmatic-hydrothermal systems of volcanoes. However, no comprehensive scheme has been established for constructing subsurface permeability structures that have a significant impact on fluid flow within the volcano. In this study, as a first step to establishing such a scheme, numerical simulations of hydrothermal fluid flow incorporating the heterogeneous properties of the permeability structure were performed utilizing the resistivity structure observed at Kusatsu-Shirane Volcano, central Japan. Although the constructed permeability structure was relatively simple, the simulation results closely reproduced some observations, such as the broad resistivity structure and the distribution and discharge patterns of hot springs around the volcano. These results suggest that the uncertainty in generating permeability structures in hydrothermal fluid flow simulations can be greatly reduced by using resistivity structures.

1. [1] Y. Ogawa et al., “Comprehensive survey of 2018 Kusatsu-Shirane Eruption,” Proc. Symp. Nat. Disaster Sci., Vol.55, pp. 25-30, 2018 (in Japanese).
2. [2] F. Barberi, A. Bertagnini, P. Landi, and C. Principe, “A review on phreatic eruptions and their precursors,” J. Volcanol. Geotherm. Res., Vol.52, No.4, pp. 231-246, doi: 10.1016/0377-0273(92)90046-G, 1992.
3. [3] P. R. L. Browne and J. V. Lawless, “Characteristics of hydrothermal eruptions, with examples from New Zealand and elsewhere,” Earth Sci. Rev., Vol.52, No.4, pp. 299-331, doi: 10.1016/S0012-8252(00)00030-1, 2001.
4. [4] J. Stix and J. M. de Moor, “Understanding and forecasting phreatic eruptions driven by magmatic degassing,” Earth Planets Space, Vol.70, No.1, Article No.83, doi: 10.1186/s40623-018-0855-z, 2018.
5. [5] S. E. Ingebritsen, S. Geiger, S. Hurwitz, and T. Driesner, “Numerical simulation of magmatic hydrothermal systems,” Rev. Geophys., Vol.48, No.1, Article No.RG1002, doi: 10.1029/2009RG000287, 2010.
6. [6] E. A. Sammel, S. E. Ingebritsen, and R. H. Mariner, “The hydrothermal system at Newberry Volcano, Oregon,” J. Geophys. Res., Vol.93, No.B9, pp. 10149-10162, 1988.
7. [7] M. U. Birch, “Groundwater flow systems and thermal regimes near cooling igneous plutons: Influence of surface topography,” Master thesis, Utah State University, doi: 10.26076/27ce-79a1, 1989.
8. [8] R. B. Hanson, “Effects of fluid production on fluid flow during regional and contact metamorphism,” J. Metamorph. Geol., Vol.10, No.1, pp. 87-97, doi: 10.1111/j.1525-1314.1992.tb00073.x, 1992.
9. [9] M. Todesco, G. Chiodini, and G. Macedonio, “Monitoring and modelling hydrothermal fluid emission at La Solfatara (Phlegrean Fields, Italy). An interdisciplinary approach to the study of diffuse degassing,” J. Volcanol. Geotherm. Res., Vol.125, Nos.1-2, pp. 57-79, doi: 10.1016/S0377-0273(03)00089-1, 2003.
10. [10] S. C. Stissi, R. Napoli, G. Currenti, A. Afanasyev, and G. Montegrossi, “Influence of permeability on the hydrothermal system at Vulcano Island (Italy): Inferences from numerical simulations,” Earth Planets Space, Vol.73, No.1, Article No.179, doi: 10.1186/s40623-021-01515-z, 2021.
11. [11] M. Todesco, J. Rutqvist, G. Chiodini, K. Pruess, and C. M. Oldenburg, “Modeling of recent volcanic episodes at Phlegrean Fields (Italy): Geochemical variations and ground deformation,” Geothermics, Vol.33, No.4, pp. 531-547, doi: 10.1016/j.geothermics.2003.08.014, 2004.
12. [12] S. Hurwitz, L. B. Christiansen, and P. A. Hsieh, “Hydrothermal fluid flow and deformation in large calderas: Inferences from numerical simulations,” J. Geophys. Res. Solid Earth, Vol.112, No.B2, Article No.B02206, doi: 10.1029/2006JB004689, 2007.
13. [13] M. Hutnak, S. Hurwitz, S. E. Ingebritsen, and P. A. Hsieh, “Numerical models of caldera deformation: Effects of multiphase and multicomponent hydrothermal fluid flow,” J. Geophys. Res. Solid Earth, Vol.114, No.B4, Article No.B04411, doi: 10.1029/2008JB006151, 2009.
14. [14] A. P. Rinaldi, M. Todesco, and M. Bonafede, “Hydrothermal instability and ground displacement at the Campi Flegrei caldera,” Phys. Earth Planet. Inter., Vol.178, Nos.3-4, pp. 155-161, doi: 10.1016/j.pepi.2009.09.005, 2010.
15. [15] G. M. Currenti and R. Napoli, “Learning about hydrothermal volcanic activity by modeling induced geophysical changes,” Front. Earth Sci., Vol.5, Article No.41, doi: 10.3389/feart.2017.00041, 2017.
16. [16] M. Todesco and G. Berrino, “Modeling hydrothermal fluid circulation and gravity signals at the Phlegraean Fields caldera,” Earth Planet. Sci. Lett., Vol.240, No.2, pp. 328-338, doi: 10.1016/j.epsl.2005.09.016, 2005.
17. [17] K. Aizawa, Y. Ogawa, and T. Ishido, “Groundwater flow and hydrothermal systems within volcanic edifices: Delineation by electric self-potential and magnetotellurics,” J. Geophys. Res. Solid Earth, Vol.114, No.B1, Article No.B01208, doi: 10.1029/2008JB005910, 2009.
18. [18] S. Byrdina et al., “Influence of the regional topography on the remote emplacement of hydrothermal systems with examples of Ticsani and Ubinas volcanoes, Southern Peru,” Earth Planet. Sci. Lett., Vol.365, pp. 152-164, doi: 10.1016/j.epsl.2013.01.018, 2013.
19. [19] S. Scott, T. Driesner, and P. Weis, “Geologic controls on supercritical geothermal resources above magmatic intrusions,” Nat. Commun., Vol.6, Article No.7837, doi: 10.1038/ncomms8837, 2015.
20. [20] S. W. Scott, “Decompression boiling and natural steam cap formation in high-enthalpy geothermal systems,” J. Volcanol. Geotherm. Res., Vol.395, Article No.106765, doi: 10.1016/j.jvolgeores.2019.106765, 2020.
21. [21] A. Afanasyev, J. Blundy, O. Melnik, and S. Sparks, “Formation of magmatic brine lenses via focussed fluid-flow beneath volcanoes,” Earth Planet. Sci. Lett., Vol.486, pp. 119-128, doi: 10.1016/j.epsl.2018.01.013, 2018.
22. [22] N. Collard, L. Peiffer, and Y. Taran, “Heat and fluid flow dynamics of a stratovolcano: The Tacaná Volcanic Complex, Mexico-Guatemala,” J. Volcanol. Geotherm. Res., Vol.400, Article No.106916, doi: 10.1016/j.jvolgeores.2020.106916, 2020.
23. [23] M. Raguenel, T. Driesner, and F. Bonneau, “Numerical modeling of the geothermal hydrology of the Volcanic Island of Basse-Terre, Guadeloupe,” Geotherm. Energy, Vol.7, Article No.28, doi: 10.1186/s40517-019-0144-5, 2019.
24. [24] Z. Petrillo et al., “Defining a 3D physical model for the hydrothermal circulation at Campi Flegrei caldera (Italy),” J. Volcanol. Geotherm. Res., Vol.264, pp. 172-182, doi: 10.1016/j.jvolgeores.2013.08.008, 2013.
25. [25] Z. Petrillo et al., “A perturbative approach for modeling short-term fluid-driven ground deformation episodes on volcanoes: A case study in the Campi Flegrei caldera (Italy),” J. Geophys. Res. Solid Earth, Vol.124, No.1, pp. 1036-1056, doi: 10.1029/2018JB015844, 2019.
26. [26] G. Gola et al., “A novel multidisciplinary approach for the thermo-rheological study of volcanic areas: The case study of Long Valley Caldera,” J. Geophys. Res. Solid Earth, Vol.126, No.2, doi: 10.1029/2020JB020331, 2021.
27. [27] T. Hashimoto et al., “Significance of electromagnetic surveys at active volcanoes: Toward evaluating the imminence of wet eruptions,” J. Disaster Res., Vol.14, No.4, pp. 580-591, doi: 10.20965/jdr.2019.p0580, 2019.
28. [28] A. Revil and M. Gresse, “Induced polarization as a tool to assess alteration in geothermal systems: A review,” Minerals, Vol.11, No.9, Article No.962, doi: 10.3390/min11090962, 2021.
29. [29] Y. Matsunaga, W. Kanda, T. Koyama, S. Takakura, and T. Nishizawa, “Large-scale magmatic–hydrothermal system of Kusatsu-Shirane Volcano, Japan, revealed by broadband magnetotellurics,” J. Volcanol. Geotherm. Res., Vol.429, Article No.107600, 2022.
30. [30] A. Terada, “Kusatsu-Shirane volcano as a site of phreatic eruptions,” J. Geol. Soc. Japan, Vol.124, No.4, pp. 251-270, doi: 10.5575/geosoc.2017.0060, 2018 (in Japanese).
31. [31] Y. Jung, G. S. H. Pau, S. Finsterle, and R. M. Pollyea, “TOUGH3: A new efficient version of the TOUGH suite of multiphase flow and transport simulators,” Comput. Geosci., Vol.108, pp. 2-7, doi: 10.1016/j.cageo.2016.09.009, 2017.
32. [32] K. Pruess, C. M. Oldenburg, and G. J. Moridis, “TOUGH2 User’s Guide Version 2,” Earth Sciences Division, Lawrence Berkeley National Laboratory, doi: 10.2172/751729, 1999.
33. [33] L. Pan, N. Spycher, C. Doughty, and K. Pruess, “ECO2N V2.0: A TOUGH2 fluid property module for modeling CO₂-H₂O-NaCl systems to elevated temperatures of up to 300°C,” Greenhouse Gas. Sci. Technol., Vol.7, No.2, pp. 313-327, doi: 10.1002/ghg.1617, 2017.
34. [34] H. Munekane, “Modeling long-term volcanic deformation at Kusatsu-Shirane and Asama volcanoes, Japan, using the GNSS coordinate time series,” Earth Planets Space, Vol.73, No.1, Article No.192, doi: 10.1186/s40623-021-01512-2, 2021.
35. [35] K. H. Tseng et al., “Anatomy of active volcanic edifice at the Kusatsu-Shirane volcano, Japan, by magnetotellurics: Hydrothermal implications for volcanic unrests,” Earth Planets Space, Vol.72, No.1, Article No.161, doi: 10.1186/s40623-020-01283-2, 2020.
36. [36] R. O. Fournier, “Hydrothermal processes related to movement of fluid from plastic into brittle rock in the magmatic-epithermal environment,” Econ. Geol., Vol.94, No.8, pp. 1193-1211, doi: 10.2113/gsecongeo.94.8.1193, 1999.
37. [37] T. Ohba et al., “Time variation in the chemical and isotopic composition of fumarolic gasses at Kusatsu-Shirane volcano, Japan,” Front. Earth Sci., Vol.7, Article No.249, doi: 10.3389/feart.2019.00249, 2019.
38. [38] M. Nakano, H. Kumagai, and B. A. Chouet, “Source mechanism of long-period events at Kusatsu-Shirane Volcano, Japan, inferred from waveform inversion of the effective excitation functions,” J. Volcanol. Geotherm. Res., Vol.122, Nos.3-4, pp. 149-164, doi: 10.1016/S0377-0273(02)00499-7, 2003.
39. [39] Gunma Prefecture, “Report on the Geothermal Survey around Kusatsu-Shirane Volcano,” The Gunma Prefecture Enterprise Bureau, 1989 (in Japanese).
40. [40] P. A. Bedrosian, J. R. Peacock, E. Bowles-Martinez, A. Schultz, and G. J. Hill, “Crustal inheritance and a top-down control on arc magmatism at Mount St Helens,” Nature Geosci., Vol.11, No.11, pp. 865-870, doi: 10.1038/s41561-018-0217-2, 2018.
41. [41] K. Aizawa et al., “Magmatic fluid pathways in the upper crust: Insights from dense magnetotelluric observations around the Kuju Volcanoes, Japan,” Geophys. J. Int., Vol.228, No.2, pp. 755-772, doi: 10.1093/gji/ggab368, 2022.
42. [42] C. E. Manning and S. E. Ingebritsen, “Permeability of the continental crust: Implications of geothermal data and metamorphic systems,” Rev. Geophys., Vol.37, No.1, pp. 127-150, doi: 10.1029/1998RG900002, 1999.
43. [43] Y. Sakamoto and A. Terada, “Evolution of a hydrothermal system of Kusatsu-Shirane volcano inferred from aerial infrared surveys in the nighttime,” Proc. of Japan Geoscience Union Meeting 2015, SVC49-09, 2015 (in Japanese).
44. [44] J. Hirabayashi and S. Mizuhashi, “The discharge rate of volatiles from Kusatsu-Shirane volcano, Japan,” Report of 4th Joint Observation of Kusatsu-Shirane Volcano, 2004 (in Japanese).
45. [45] Z. Hashin and S. Shtrikman, “A variational approach to the theory of the effective magnetic permeability of multiphase materials,” J. Appl. Phys., Vol.33, No.10, pp. 3125-3131, doi: 10.1063/1.1728579, 1962.
46. [46] A. S. Quist and W. L. Marshall, “Electrical conductances of aqueous sodium chloride solutions from 0 to 800° and at pressures to 4000 bars,” J. Phys. Chem., Vol.72, No.2, pp. 684-703, doi: 10.1021/j100848a050, 1968.
47. [47] R. Sinmyo and H. Keppler, “Electrical conductivity of NaCl-bearing aqueous fluids to 600°C and 1 GPa,” Contrib. Mineral. Petrol., Vol.172, No.1, Article No.4, doi: 10.1007/s00410-016-1323-z, 2017.
48. [48] Y. Hayakawa and M. Yui, “Eruptive history of the Kusatsu Shirane volcano,” The Quaternary Research (Daiyonki-Kenkyu), Vol.28, No.1, pp. 1-17, doi: 10.4116/jaqua.28.1, 1989 (in Japanese).
49. [49] S. Mizuhashi, “Hydrothermal system of Mt. Kusatsu-Shirane inferred from volatile emissions,” M.S. thesis, Tokyo Institute of Technology, 2004.
50. [50] C. Haukwa, “AMESH A mesh creating program for the integral finite difference method: A User’s Manual,” Lawrence Berkeley National Laboratory, doi: 10.2172/892927, 1998.
51. [51] A. E. Croucher, “PyTOUGH: A Python scripting library for automating TOUGH2 simulations,” Proc. of the New Zealand Geothermal Workshop, 2011.