An estimation method for debris flow potential is proposed to evaluate the possibility of the occurrence of rain-triggered debris flows. Sakurajima volcano has repeatedly erupted (Vulcanian type) and has continuously emitted volcanic ash at the Minamidake summit crater or Showa crater east of the summit since 1955, and debris flows have frequently occurred at rates of 10 to 111 events per year. Ground deformation associated with debris flows along the Arimura River were analyzed for the period from 2009 to 2016. Downward tilt (10–450 nrad) in the direction of the river and extensional strain (3–138 nstrain) were detected during occurrence of the debris flows. The tilt and strain changes were modeled using a point load caused by debris flow deposition beside a sabo dam. Depositional weights of individual debris flow events were estimated to range from 6 to 276 kt. The total weight of the debris flows was 2,154 kt, which is approximately 5% of the total weight of volcanic ash ejected from the craters during the study period. Debris flow potential (DFP) was defined as the difference in the volcanic ash deposits along the upper stream of the river (5% of the total) and the lower stream of the river, and the temporal change of the debris flow potential was investigated. When the debris flow potential reached a level of 0.4 Mt resulting from an increase in eruptive activity, debris flows frequently occurred or large debris flows were induced during rainy seasons. The concept of debris flow potential was applied to volcanoes in Indonesia as lahar potential. After the 2010 eruption at Merapi volcano, lahar potential, perhaps, quasi-exponentially decays during the dormant period. The lahar potential of Sinabung volcano complicatedly varies because of long-term eruptivity beginning in 2014.

1. [1] M. Iguchi, “Method for real-time evaluation of discharge rate of volcanic ash – case study on intermittent eruptions at the Sakurajima volcano, Japan –,” J. Disaster Res., Vol.11, No.1, pp. 4-14, 2016.
2. [2] B. Walsh, A. D. Jolly, and J. N. Procter, “Seismic analysis of the 13 October 2012 Te Maari, New Zealand, lake breakout lahar: Insights into flow dynamics and the implications on mass flow monitoring,” J. Volcanol. Geotherm. Res., Vol.324, pp. 144-155, 2016.
3. [3] K. Kumagai et al., “Seismic tracking of lahars using tremor signals,” J. Volcanol. Geotherm. Res., Vol.183, pp. 112-121, 2009.
4. [4] A. Ratdomopurbo, “Overview of the 2006 eruption of Mt. Merapi,” J. Volcanol. Geotherm. Res., Vol.261, pp. 87-97, 2013.
5. [5] T. Mizuyama and S. Uehara, “Observed data of the depth and velocity of debris flow,” J. Japan Society of Erosion Control Engineering, Vol.37, pp. 23-26, 1984 (in Japanese).
6. [6] T. Yamada, N. Minami, and H. Mizuno, “Developing of debris flows prediction and detection technology, their future issues,” J. Japan Society of Erosion Control Engineering, Vol.50, pp. 60-64, 1998 (in Japanese).
7. [7] K. Kamo and K. Ishihara, “A preliminary experiment on automated judgment of the stages of eruptive activity using tiltmeter records at Sakurajima, Japan,” J. H. Latter (ed.), Volcanic Hazards, IAVCEI Proc. in Volcanology 1, Springer-Verlag, pp. 585-598, 1989.
8. [8] M. Iguchi et al., “Characteristics of volcanic activity at Sakurajima volcano’s Showa crater during the period 2006 to 2011,” Bull. Volcanol. Soc. Japan, Vol.58, pp. 115-135, 2013.
9. [9] M. Iguchi, “Quantitative detection of debris flow by using tilt and strain meters,” Abstracts of JpGU-AGU Joint Meeting 2017, SVC49-01, 2017.
10. [10] S. Yoshinaga et al., “Field observation of the ground vibration generated by debris flow using the transportable vibration sensor,” J. Japan Society of Computational Fluid Dynamics, Vol.68, pp. 52-59, 2015 (in Japanese).
11. [11] M. Iguchi et al., “Mechanism of explosive eruption revealed by geophysical observations at the Sakurajima, Suwanosejima and Semeru volcanoes,” J. Volcanol. Geotherm. Res., Vol.178, pp. 1-9, 2008.
12. [12] K. Ishihara, “Pressure sources and induced ground deformation associated with explosive eruptions at an andesitic volcano: Sakurajima volcano, Japan,” M. P. Ryan (ed.), Magma Transport and Storage, John Wiley & Sons, pp. 335-356, 1990.
13. [13] K. Yamashina, “Stress fields and volcanic eruption,” Bull. Volcanol. Soc. Japan, Vol.30, special issue, pp. S101-S119, 1986.
14. [14] S. Egashira and T. Itoh, “Numerial simulation of debris flow,” J. Japan Society of Computational Fluid Dynamics, Vol.12, pp. 33-43, 2004.
15. [15] Surono et al., “The 2010 explosive eruption of Java’s Merapi volcano – A ‘100-year’ event,” J. Volcanol. Geotherm. Res., Vol.241-242, pp. 121-135, 2012.
16. [16] E. de Belizal et al., “Rain-triggered lahars following the 2010 eruption of Merapi volcano, Indonesia: A major risk,” J. Volcanol. Geotherm. Res., Vol.261, pp. 330-347, 2013.
17. [17] F. Maeno et al., “A sequence of a Plinian eruption preceded by dome destruction at Kelud volcano, Indonesia, on 13 February 2014, revealed from tephra fallout and pyroclastic density current deposits,” J. Volcanol. Geotherm. Res., doi: https://doi.org/10.1016/j.jvolgeores.2017.03.002, (in press).
18. [18] S. Nakada et al., “Growth process of the lava dome/flow complex at Sinabung Volcano during 2013-2016,” J. Volcanol. Geotherm. Res., doi: https://doi.org/10.1016/j.jvolgeores.2017.06.012, (in press).
19. [19] S. Nakada, H. Shimizu, and K. Ohta, “Overview of the 1990–1995 eruption at Unzen Volcano,” J. Volcanol. Geotherm. Res., Vol.89, pp. 1-22, 1999.
20. [20] H. L. Tanaka and M. Iguchi, “Numerical Simulations of Volcanic Ash Plume Dispersal for Sakura-Jima Using Real-Time Emission Rate Estimation,” J. Disaster Res., Vol.14, No.1, 2019.
21. [21] M. Shimomura et al., “Numerical Simulation of Mt. Merapi Pyroclastic Flow in 2010,” J. Disaster Res., Vol.14, No.1, 2019.