Physical vulnerability assessment of settlements in lahar hazard prone areas along the river originated from Merapi Volcano

Merapi Vulcano is one of the most active volcanoes in the world. This volcano has a secondary hazard that is lahar flow. This research refers to the 2010 eruption of Merapi Vulcano in Indonesia. Eventhough there was a phreatic eruption in 2018, it did not increase the deposition of lahar material on its slopes. The communities living along the lahar prone rivers have various vulnerabilities including physical, social, economic and environmental vulnerability. This research aims to determine the physical vulnerability distribution of settlement in lahar hazard prone areas alongside Pabelan River. This study is a descriptive quantitative research using secondary data and spatial analysis. The results of this research show that the high vulnerability distribution covers the area of Mungkid Sub-district and Sawangan Sub-district as many as 5 clusters or equal to 7.49% of the research area. The medium level of physical vulnerability is Dukun Sub-district, Mungkid Sub-district and Muntilan Sub-district as many as 26 clusters or equal to 41.27% of the research area. The low physical vulnerability of settlements is scattered in Dukun Sub-district, Mungkid Sub-district, Muntilan Sub-district and Sawangan Sub-district as much as 32 clusters or equal to 50.79% of the research area. It can be concluded that the physical vulnerability of settlements in lahar hazar prone along the Pabelan river after the 2010 eruption of Merapi Vulcano is dominated by low vulnerability.

Delineating a Volcanic Aquifer Using Groundwater-induced Gravity Changes in the Tatun Volcano Group, Taiwan

The Tatun Volcanic Group (TVG) is an active volcano that could cause volcanic hazards in northern Taiwan. The latest phreatic eruption of the TVG occurred some 6000 years ago. Understanding the state of groundwater around the TVG can be a crucial step towards effectively assessing the risk of phreatic explosion by providing information about the sources of groundwater and the media it flows. We measured gravity changes at a superconducting gravity station and several groundwater-sensitive sites to examine the way the groundwater altered the gravity values around the TVG. Groundwater-induced gravity changes are simulated by two hydrological models (A and B). Both models show coherent seasonal variations in groundwater level and gravity value in the center of the TVG (Chintiengang). This coherence indicates inter-connected porous media for free groundwater flows below Chintiengang. However, inconsistencies between the modeled and observed gravity changes occurred in the eastern part of the TVG, suggesting here highly heterogeneous formations with fractures and barriers may exist below Chihsinshan and Dayoukeng. The gravity consistencies and inconsistencies between the observations and the models are used to delineate a volcanic aquifer, which can provide additional information for assessing the probability of a potential phreatic eruption over the TVG.

The 2018 phreatic eruption at Mt. Motoshirane of Kusatsu–Shirane volcano, Japan: eruption and intrusion of hydrothermal fluid observed by a borehole tiltmeter network

We estimate the mass and energy budgets for the 2018 phreatic eruption of Mt. Motoshirane on Kusatsu–Shirane volcano, Japan, based on data obtained from a network of eight tiltmeters and weather radar echoes. The tilt records can be explained by a subvertical crack model. Small craters that were formed by previous eruptions are aligned WNW–ESE, which is consistent with the strike of the crack modeled in this study. The direction of maximum compressive stress in this region is horizontal and oriented WNW–ESE, allowing fluid to intrude from depth through a crack with this orientation. Based on the crack model, hypocenter distribution, and MT resistivity structure, we infer that fluid from a hydrothermal reservoir at a depth of 2 km below Kusatsu–Shirane volcano has repeatedly ascended through a pre-existing subvertical crack. The inflation and deflation volumes during the 2018 eruption are estimated to have been 5.1 × 105 and 3.6 × 105 m3, respectively, meaning that 1.5 × 105 m3 of expanded volume formed underground. The total heat associated with the expanded volume is estimated to have been ≥ 1014 J, similar to or exceeding the annual heat released from Yugama Crater Lake of Mt. Shirane and that from the largest eruption during the past 130 year. Although the ejecta mass of the 2018 phreatic eruption was small, the eruption at Mt. Motoshirane was not negligible in terms of the energy budget of Kusatsu–Shirane volcano. A water mass of 0.1–2.0 × 107 kg was discharged as a volcanic cloud, based on weather radar echoes, which is smaller than the mass associated with the deflation. We suggest that underground water acted as a buffer against the sudden intrusion of hydrothermal fluids, absorbing some of the fluid that ascended through the crack.

Total mass estimate of the January 23, 2018, phreatic eruption of Kusatsu-Shirane Volcano, central Japan

On January 23, 2018, a small phreatic eruption (VEI = 1) occurred at the Motoshirane Pyroclastic Cone Group in the southern part of Kusatsu-Shirane Volcano in central Japan. The eruption ejected ash, lapillus, and volcanic blocks from three newly opened craters: the main crater (MC), west crater (WC), and south crater (SC). Volcanic blocks were deposited up to 0.5 km from each crater. In contrast, the ash released during this eruption fell up to 25 km ENE of the volcano. The total mass of the fall deposit generated by the eruption was estimated using two methods, yielding total masses of 3.4 × 104 t (segment integration method) and 2.4 × 104 t (Weibull fitting method). The calculations indicate that approximately 70% of the fall deposit was located within 0.5 km of the craters, which was mainly attributed to the low height of the eruption plume.

The 2018 phreatic eruption at Mt. Motoshirane of Kusatsu–Shirane volcano, Japan: Eruption and intrusion of hydrothermal fluid observed by a borehole tiltmeter network

We estimate the mass and energy budgets for the 2018 phreatic eruption of Mt. Motoshirane on Kusatsu–Shirane volcano, Japan, based on data obtained from a network of eight tiltmeters and weather radar echoes. The tilt records can be explained by a subvertical crack model. Small craters that were formed by previous eruptions are aligned WNW–ESE, which is consistent with the crack azimuth modeled in this study. The direction of maximum compressive stress in this region is horizontal and oriented WNW–ESE, allowing fluid to intrude from depth through a crack with this orientation. Based on the crack model, hypocenter distribution, and MT resistivity structure, we infer that fluid from a hydrothermal reservoir at a depth of 2 km below Kusatsu–Shirane volcano has repeatedly ascended through a pre-existing subvertical crack. The inflation and deflation volumes during the 2018 eruption are estimated to have been 5.1 * 10 5 and 3.6 * 10 5 m 3 , respectively, meaning that 1.5 * 10 5 m 3 of expanded volume formed underground. The total heat associated with the expanded volume is estimated to have been ≥10 14 J, similar to or exceeding the annual heat released from Yugama Crater Lake of Mt. Shirane and that from the largest eruption during the past 130 yr. Although the ejecta mass of the 2018 phreatic eruption was small, the 2018 MPCG eruption was not negligible in terms of the energy budget of Kusatsu–Shirane volcano. A water mass of 0.1–2.0 * 10 7 kg was discharged as a volcanic cloud, based on weather radar echoes, which is smaller than the mass associated with the deflation. We suggest that underground water acted as a buffer against the sudden intrusion of hydrothermal fluids, absorbing some of the fluid that ascended through the crack.