We conducted an experiment on the impact absorption effect of pumice layer on wooden and reinforced concrete (RC) structures using a simulated ballistic ejecta with a mass of 2.66 kg. In this study, we used artificial pumice with a composition similar to that of natural pumice. The result of experimentation, we confirmed the impact absorption effect of artificial pumice layer about both structures, and increasing the thickness of the artificial pumice layer improves the effect. However, with wooden structures, there are limits to the dead load, so care must be taken about the weight of the artificial pumice. On the other hand, RC structure has enough dead load capacity, therefore, it is more effective to cover it with artificial pumice layer as much as possible. In conclusion, we recommend the usage of pumice layer as a low-cost reinforcement method of impact resistance against ballistic ejecta.

Mitsudake Volcano is located in the northern part of Senjogahara Marsh, Nikko, Tochigi Prefecture, and comprises several morphologically fresh lava domes of unknown age. In this study, pit excavation surveys and ¹⁴C dating of the eolian strata overlying the lava domes were carried out to determine the upper age of the lava domes. Existing drill cores that penetrated the lava were re-described and used for ¹⁴C dating, which has resulted in refining the eruption age of Mitsudake Volcano to around 5.0 ka. A new tephra from this volcano was discovered in the nearby area and named the Mitsudake-1 Pyroclastic Fall Deposit (Mt-1). This tephra contains vesicular glassy particles, and its chemical composition indicates that it originated from the magma of Mitsudake Volcano. The ¹⁴C age of Mt-1 is around 5.0 ka, which is the same age as the lava domes. The morphology of the glassy particles suggests rapid quenching and Mt-1 is interpreted to be the product of a phreatomagmatic eruption.

Reinforced concrete shelters against ballistic ejecta are installed at active volcanoes in Japan. The impact resistance ability has been researched and developed, but the structural integrity after the impact has not been sufficiently evaluated. Thus, we proposed non-destructive inspection methods with infrared thermography as an effective integrity method in this study. The prepared specimen was assumed to be a box culvert RC shelter impacted by ballistic ejecta. There were two types of specimens: an RC specimen directly impacted by ejecta before the inspection, and an RC specimen impacted by ejecta through an absorbent material. It was not easy for a visual test to detect the cracks and flaws caused by the impact. Firstly, a conventional infrared thermographic testing with halogen spot lamp heating was carried out. This is a measurement technique based on an adiabatic temperature field. In raw thermal images without image processing, false detections occurred due to uneven temperature distribution caused by non-uniform emissivity of the concrete surface, and cracks and flaws were not clearly visible. However, by subtracting the initial thermal image before the heating from the thermal image after the heating, cracks can be clearly extracted from the temperature difference image. Next, a sequential phase analysis method was applied to the thermal data. This method is a type of phase analysis technique that can reduce the influence of background reflections and non-uniform emissivity. In addition, by sequentially using thermal data during the experiments, it is possible to reduce the processing time and identify the depth of flaws. By applying this method to the RC specimen that was impacted by ejecta, we could quantitatively identify the flaws that occurred between the reinforced steel and the concrete. The destruction of RC shelter impacted by ballistic ejecta can be classified into two mechanisms: a vertical crack generated by bending deformation, and a diagonal crack and a flaw caused by a spall fracture. The proposed infrared thermography methods can detect the cracks and the flaws and identify those two mechanisms.

Miike Lake is a maar about 1 km in diameter located in the southeastern part of Kirishima Volcano, and was formed by one of the largest Plinian eruption of the volcano. Here, we report the ¹⁴C date of 4078±28 BP of the charcoal (240220 A-C01) found in the Miike tephra. The calibrated age is consistent with the previous reported dates from humic soils just above and below the Miike tephra. The sampled outcrop (MZ2419) is located approximately 0.8 km south of Miike crater rim. We can observe well-vesiculated red pumice, indicating high-temperature oxidation, in the stratigraphic level from which we collected 240220 A-C01. In addition, there is no strong evidence of organic substances contamination due to disturbance of the layers after the tephra deposits has settled. These characteristics suggest that 240220 A-C01 was carbonized by deposition of this tephra under high-temperature condition. Therefore, the ¹⁴C age obtained in this study is considered to be a fairly accurate eruption age of Miike tephra. The reported age will be useful in tephrochronology in southern Kyushu.