5:15 PM - 6:45 PM
[SVC26-P14] Reconstruction of 19th century eruptive activity on the Izu-Oshima Volcanic Island by using historical documents
Keywords:Izu-Oshima Volcanic Island, Historical document, The Y0.8 eruption
The Y0.8 eruption, occurred around the 19th century on early modern Izu Oshima a volcanic island 110 km SW of Tokyo, is a medium scale eruption defined by the gray volcanic ash layer (Koyama and Hayakawa 1996). On the other hand, records of eruptions corresponding to the Y0.8 eruption in old documents are unclear, although several eruption records exist for this period. In Izu-Oshima volcano, no eruptions that mainly erupted volcanic ash, such as the Y0.8 eruption, have occurred since the Y0.8 eruption. Therefore, it is important to clarify precise eruption age of the Y0.8 eruption. The small to medium eruptions are sometimes difficult to identify pyroclastic fall deposits because of their small eruptive volume. However, there is a possibility that eruptions occurred in the historical period are recorded in historical documents. In this study, we comprehensively investigated the eruption age of the Y0.8 eruption by observation and analysis based on outcrop survey and historical documents.
We confirmed the existences of the Y0.8 pyroclastic deposit and distinct volcanic soil deposits sandwiching the Y0.8 pyroclastic deposits, indicating quiescent of volcanic activity before and after the Y0.8 eruption. Thus, we reexamined whether the layer of the Y0.8 pyroclastic deposits is the result of a single eruptive event. In the survey of historical documents, we reconsidered those used in previous studies and used original sources. In the analysis, we judged the reliability of the eruption records comprehensively based on the date, the author, and the writing background of the records.
We confirmed that the Y0.8 pyroclastic deposits preserved between the Y1 pyroclastic deposits and the 1986 scoria fall deposits within a well-developed volcanic soil deposits. Moreover, no eruptions were identified between the Y1 and 1986 eruptions. It was found that the 1846 eruption record mentioned in Yamazaki(1896) is most likely to be pre-1766 eruption record. We also found new record indicating an eruption and ash fall occurred in 1812. In addition, although the original source of the 1822-24 eruption record described by Yamazaki (1896) has been unknown so far, we found descriptions of this eruption in the "Koka 3(1846) Oshima Sashidashicho(Oshima officials’ documents in Edo period)" and "Izunokuni Oshima Sashidashicho". In addition, we found records of eruptions in 1803 and 1837-1838, which were mentioned in previous studies.
We concluded that the Y0.8 eruption was a single eruptive event. Based on the distribution and thickness of the Y0.8 pyroclastic deposits, we estimated the volume of ejecta to be 2.2 × 1010kg. Therefore Y0.8 eruption is medium-scale eruption event. Y0.8 eruption is considered to be a significantly larger eruption than other small- to medium-scale eruptions, because there are no other eruptions preserved their ejecta between the Y1 and 1986 eruptions deposits except Y0.8 eruption. Considering the distribution of Y0.8 pyroclastic deposits over the entire area of Izu-Oshima Island, Y0.8 tephra mainly composed of volcanic ash was provided in various directions during multiple eruptions. Furthermore, eruptions recorded in official local historical records are likely to have actually occurred. Based on these results, it is likely that the 1822-24 eruption record corresponds to Y0.8 eruption, as the historical records are highly reliable and indicate the eruptions with ash fall for a few years.
We confirmed the existences of the Y0.8 pyroclastic deposit and distinct volcanic soil deposits sandwiching the Y0.8 pyroclastic deposits, indicating quiescent of volcanic activity before and after the Y0.8 eruption. Thus, we reexamined whether the layer of the Y0.8 pyroclastic deposits is the result of a single eruptive event. In the survey of historical documents, we reconsidered those used in previous studies and used original sources. In the analysis, we judged the reliability of the eruption records comprehensively based on the date, the author, and the writing background of the records.
We confirmed that the Y0.8 pyroclastic deposits preserved between the Y1 pyroclastic deposits and the 1986 scoria fall deposits within a well-developed volcanic soil deposits. Moreover, no eruptions were identified between the Y1 and 1986 eruptions. It was found that the 1846 eruption record mentioned in Yamazaki(1896) is most likely to be pre-1766 eruption record. We also found new record indicating an eruption and ash fall occurred in 1812. In addition, although the original source of the 1822-24 eruption record described by Yamazaki (1896) has been unknown so far, we found descriptions of this eruption in the "Koka 3(1846) Oshima Sashidashicho(Oshima officials’ documents in Edo period)" and "Izunokuni Oshima Sashidashicho". In addition, we found records of eruptions in 1803 and 1837-1838, which were mentioned in previous studies.
We concluded that the Y0.8 eruption was a single eruptive event. Based on the distribution and thickness of the Y0.8 pyroclastic deposits, we estimated the volume of ejecta to be 2.2 × 1010kg. Therefore Y0.8 eruption is medium-scale eruption event. Y0.8 eruption is considered to be a significantly larger eruption than other small- to medium-scale eruptions, because there are no other eruptions preserved their ejecta between the Y1 and 1986 eruptions deposits except Y0.8 eruption. Considering the distribution of Y0.8 pyroclastic deposits over the entire area of Izu-Oshima Island, Y0.8 tephra mainly composed of volcanic ash was provided in various directions during multiple eruptions. Furthermore, eruptions recorded in official local historical records are likely to have actually occurred. Based on these results, it is likely that the 1822-24 eruption record corresponds to Y0.8 eruption, as the historical records are highly reliable and indicate the eruptions with ash fall for a few years.