[S-VC35] Integrated Program for Next Generation Volcano Research and Human Resource Development

[SVC35-P13] Proposal of the procedure of creating the time – accumulated eruption volume step diagram

*Mitsuhiro Nakagawa¹, Yasuo Miyabuchi², Takeshi Hasegawa³, Ryuta FURUKAWA⁴, Shimpei Uesawa⁵, Masashi NAGAI⁶ (1.Department of Natural History of Science, Faculty of Science, Hokkaido University, 2.Center for Water Cycle, Marine Environment and Disaster Management, Kumamoto University, 3.Department of Earth Sciences, College of Science, Ibaraki University, 4.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 5.Central Research Institute of Electric Power Industry, 6.Volcano Research Department, National Research Institute for Earth Science and Disaster Resilience)

Keywords:Integrated Program for Next Generation Volcano Research, Time – accumulated eruption volume step diagram, Step diagram, Volcanic eruption, Eruption history

The time – accumulated eruption volume step diagram (hereinafter as the step diagram), which shows the relationship between the eruption volume and the eruption intervals, can comprehensively show the eruption history of each individual volcano. It is important for evaluating the long-term activity of volcanoes. From that point of view, in the subtheme C-2: “Medium-to-long term forecasting of volcanic eruptions through investigation of eruptive history, and creation of Eruption Event Tree based on investigation of eruption sequence” of the project, “Integrated Program for Next Generation Volcano Research”, we first create high precise step diagrams for each volcano. Although many step diagrams have been created since Nakamura (1964) first published the diagram for Izu-Oshima volcano, the formats varies depending on each researcher and has not been standardized. However, in order to evaluate the activity level and perform the medium- to long-term eruption forecasting based on the same standard, it is necessary to create the step diagram with similar quality in the same format. Therefore, the subtheme C-2 formed a working group to discuss guidelines for creating step diagrams and eruption event trees, as well as database for these diagrams. In this presentation, we introduce the creation procedure of the step diagram summarized by the working group. The definitions of eruptions, eruption dates, etc., which form the basis of database creation, are reviewed according to the following criteria. (1) An eruption is an activity that releases tephra and volcanic edifices as ejecta, and a single eruption or a series of eruption is recognized as one eruption. The activity producing an edifice collapse alone is also regarded as one eruption. If eruptions occur sporadically over a period of time and there is a time break of one year or more between eruptions, each eruption is considered as a distinct eruption. (2) If tephra and volcanic edifices, such as lava flows, domes, pyroclastic cones, etc., cannot be correlated, these deposits are treated as eruptive deposits derived from distinct eruptions. (3) Eruption products whose dates cannot be determined well are considered as those of activity over a certain time span in the step diagram. In the database, the median value of the range is indicated as the eruption age, and the range is noted in the remarks. (4) Data on individual eruptions are classified into the following four ranks. A: eruptions that have been recorded, B: eruptions that have no records but whose age and sequence can be clarified by geological methods, C: eruptions that have no detail informations but whose eruptive center and deposits can be mapped on the geological map, D: eruptive deposits that can be recognized as products of a certain active stage. For each eruption, database includes the name od eruption (or stage), the data rank, eruption age (determined method and chronology), activity period, eruption style, types of eruptive deposits, eruption volume, petrological features, etc. For these data, detailed guidelines are also provided. The formats of the step diagram are as follows. (1) A set of two types of the step diagram is created, such as long-term and short-term. The long-term step diagram covers the period from the beginning of the activity of a volcano to the present and is created using data rank A to D above. On the other hand, the short-term step diagram covers a relatively young certain period up to the present, and is the basis for creating an eruption event tree. For each eruption during the short-term period, it is necessary to have a relatively high level of clarification of the age and eruption sequence, so the above A and B rank data should be used as much as possible to create the step diagram. (2) As much as possible, the step diagram for each volcano is created in sets of the following three types. They are tephra only, volcanic edifice (lava flows, lava domes, pyroclastic cones, etc.) only, and a combination of both.