The magma discharge step-diagram shows the relationship between time and cumulative eruption volume/mass of a volcano and is thought to be useful not only for evaluating volcanic activity but also for forecasting medium- to long-term future activity. However, the procedure and method for making the step-diagram has not been well described. Therefore, a unified procedure and method for making the step-diagram is proposed. First, the spatial distribution and activity period of eruption centers of a target volcano or volcano group should be defined. Then, the database that covers all the data of the eruption episode and/or eruptive stage of the volcano, such as age/duration of eruptive activity, mode of eruption, eruption volume and so on, is completed. Based on the database, several types of the step-diagrams can be created according to the purpose. Finally, it is recommended to publish both the diagram and the database as a set. As an example, we present the procedure to create the database and two types of step-diagrams of Usu volcano, long- and short-term ones. The short-term step-diagram shows the temporal change of the magma discharge rate during the historical activity since the 17^th century, suggesting that the rate has been obviously decreased during the last 100 years of activity.

Sakurajima Volcano, located in the southern part of Aira caldera, southern Kyushu, Japan, has caused at least 17 major eruptive events since about 26 cal ka BP, after the Aira caldera-forming eruption. The Satsuma eruption in 12.8 cal ka BP is the largest activity of this volcano. This study describes the distribution and stratigraphy of the Satsuma eruption deposits based on facies of deposits and component characteristics. The Satsuma eruption deposits are divided into 17 units (P14A to P14Q, in ascending order): P14A to P14E (Group 1: total tephra volume 7.4 km³ (based on Fierstein and Nathenson (1992)) are distributed around southern Kyushu, while P14F to P14 K (Group 2: total 0.48 km³) and P14 L to P14Q (Group 3: total 0.73 km³) are distributed in the west (Satsuma Peninsula) and east (Osumi Peninsula) from Sakurajima Volcano, respectively. The Group 1 shows a transition from a large-scale plinian eruption to a pyroclastic density current (PDC)-generating eruption, followed by widespread poorly-sorted fine ash including accretionary lapilli, and finally another plinian eruption. The PDC deposit (P14B) reaches an approximate radius of 10‒18 km from Sakurajima Volcano. The Group 2 consists mainly of small pumiceous deposits and well-sorted coarse ash deposits, which represent two repetitions of vulcanian and (sub) plinian eruption phases. The Group 3 are composed of relatively small pumiceous deposits, well-sorted coarse- and poorly-sorted fine ash in alternating layers, which indicates the occurrence of vulcanian, (sub) plinian, and phreatomagmatic eruptions. In addition, the Groups 2 and 3 characteristically contain fresh glassy lithics and a few dark-colored vesiculated particles. These suggest that the latter half of Satsuma eruption underwent a variety of eruptive styles due to differences in conduit processes and other factors.