**1. Introduction**

Explosive volcanic eruptions that do not contain juvenile materials have been called by several names, e.g., steam, phreatic or hydrothermal eruption. Although the terms phreatic eruption and hydrothermal eruption have similar meanings, the two are sometimes confused [1]. Hydrothermal eruption is often used in the geothermal fields in New Zealand and in the USA. The eruption termed here has both shallow and deep origins [2,3]. The general nomenclature for steam-driven eruptions is categorized into two types. First, phreatic eruption, which is an eruption that is caused by the heating and flashing of water produced when magma comes in contact with water. This results in the ejection of only country rock or overburden, i.e., no juvenile magmatic material. Second, hydrothermal

eruption, which is an eruption that ejects at least some solid material and whose energy derives solely from heat loss and phase changes in a convecting hot water or steam-dominated hydrothermal system [3]. Steam-driven events are the most common phenomena in volcanic activity and are not easy to forecast, although there are many investigations on their precursory signals [1,4]. To explain steam-driven eruptions that contain non-juvenile materials, several mechanisms have been considered. The current perspective is that the eruption is triggered by the decompression driven boiling of ground hydrothermal fluids [2,3,5]. This type of mechanism is referred to as an abrupt departure from equilibrium conditions and has been experimentally produced by engineering researchers in Japan [6–8]. The 2014 eruption at Ontake Volcano in central Japan is considered one example of an abrupt departure from equilibrium conditions [9–11]. However, the true trigger of the eruption have not ye<sup>t</sup> been made clear. One of the triggers of eruption in general may be pressurization of a hydrothermal system heated by input of magmatic fluids from the depths (bottom–up gas/fluid pressurization) [12]. In this case, an eruption is caused by the fluid pressure exceeding the lithostatic pressure [3,13]. Another trigger is decompression of the hydrothermal reservoir for external reasons. For example, a water level drop, drilling or a landslide can cause the flashing from the top to downward (top–down flashing) [12,14–16]. These processes are not simple, depending on the material heterogeneity under the ground, groundwater distribution and caprock development. In fact, the caprock structure is considered important for storing hydrothermal liquids. The caprock formation of silica or hydrothermal minerals is considered to be the layer that controls the sealing and release of hydrothermal fluids within di fferent depths of a volcanic edifice e.g., [3,17–19]. In recent years, magnetotelluric (MT) surveys revealed the existence of highly conductive layers in the shallow parts of volcanic edifices, which are considered to be hydrothermally altered zones with an abundance of clay minerals such as smectite [20–24]. In Ebinokogen Ioyama Volcano, this conductive layer is located 200 to 700 m below the surface, with a pressure source below it [25]. It has been suggested that this layer is the low permeability.

Many casualties have been previously reported from steam-driven (phreatic) eruptions in Japan, including from events at Adatara Volcano in 1900, Kusatsu-Shirane Volcano in 1932, Aso Volcano in 1958 and Ontake Volcano in 2014 [10,26–28]. Meanwhile, geothermal areas—with the potential for steam-driven eruptions—often develop beneath a shallower hydrothermal system in a volcano and are beneficial as sightseeing spots. In the past, many laborers mined sulfur in these fumarolic areas in Japan. Expeditious information on the resumption of volcanic activity in such geothermal areas is crucial, and lack of timeliness has grave consequences. Sometimes, the occurrence of an explosive steam-driven eruption is preceded by a few signs. However, in other cases, an eruption may fail, even during an increase in geothermal activity. There are various precursory phenomena and event sequences that precede steam-driven eruptions: it is necessary to clarify what causes such a variety.

In Kirishima Volcano Group, steam-driven eruptions occurred at Shinmoedake in 1959, 1991–1992, 2008 and 2010 [29–31]. And they occurred at Ebinokogen Ioyama in 1768 [32]. These eruptions di ffered in impact and chronology. For example, at Shinmoedake Volcano, a phreatic eruption in 1959 was a strong explosive event with 8.6 million tons of ejecta in a short time and was accompanied by a blast [33]. However, the 1991–1992 eruption was extremely small, producing 360 tons of ejecta and leaving climbers unharmed on the summit crater rim [30]. Steam-driven eruptions have been dominant at Ebinokogen Ioyama Volcano over the long term, and there has been geothermal activity around there in recent times [32,34,35]. At Ebinokogen Ioyama located near tourist spots and mountain trails, volcanic activity has been observed since the 1910s. We recorded the activity just before the latest steam-driven eruption at Ebinokogen Ioyama Volcano in April 2018. Here, we discuss the sequence of geothermal activity and the possible phenomenology, especially focusing on the hydrothermal aquifer depth and the precursory signals of the eruption.

We use the above-mentioned terms of phreatic eruption and hydrothermal eruption, as suggested by Brown and Lawless [3]. Since juvenile materials were not found in the products from the 2018 Ebinokogen Ioyama eruption, hydromagmatic and phreatomagmatic eruptions are not considered here. In this study, a fumarolic jet is described as the phenomenon in which geothermal jetting occurs with jet-like roaring sounds [36], and the vent of such a fumarole is referred to as a jet fumarole vent. The term "fumarole area" refers to a place that contains many relatively weak to strong jet fumaroles. A small hole from which hot muddy hydrothermal fluids well up is called a mud pot [37], and a larger irregular-shaped pool with hydrothermal fluids is called a hydrothermal pond. The time description in this study is the Japan Standard Time (JST), UTC+9.
