Lahar is a general term for the rapid flow phenomenon of volcanic debris and water mixtures occurring around volcanoes (Smith and Fritz, 1989). Lahars have occurred at numerous volcanoes worldwide, ranking among the most hazardous volcanic phenomena, resulting in approximately 30,000 deaths during the past century, second only to pyroclastic flows (Schmincke, 2004). The devastating impact of lahars is evident; for example, during the 1991 eruption of Mount Pinatubo in the Philippines, approximately 50,000 people were evacuated due to lahars (Vallance, 2000). Another instance occurred during the 1985 eruption of Nevado del Ruiz in Colombia, where about 25,000 fatalities and the burial of approximately 5,000 houses resulted from lahars (Vallance, 2000). Lahars not only cause human casualties but also significantly impact existing structures and infrastructure, making them a crucial focus of volcanic disaster management. To mitigate lahars effectively, a comprehensive understanding of their scale, distribution, flow behavior, and causes is essential, highlighting the significance of detailed studies on past lahars for volcanic hazard mitigation.
Because lahar is a general term, it can be subdivided into several types based on rheology and sediment concentration, proportion of clay component, and their trigger events. Firstly, with respect to the rheology and sediment concentration, lahar includes several flow types such as debris flow, hyperconcentrated flow, streamflow, and muddy streamflow (Smith and Fritz, 1989; Smith and Lowe, 1991; Vallance and Iverson, 2015). Secondly, with respect to their proportion of clay component, lahar can be subdivided into clay-rich cohesive debris flow and clay-poor cohesionless lahar (Vallance and Iverson, 2015). In general, clay-rich cohesive debris flows are caused by edifice collapse of water-saturated altered volcanoes (Vallance and Iverson, 2015) and reworking of clay-rich volcanic materials, such as phreatic and phreatomagmatic eruption products (e.g. Minami et al. 2015 and 2019; Kataoka et al. 2018). Finally, lahars are classed based on their trigger events such as syn-eruptive (e.g. snow-melting: Vallance and Iverson, 2015; Uesawa, 2014) and post-eruptive types (e.g. rain-fall triggered, rain-on-snow, slush, lake breakout: Manville and White 2003; Manville and Cronin 2007; Vallance and Iverson 2015). These criteria about lahars can overlap with each other. For example, the deposit of a lahar caused by rain-fall after a magmatic eruption, generally consists of fresh juvenile materials and is poor in clay component. Such a deposit may be termed a syn-eruptive, cohesionless, rain-fall triggered, debris-flow and/or hyperconcentrated flow deposit.
These criteria can be useful to recent and historical events such as the dam break lahar on March 2007 at Ruapehu Volcano (Manville and Cronin, 2007), rain-fall triggered, rain-on-snow and slush lahars at Ontake Volcano after the 2014 eruption (Kataoka et al. 2018 and 2019), rain-fall triggered lahars at Chokai Volcano after the 1801 eruption (Minami et al. 2019). Few previous studies have reported critical triggers of lahars (i.e., the origin of the water source) based only on geological records (Capra and Macías 2002; Giordano et al. 2002). From studying the facies of a deposit, the water/debris ratio can be semi-quantitatively defined, which allows for interpretations to be made about the water source. The origin of water generally is not directly recorded in lahar deposits, thus it is important to utilize studies of the geomorphology and geology of the summit area of an active volcano to constrain the water source.
The target area of this study is Akita-Yakeyama Volcano (elevation: 1366 m asl), situated in the Northeast Honshu Island, Japan (Fig. 1). This stratovolcano experienced a hydrothermal eruption with accompanying landslides near Sumikawa Onsen at the northeast foot of the mountain on May 11, 1997, and a phreatic eruption Karanuma vent near the summit on August 16, 1997 (Fig. 2: Ohba et al., 2007). Since then, volcanic earthquakes and volcanic tremors have continued to occur in the vicinity of the summit, indicating its active state (Japan Meteorological Agency, 2013). This study focuses on a prehistoric crater lake breakout lahar, which is reported to be more extensive than the 1997 lahars (Minami et al. under review), implying that any potential future lahar disasters may be more significant than those observed in 1997. Investigating the occurrence mechanisms and scales of lahars through geological records is vital not only for understanding past volcanic disasters but also for disaster preparedness, particularly concerning historical volcanic events without written records.
Geological Setting and Historical Eruptions
Akita-Yakeyama Volcano's activity can be divided into three major periods: the early period (520 ka to 330 ka), the middle period (100 ka to 80 ka), and the younger period (10 ka to present) (Ohba 1991; Minami et al. under review). The study area includes the current summit of Akita-Yakeyama, the vicinity of the crater lake, and the northward-flowing watersheds of the Yunosawa, Nakanosawa, Kumazawa, and Yoneshiro rivers (Fig. 2). In this region, remnants of volcanic deposits from the recent period are found around the current summit and the crater lake, including Nagoritoge pyroclastics (age > 6 ka: Minami et al., 2023), Sakebisawa pyroclastics, Togamori-Nishi lava dome, and Onigajo lava dome (ca. 2 ka; Ohba, 1991; Minami et al., 2023).
The Nakanosawa lava and Sakebisawa lava, formed during the middle period, are distributed around the Yunosawa and Nakanosawa watershed (Ohba, 1991), while the Kumazawa Formation from the Neogene, mainly comprising subaqueous pyroclastics, is distributed at the confluence of Nakanosawa and Kumazawa Rivers (Suto, 1992). North of the confluence of Kumazawa and Akagawa Rivers, the Neogene Oinokawa Formation, primarily composed of mudstone, is found (Fig. 2: Suto, 1992).
The historical eruptive activity of Akita-Yakeyama Volcano documented by written records were reviewed by Minami et al. (2023). The most reliable eruptive events are dated to 1678, 1868, the 1890s, 1949, 1951, and May 11 and August 16, 1997. The occurrence of eruptions in 807 AD and 1867 AD has been ruled out, while the reliability of events in 1929, 1948, and 1957 remains uncertain. In the 1997 eruption, lahars were reported to have occurred on May 11, triggered by a landslide accompanying a hydrothermal eruption near Sumikawa hot spring at the foot of the volcano (Fig. 1). Initially, it was observed that rockslide debris flowed down the cliff, forming a debris avalanche-like flow, traveling approximately 450 m from the landslide cliff (Tsukamoto, 1997). At least three volcanic plumes were confirmed before and after the rockslide event (Tsukamoto, 1997). Lahars were also reported to have occurred before and after the rockslide (Tsukamoto, 1997). The resulting lahars devastated the facilities of Sumikawa hot spring, located on the northeastern foothills of Akita-Yakeyama, and crossed three rivers, Sumikawa, Akagawa, and Kumazawa, over a distance of approximately 1400 m in a straight line. This lahar caused the closure of National Route 341, connecting Kazuno City and Senboku City in Akita Prefecture, significantly impacting the livelihood and tourism industry of the local residents. Subsequent investigations by Tsukamoto (1997) revealed the formation of several flow hills along the course of Sumikawa and Akagawa Rivers, but these geomorphic traces have since been largely obscured due to artificial modifications from the construction of sediment control dams and multiple occurrences of post-eruption debris flows.
The eruption in August 1997 occurred at the Karanuma crater (Ohba et al., 2007). The larger Yunuma crater contains a permanent volcanic lake and exhibits vigorous fumarolic activity and hot spring discharge in its vicinity. As the Onigajo lava dome is present within the Yunuma crater, it is believed that the eruptions associated with the Yunuma crater occurred before the formation of the Onigajo lava dome. On the other hand, the Karanuma crater appears to be a part of the Onigajo lava dome, suggesting that eruptions related to the Karanuma crater occurred after the formation of the Onigajo lava dome (Ohba, 1991).
Geology and Geomorphology of the Summit Area
Akita-Yakeyama Volcano features a large depression near its summit, measuring approximately 1 km in diameter. The geomorphology of this depression is a combination of the almost orbicular western part (peaks marked by the dashed yellow line in Fig. 3) and the irregular-shaped eastern part (peaks marked by the dashed blue line in Fig. 3). The western part corresponds to the rim of Yunuma crater (diameter ~ 100 m). The eastern part can be interpreted as resulting from erosion. The Yunosawa-Nakanosawa River drains the summit crater lake from its northern wall, although contact between the river and the crater lake has been artificially modified by a trench formed in the1960s during sulfur mining (orange line in Fig. 3). Before this artificial trench was formed, the water level of the crater lake was much higher than the present level (1240 m asl), although the past water level was not officially documented.
Within the summit depression are the Yunuma crater (diameter ~ 100 m) and the Onigajo lava dome. Additionally, a smaller crater named Karanuma (diameter ~ 80 m) is located on the northern edge of the Onigajo lava dome (Fig. 3). The Yunuma crater contains a stable volcanic lake and exhibits vigorous fumarolic activity in its surroundings. The lake water appears turbid due to suspended white altered materials supplied from nearby hydrothermal alteration zones and the crystallization of sulfate minerals and sulfur from volcanic gases (details provided later; Fig. 3). The formation of organic soils is nearly absent in the area. Sumi and Takashima (1978) reported a radiocarbon date of 980 ± 100 year BP for charcoal fragments found in lacustrine sediments from an outcrop in the northeastern part of the Yunuma crater, suggesting that the formation of this crater lake occurred at least 1000 years ago.
Yunosawa-Nakanosawa River is confluent with Kumazawagawa River at a distance of 3.7 km north from the crater lake (Loc. 3 on Figs. 1 and 4). Kumazawagawa River originates from Ohbayachi swamp located northeast of the volcanic edifice (Fig. 1). The gradient of slope of Yunosawa-Nakanosawa River (from the crater to Loc. 3; Fig. 1) is 12.8 m elevation change per 100 m (Fig. 4). On the other hand, the gradient of slope of Kumazawagawa River is 9.1 m elevation change per 100 m (Fig. 4), indicating that Yunosawa-Nakanosawa River has a steeper gradient. After Yunosawa-Nakanosawa River and Kumazawagawa River are confluent (Loc. 3 in Fig. 4), their gradient of slope is 5.4 m elevation change per 100 m (Fig. 4).