10:45 AM - 12:15 PM
[SVC31-P04] Distribution and sources of the tephra fall deposits scattered by the phreatic eruptions since the mid-Holocene at the Kurikoma Volcano, northeastern Japan
Keywords:Phreatic eruption, Tephra fall, Volcanic hazard, Holocene, Kurikoma volcano
Phreatic eruptions occurs in some smaller scales and more frequently than magmatic eruptions generally, but they can cause significant hazards depending on location and timing of the eruption, as in the 2014 eruption of the Ontake volcano. It is necessary to construct the eruption histories in higher-resolution at each volcano in the Tohoku region where many phreatic eruption events have occurred in historic age in order to evaluate its activity. At the Kurikoma volcano, located in the central Ou Mountains, although the number of phreatic eruption events and their ages in the Holocene have been briefly reported in previous studies, their spatial scale and the distribution of ash fall associated with each eruption have been unrevealed. We estimate the source craters of each event of phreatic eruption based on the age, distribution of tephra fall deposits associated with phreatic eruption events and their thickness, and reconstruct the phreatic eruption history of the Kurikoma volcano since the mid-Holocene.
Based on topographic maps and aerial photographs generated from 2 m and 5 m mesh DEMs, at least 50 crater-like concave features were identified in the northern foot of Kurikoma volcano. We found glassy tephra beds, and mud tephra beds. Mud tephra is consisted with lapilli of does not contain volcanic glass shards is probably phreatic eruption fall deposit scattered from above craters layers get stuck in the soil and to estimate their ages. Result of that, at least two layers of PED were identified above the Towada-a tephra (To-a; AD 915), at least four layers between the To-a and the Towada-Chuseri tephra (To-Cu; 5,986 to 5,899 cal BP), and at least one layer below the To-Cu. The PED after the To-a fall was determined to be due to the 1744 eruption and the 13th century eruption based on previous studies and soil thickness. The PEDs prior to the To-a fall were deposited by eruptions at approximately 1 ka, 3 ka, 4 ka, 6 ka, and 7 ka based on radiocarbon dates of the soils immediately.
These results indicate that phreatic eruptions have occurred at Kurikoma volcano at least eight to nine times during the past 8,000 years and that ash fall has occurred, including the Showa Lake phreatic eruption event in 1944. In particular, a total of three phreatic eruptions have occurred in the last 1,000 years, indicating that the frequency of these eruptions is increasing. However, because phreatic eruptions are small in scale, the thickness of the sediments is thin, and the older layers are more easily lost due to erosion etc., it is possible that phreatic eruptions occurred at the same frequency in the past, and further detailed investigation is needed. We also created isopach maps of the falling tephra from each phreatic eruption to estimate the source of the eruption, the range of falling tephra, and the spatial distribution of the tephra. As a result, the source of each PED is estimated to be the eastern crater group for the 13th century eruption, the western crater group for the 1744, 3 ka, and 4 ka phreatic eruptions, the western crater group or Konigozawa crater for the 7 ka phreatic eruption, and the southern foot of Kurikoma volcano for the 1 ka phreatic eruption. Furthermore, the eruptions that occurred in 1744, about 4 ka and 7 ka, respectively, may have been relatively large phreatic eruptions that left a descending tephra layer at least 6 cm thick. This indicates that relatively large phreatic eruptions have occurred at Kurikoma volcano every 3,000 to 4,000 years. Furthermore, the ash falls from a phreatic eruption of about 4 ka, which is considered to be the largest at Kurikoma Volcano, can be estimated to be about 1.85 × 106 m3 based on the empirical formula by Hayakawa (1985), which is the same size or larger than the 2014 Ontake eruption, when the tephra outburst was estimated to be about 106 m3.
From the above, the scale and frequency of phreatic eruptions at Kurikoma volcano since the middle Holocene are comparable to those of other volcanoes. Given the fact that climbers and other visitors enter the area around the crater of Kurikoma Volcano, this suggests that there is a risk of a large-scale disaster in the event of a phreatic eruption. After this, it is necessary to increase the number of sites where tephra is described and to clarify the range of ash fall and isopach lines in more detail, taking into account the effects of erosion and other factors.
This study was financially supported by Mt. Kurikoma Area Geopark.
Based on topographic maps and aerial photographs generated from 2 m and 5 m mesh DEMs, at least 50 crater-like concave features were identified in the northern foot of Kurikoma volcano. We found glassy tephra beds, and mud tephra beds. Mud tephra is consisted with lapilli of does not contain volcanic glass shards is probably phreatic eruption fall deposit scattered from above craters layers get stuck in the soil and to estimate their ages. Result of that, at least two layers of PED were identified above the Towada-a tephra (To-a; AD 915), at least four layers between the To-a and the Towada-Chuseri tephra (To-Cu; 5,986 to 5,899 cal BP), and at least one layer below the To-Cu. The PED after the To-a fall was determined to be due to the 1744 eruption and the 13th century eruption based on previous studies and soil thickness. The PEDs prior to the To-a fall were deposited by eruptions at approximately 1 ka, 3 ka, 4 ka, 6 ka, and 7 ka based on radiocarbon dates of the soils immediately.
These results indicate that phreatic eruptions have occurred at Kurikoma volcano at least eight to nine times during the past 8,000 years and that ash fall has occurred, including the Showa Lake phreatic eruption event in 1944. In particular, a total of three phreatic eruptions have occurred in the last 1,000 years, indicating that the frequency of these eruptions is increasing. However, because phreatic eruptions are small in scale, the thickness of the sediments is thin, and the older layers are more easily lost due to erosion etc., it is possible that phreatic eruptions occurred at the same frequency in the past, and further detailed investigation is needed. We also created isopach maps of the falling tephra from each phreatic eruption to estimate the source of the eruption, the range of falling tephra, and the spatial distribution of the tephra. As a result, the source of each PED is estimated to be the eastern crater group for the 13th century eruption, the western crater group for the 1744, 3 ka, and 4 ka phreatic eruptions, the western crater group or Konigozawa crater for the 7 ka phreatic eruption, and the southern foot of Kurikoma volcano for the 1 ka phreatic eruption. Furthermore, the eruptions that occurred in 1744, about 4 ka and 7 ka, respectively, may have been relatively large phreatic eruptions that left a descending tephra layer at least 6 cm thick. This indicates that relatively large phreatic eruptions have occurred at Kurikoma volcano every 3,000 to 4,000 years. Furthermore, the ash falls from a phreatic eruption of about 4 ka, which is considered to be the largest at Kurikoma Volcano, can be estimated to be about 1.85 × 106 m3 based on the empirical formula by Hayakawa (1985), which is the same size or larger than the 2014 Ontake eruption, when the tephra outburst was estimated to be about 106 m3.
From the above, the scale and frequency of phreatic eruptions at Kurikoma volcano since the middle Holocene are comparable to those of other volcanoes. Given the fact that climbers and other visitors enter the area around the crater of Kurikoma Volcano, this suggests that there is a risk of a large-scale disaster in the event of a phreatic eruption. After this, it is necessary to increase the number of sites where tephra is described and to clarify the range of ash fall and isopach lines in more detail, taking into account the effects of erosion and other factors.
This study was financially supported by Mt. Kurikoma Area Geopark.