1:45 PM - 2:00 PM
[SVC35-05] Preparation of step diagram and volcanic event tree based on the detailed eruptive history of Asama-Maekake volcano
Keywords:Asama-Maekake Volcano, eruptive history, step diagram, volcanic event tree
Pyroclastic flows and lava flows generally flow down to topographically low places, and pyroclastic materials derived from eruption clouds are deposited downwind under the meteorological conditions at the time of the eruption. In the pyroclastic fall deposits of Asama-Maekake volcano, pumice fall deposits of about 6,300 years ago can be seen at the southern foot and the north-northwest foot of the volcano. Their depths from the ground surface are shallow and relatively easy to recognize. On the other hand, there are few opportunities to observe the outcrops of old deposits that are buried under the ground because the deposits of newer ages such as As-A (18th century) and As-B (12th century) are thicker on the eastern foot of the volcano. In addition, it was very difficult to correlate pumice layers of similar lithology among different sites because it was rare for each eruption event to show characteristic lithology or unique magma composition. Considering these problems, in this study, trench excavation at 26 locations above 180 degrees around the summit crater and 14C dating of more than 100 samples were carried out to clarify the distribution and age of the main deposits and to determine the entire stratigraphy of Asama-Maekake Volcano. As-C, Group E, and As-F are thought to have erupted around 1,800, 6,300, and 8,300 years ago, respectively. The soil between As-C and Grouop E contains many pumice layers of 3,000 to 6,000 years ago, defined as group D. Based on the comparison of the distribution areas shown in the isopach maps, the scale of the eruption was classified from Class 1, in which the pumice layer was widely distributed, to Class 4, in which the pumice was scattered in the soil. Class 3 and 4 eruptions occurred intermittently between 9400 and 3000 years ago (eruption stage I and II), and class 1 eruptions occurred three times after 2000 years ago (stage III). From the above, it was shown that the combination of extensive trench excavation survey and multiple dating is effective for reconstructing eruption history with high resolution.
A step diagram showing the relationship between eruption volumes (DRE) and ages is proposed, which indicates that active stage III is not time-predictable but is volume-predictable; if the large-scale pumice eruption occurs in 2023AD, the forecasted eruptive volume is about 0.21km3. The eruption rate varies depending on the active period, and the average eruption rate of active period III (0.0011 km3/year) is more than one order of magnitude larger than that of active periods I (0.00006 km3/year) and II (0.0001 km3/year). After Active Stage III, the style is considered to have changed to that large-scale sub-plinian eruptions accompanied by pyroclastic flows and lava flows occurred at a low frequency.
A volcanic event tree of Asama-Maekake volcano is constructed using documented eruption events since 1527AD. A volcanic event starts with unrest caused by magma intrusion. Volcano unrest caused by a magma intrusion results in either no eruption (with a probability of 0.63) or an eruption (with a probability of 0.37). The eruption itself is either small-scale (0.22), intermediate-scale (0.75) or forms a Class-B (Class-3-4) large-scale eruption (0.03). The intermediate-scale eruption continues intermittently over a period of five or more years (0.29) or is a single eruption (0.67). Intermediate-scale eruptions rarely develop into Class-A (Class-1-2) large-scale eruption (0.04). According to our event tree, and assuming previous eruption trends since 1527 A.D. do not change, the probability of a Class-A large-scale eruption occurring from the onset of unrest by magma intrusion is 0.008.
A step diagram showing the relationship between eruption volumes (DRE) and ages is proposed, which indicates that active stage III is not time-predictable but is volume-predictable; if the large-scale pumice eruption occurs in 2023AD, the forecasted eruptive volume is about 0.21km3. The eruption rate varies depending on the active period, and the average eruption rate of active period III (0.0011 km3/year) is more than one order of magnitude larger than that of active periods I (0.00006 km3/year) and II (0.0001 km3/year). After Active Stage III, the style is considered to have changed to that large-scale sub-plinian eruptions accompanied by pyroclastic flows and lava flows occurred at a low frequency.
A volcanic event tree of Asama-Maekake volcano is constructed using documented eruption events since 1527AD. A volcanic event starts with unrest caused by magma intrusion. Volcano unrest caused by a magma intrusion results in either no eruption (with a probability of 0.63) or an eruption (with a probability of 0.37). The eruption itself is either small-scale (0.22), intermediate-scale (0.75) or forms a Class-B (Class-3-4) large-scale eruption (0.03). The intermediate-scale eruption continues intermittently over a period of five or more years (0.29) or is a single eruption (0.67). Intermediate-scale eruptions rarely develop into Class-A (Class-1-2) large-scale eruption (0.04). According to our event tree, and assuming previous eruption trends since 1527 A.D. do not change, the probability of a Class-A large-scale eruption occurring from the onset of unrest by magma intrusion is 0.008.