Aso Volcano Research Articles

RETRACTED: Coseismic rupturing stopped by Aso volcano during the 2016 Mw 7.1 Kumamoto earthquake, Japan.

Field investigations and seismic data show that the 16 April 2016 moment magnitude (Mw) 7.1 Kumamoto earthquake produced a ~40-kilometer-long surface rupture zone along the northeast-southwest-striking Hinagu-Futagawa strike-slip fault zone and newly identified faults on the western side of Aso caldera, Kyushu Island, Japan. The coseismic surface ruptures cut Aso caldera, including two volcanic cones inside it, but terminate therein. The data show that northeastward propagation of coseismic rupturing terminated in Aso caldera because of the presence of magma beneath the Aso volcanic cluster. The seismogenic faults of the 2016 Kumamoto earthquake may require reassessment of the volcanic hazard in the vicinity of Aso volcano.

阿蘇火山北西部に分布するAso-4火砕流堆積物，弁利サブユニットの層序と岩石学的特徴

阿蘇火山北西部に分布するAso-4火砕流堆積物，弁利サブユニットの層序と岩石学的特徴

Rupture process of the 2016 Kumamoto earthquake in relation to the thermal structure around Aso volcano

We constructed the rupture process model for the 2016 Kumamoto, Japan, earthquake from broadband teleseismic body waveforms (P-waves) by using a novel waveform inversion method that takes into account the uncertainty of Green’s function. The estimated source parameters are: seismic moment = 5.1 × 1019 Nm (Mw = 7.1), fault length = 40 km, and fault width = 15 km. The mainshock rupture mainly propagated northeastward from the epicenter, for about 30 km, along an active strike-slip fault. The rupture propagation of the mainshock decelerated and terminated near the southwest side of the Aso volcano; the aftershock activity was low around the northeastern edge of the major slip area. Our results suggest that the rupture process of the mainshock and the distribution of aftershocks were influenced by the high-temperature area around the magma chamber of Mt. Aso.

Geomorphological setting of shallow disrupted slides in Aso Volcano, Southwest Japan

Geomorphological setting of shallow disrupted slides in Aso Volcano, Southwest Japan

平成28年（2016年）熊本地震によって南阿蘇村周辺域で発生した斜面災害

The Kumamoto earthquake (Mj 7.3) on April 16, 2016 triggered numerous landslides in and around Minamiaso Village, which is located at the western part of Aso caldera, southwestern Japan. The landslides were divided into two types: landslides occurring at steep caldera walls and landslides generated on the slopes of post-caldera central cones of Aso Volcano. Several landslides occurred on slopes steeper than 25° at the northwestern to western caldera walls, which comprise pre-Aso volcanic rocks (lavas and pyroclastics). The largest landslide (ca. 300 m high, 130–200 m wide) occurred on the western caldera wall, damaging National Route 57 and the Hohi line of the Japan Railway. Because a clear rupture surface could not be observed, unstable blocks which had been divided by cracks, were likely to collapse due to the intense earthquake on April 16. At the post-caldera central cones of Aso Volcano, earthquake-induced landslides were generated not only on steep slopes but also on slopes gentler than 10°. They occurred in unconsolidated superficial tephra deposits overlying lavas and agglutinates, and the thickness of the slides usually ranged from 4 to 8 m. The sliding masses traveled long distances (＜600 m), compared to small differences in elevation. The deposits were composed of tephra blocks of a few meters and there was no evidence that they were transported by water. These facts suggest that some landslides mobilized rapidly into debris avalanches, traveling a few hundred meters. The associated debris avalanche resulted in five casualties and severe damage to houses at the foot of the Takanoobane lava dome. The characteristics of the April 16, 2016 earthquake-induced landslides differ from those of rainfall-induced landslides in July 2012, June 2001, and July 1990 at Aso Volcano, and provide important information for preventing or mitigating future landslide disasters in the Aso caldera region.

VLF -MT Survey around Nakadake crater at Aso Volcano

Shallow resistivity structure around Nakadake craters has been revealed by extensive VLF- MT survey from August to October 2006. This structure comprises a varied resistivity value at the upper and bottom layer of the western flank of craters; a conductive upper layer at the fourth Crater; and a resistive upper layer at southern part of craters. The conductive value of second layer at a line in the western flank and southern part of craters at several ten meter depth, interpreted as water saturated, and high resistivity at second line of western flank of craters as sheet of lava and at fourth crater is interpreted as lava rock. At western flank of craters, we found the resistivity value and the depth of second layer decrease in depth with decreasing of distance to craters line.

The oscillation model of hydrothermal dynamics beneath Aso volcano, southwest Japan after small eruption on May 2011: A new understanding model using repeated absolute and relative gravity measurement

At the end of 2010, the seismic activity in Aso volcano intensely increased and water level in the Nakadake crater decreased until early in 2011, then was followed by a small eruption in May 2011. After the eruption and heavy rain, the volcanic activity subsided to calm period, crater bottom was refilled with water, and water level increased in the Nakadake crater. The next tremor reappeared in 2014 and tracked to eruption in November 2014. This eruptive pattern and water level variation in the crater repeatedly appeared on the surface, and it should be related to the hydrothermal dynamics beneath Aso volcano.We initiated the gravity measurements in relation to hydrothermal dynamics in the subsurface of Aso volcano using Scintrex CG-5 (549) and LaCoste Romberg type G-1016 relative gravimeter at 28 benchmarks in April 2011, one month before the eruption. The repeated gravity measurements continue to monitor Aso volcano with a series of the measurement after the eruption in every three months to a half year. We analyze the gravity variation from 2011 to 2014 between the time of the phreatic and strombolian eruption. The measurements covered the area more than 60km2 in the west side of Aso caldera. A new gravity network was also installed in May 2010 at seven benchmarks using A10-017 absolute gravimeter, which re-occupied in October 2010, June 2011 and two benchmarks in June 2014.As a result, the gravity changes distinguish hydrothermal dynamic in the subsurface, which has a direct correlation to water level fluctuation in the crater, after the first eruption and before the second discharge. The monitoring data notice large gravity changes between the surveys at benchmarks around Nakadake crater and Kusasenri area. The simple 3D inversion models of the 4-D gravity data deduce the density contrast distribution beneath Aso volcano. The inversion and mass change result generate the oscillation typical as a new understanding model. The variation of the mass shows a similar trend with the hydrothermal input rate to the crater of past research. The third year monitoring from April 2013 displays a large gravity and mass variation, while precipitation data in this period is smaller than the previous season. The largest increased mass about 43million tons by Gaussian method occurred between May 2013 and September 2013. According to the three year gravity monitoring, the calm period in Aso volcano happens after May 2011 eruption until September 2013, which is followed by the active period, before the November 2014 eruption. This result will contribute to understand the process of eruption.

Sudden changes in the amplitude‐frequency distribution of long‐period tremors at Aso volcano, southwest Japan

AbstractWe observed the activity of long‐period tremors (LPTs) with a period of ~15 s at Aso volcano, Japan, during a 3 year period including the 2014 eruptions. The number of LPTs detected systematically increased 3 months before the Strombolian eruptions. LPT activity can be divided into five stages based on rapid changes in the maximum LPT amplitude. The amplitude‐frequency relation follows an exponential distribution during each stage before the Strombolian eruptions, with different characteristic amplitudes for each stage, indicating that the scale of the source property changed in stages. However, during a stage that persisted for 6 days after the onset of Strombolian activity, the amplitude‐frequency relation temporarily followed a power law distribution, indicating that the LPT source process no longer had a characteristic scale. In the last stage, the amplitude‐frequency relation returned to an exponential distribution. We therefore conclude that the physical source of volcanic LPTs changed during the eruption period.

Relationship of tephra stratigraphy and hydraulic conductivity with slide depth in rainfall-induced shallow landslides in Aso Volcano, Japan

Intense rainfall on July 12, 2012, triggered numerous shallow landslides on steep grassy hillslopes of Aso Volcano, Kyushu, Japan. The hillslopes are mantled by several meters thickness of fallout tephra accumulation from Holocene eruptions. The landslides occurred about 1 m deep in surficial tephra deposits. Stratigraphic surveys of three landslides showed that the tephra deposits beneath the ground consist of two layers, an upper blackish and a lower yellow-brown layer, and that the upper layer represents the accumulation of tephra during the last 1000 years. The surveys also demonstrated that the slip surfaces were formed near the boundary of the two layers, resulting in the sliding of the upper layer. We measured the saturated hydraulic conductivities of both the layers. The hydraulic conductivities of the lower layer are 1 to 2 orders of magnitude lower than those of the upper layer, suggesting that the lower layer acts as an aquiclude. Therefore, pore water pressure locally increases near the boundary between the two layers and failure occurs. We also examined the soil hardness, which has a high correlation with soil shear strength parameters, of the tephra layers at the three landslides. The soil hardness of the lower layer is greater than that of the upper layer in two of the landslides, suggesting that the lower layer collapses less readily than the upper layer. Comparison with previous landslides in the study area demonstrates that this type of rainfall-induced landslide event has occurred in the past and will recur in the future.

90,000-year phytolith records from caldera rim to western foot of Aso Volcano, Japan: Implications for vegetation history since catastrophic eruption

Vegetation history after the Aso-4 ignimbrite eruption (89 ka) has been reconstructed using phytolith records obtained from three tephra sections from caldera rim to the western foot of Aso Volcano, southwestern Japan. The ca. 90,000-year vegetation transitions vary between these sites and elevations. At the northern caldera rim (800–900 m asl), sparse vegetation composed of Sasa sect. Crassinodi (cool-temperate dwarf bamboo) and Zoysia was considered to be established within 1000 years after the catastrophic Aso-4 eruption. The Sasa grassland dominated by Sasa sect. Crassinodi existed continuously at the northeastern caldera rim between 89 and 13.5 ka, whereas Pleioblastus sect. Nezasa (warm-temperate dwarf bamboo), Andropogoneae (including Miscanthus Japanese pampas grass) and Zoysia existed in the Sasa grassland between 66 and 30 ka. Although Sasa grassland existed after 13.5 ka, Pleioblastus dwarf bamboo became a constituent of the grassland around 8 ka and continued flourishing until the present. On the western slope (ca. 150 m asl), sparse vegetation existed prior to 55 ka after a large outburst flood generated by a breach of caldera rim of intra-caldera lake (ca. 89–72 ka). The vegetation after 55 ka was dominated by grassland accompanied with moderate amounts of trees. Zoysia prevailed from about 55 to 40 ka, although Sasa was predominant during the period of 40–5 ka. The Sasa grassland containing Andropogoneae pampas grass declined around 5 ka, and Pleioblastus dwarf bamboo became a constituent of the grassland. The Pleioblastus grassland including Miscanthus Japanese pampas grass, Paniceae and trees (Lauraceae etc.) continued to the present. At the western foot of the volcano (∼100 m asl), Zoysia grassland became established after the large outburst flood. The grassland gradually declined from ca. 60 ka and almost disappeared around 40 ka. Instead, Sasa dwarf bamboo grassland, which accompanied Pleioblastus dwarf bamboo, increased after 60 ka. The Sasa grassland accompanying sparse trees continued, but decreased after 13.1 ka. Miscanthus grassland occurred after 29 ka, and flourished especially in the last 8 ka. The consistent presence of Miscanthus grassland for 29,000 years may be related to frequent fire episodes due to human activities including burning at the low elevation of the western foot of Aso Volcano.

P36 2014年阿蘇火山噴火に伴う電磁気観測の結果について(ポスターセッション)

P36 2014年阿蘇火山噴火に伴う電磁気観測の結果について(ポスターセッション)

A1-06 阿蘇ジオパークにおける2014-15年噴火への対応(ジオパーク,口頭発表)

A1-06 阿蘇ジオパークにおける2014-15年噴火への対応(ジオパーク,口頭発表)

P50 阿蘇火山の火口底地形変化に伴う重力変化の見積もり(ポスターセッション)

P50 阿蘇火山の火口底地形変化に伴う重力変化の見積もり(ポスターセッション)

Origin and deformation of high porosity bands in the Takanoobane Rhyolite lava of Aso volcano, Japan

In rhyolite lavas, the high porosity bands are often developed. They potentially act as pathways for gas movement to the lava surface. Since explosive activities of lavas are generally considered to be controlled by degassing system, understanding the origin and deformation process of the high porosity bands is important to assessing volcanic hazards. The Takanoobane rhyolite lava in the middle of Kyushu Island in SW Japan is effused at 51±5ka. The volume, flow length, and thickness are 0.14km3, >2km, and about 90m, respectively. The central crystalline part of the lava is characterized by the light-colored bands defined by the high porosity zone (HPZ). On the basis of geological and petrographical studies, we revealed that the HPZ was primary formed by ductile–brittle tearing of the lava (known as cavitation). According to the AMS results, the HPZs were subsequently stretched and flattened laterally during the concentric spreading of the lava. This deformation process could stretch the HPZ not only radially but also laterally. This effective stretching developed the HPZ into pervasive thin bands. Since the HPZs act as degassing pathways to the lava surface, the pervasive HPZ bands may play a role in providing volcanic gasses to void spaces created in surface fold hinges of rhyolite lavas. Thus, this degassing system may promote explosive activity of the lava surface.

Magma plumbing system of the Aso-3 large pyroclastic eruption cycle at Aso volcano, Southwest Japan: Petrological constraint on the formation of a compositionally stratified magma chamber

Aso volcano has the largest caldera (18×25km in diameter) in the southwestern Japan Island Arc, and it formed as the result of four large (VEI=6–7) pyroclastic-eruption cycles. We study the penultimate large eruption cycle, the Aso-3 cycle, which occurred 123ka with an ejecta volume of more than 150km3. The processes in the pre-eruptive magma chamber and the magma genesis of the Aso-3 cycle were inferred from geological data, phenocryst chemistry, and whole-rock chemical and Sr-, Nd-, and Pb isotopic analyses of juvenile clasts. The geological and petrological data indicate that the pre-eruptive magma chamber was stratified compositionally into three layers: from top to bottom, silicic, intermediate, and mafic magma layers. The three magma layers had a uniform isotope composition, suggesting that all the magmas were generated from a single source. The silicic and intermediate magmas were not generated from the mafic magma by fractional crystallization. The silicic magma has higher Ni content (compatible element) than the mafic magma. This suggests that these magmas were produced by partial melting of the same mafic crust but with differing amounts of partial melting: the silicic magma was produced by a low degree of partial melting of the source rock without fractional crystallization, and the mafic magma was produced by a large degree of partial melting followed by fractional crystallization. The intermediate magma compositions plot on the tie line between the silicic magma and the melt of the mafic magma in variation diagrams, and the intermediate magma has phenocrysts whose compositions are identical with those in the silicic magma. This observation indicates that, before the Aso-3 eruption cycle, a two-layer stratified magma chamber of the silicic and mafic magmas was formed as a result of melting of the mafic crust, which was followed by formation of the intermediate layer as a result of interfacial mixing between the silicic magma and the melt of mafic magma. During the eruption the three-layer stratified magma chamber was tapped from the top to the bottom.

Mechanics of water collection in plants via morphology change of conical hairs

In an arid area like the Namib Desert, plants and animals obtain moisture needed for life from mist in the air. There, some plants have hairs or fibrous structures on their leaf surface that reportedly collect fresh water from the air. We examined the morphology and function of leaf hairs of plants during water collection under different circumstances. We studied the water collecting mechanics of several plants having fibrous hairs on their leaves: tomato, balsam pear, Berkheya purpurea, and Lychnis sieboldii. This plant was selected for detailed investigation as a model because this plant originated from dry grassland near Mount Aso in Kyusyu, Japan. We found a unique feature of water collection and release in this plant. The cone-shaped hairs having inner microfibers were reversibly converted to crushed plates that were twisted perpendicularly in dry conditions. Microfibers found in the hairs seem to be responsible for water storage and release. Their unique reciprocal morphological changes, cone-shaped hairs transformed into perpendicularly twisted shapes, depend on the moisture level in the air, and water stored during wet external conditions was released onto the leaf in drier conditions. These morphological changes were recorded as a movie. Simulations explained the formation of the twisted structure. In theoretical analyses, twisted structures were found to give higher mechanical strength. Similar phenomena were found in the other plants described above. These findings pave the way to new bioinspired technology for alleviating global water shortages.

Frequency of occurrence of debris flow at the caldera wall slopes of Aso volcano, Central Kyushu

Frequency of occurrence of debris flow at the caldera wall slopes of Aso volcano, Central Kyushu

The 2011 eruption of Aso volcano, Japan, and its signature on the geomagnetic field measurements

High-resolution (1-s) geomagnetic data from the MAGDAS Network, Kyushu University, Japan, were examined to detect any local anomalous geomagnetic behavior that is possibly linked with the eruption of the Aso volcano, Kyushu Island, Japan, during 2011. Geomagnetic data recorded in the vicinity of the Aso volcano at the Kuju (KUJ) station, Japan, were analyzed and compared with those recorded at the Darwin (DAW) station in Australia, near the conjugate point of the KUJ station. The KUJ station is located about 20 km northeast of the Aso volcano. The amplitude range of daily variations was calculated and examined for the three geomagnetic components (H-, D-, and Z-components) along with the amplitude of ultra–low-frequency emissions in the Pc3-4 ranges (Pc3; 10–45 s and Pc4; 45–150 s). Data possessing and analysis reveal some local anomalous geomagnetic variations at the KUJ station during the 2011 volcanic activities of the Aso volcano. On the other hand, no anomalous changes were observed at the DAW station during that time. Therefore, the observed anomalous geomagnetic variations are considered to be of a local volcanic origin.

Eruption at the Nakadake 1st Crater of Aso Volcano Started in November 2014

Eruption at the Nakadake 1st Crater of Aso Volcano Started in November 2014

P1-28 ACTIVEによる阿蘇中岳火口における繰り返し比抵抗観測 : 2011年〜2014年の観測結果について(ポスターセッション)

P1-28 ACTIVEによる阿蘇中岳火口における繰り返し比抵抗観測 : 2011年〜2014年の観測結果について(ポスターセッション)