Kosei Takishita

Kyoto University

Author Statistics

Papers

Citation

H-Index

i-10 index

Research Trends

Author Order

Document Type

Co-Authors

Masato Iguchi

Kyoto University

Alexandros P. Poulidis

University of Bremen

Yoshinosuke Kamiya

Kyushu University

Takashi Okuda

Aichi Prefectural University

Shinichiro Horikawa

Shiga University of Medical Science

Kazunari Uchida

Kyushu University

Kazuho Nakai

Kyoto University of Education

Yuta Maeda

Nagoya University

Takuo Shibutani

Kyoto University

Takahiro Shiina

National Institute of Advanced Industrial Science and Technology

Cooperative Institutions

Kyoto University

The University of Tokyo

Association for the Development of Earthquake Prediction

Graduate School USA

Nagoya University

Hokkaido University

Rigaku (Japan)

Kyoto University of Education

Tohoku University

Institute of Seismology

Author Statistics

Papers

Citation

H-Index

i-10 index

Research Field

Tephra segregation profiles from Vulcanian eruptions on Sakurajima volcano, Japan, based on settling velocity observations

Japan Geoscience Union (2020)

Kosei Takishita Masato Iguchi Alexandros P. Poulidis Tadaomi Sonoda

Settling

Vulcanian eruption

Source

Cite

Citations (0)

In-situ measurement of tephra deposit load based on a disdrometer network at Sakurajima volcano, Japan

Journal of Volcanology and Geothermal Research (2021)

Kosei Takishita Alexandros P. Poulidis Masato Iguchi

In recent years, optical disdrometers have been used to observe tephra sedimentation at several volcanoes, but a method for calibrating disdrometer observations to accurately match corresponding samples has yet to be determined. In this study, tephra sedimentation samples were taken and disdrometer measurements were made simultaneously for more than 100 eruptions at Sakurajima volcano. Collected tephra samples were sieved and classified into two groups, larger than or smaller than 0.25 mm, the assumed detection threshold of the disdrometer used in this study. A comparison between disdrometer observations and collected samples revealed that particles smaller than 0.25 mm were detected when they formed aggregates or when many particles fell close enough together to be falsely registered as a single particle, even though they were individually smaller than the detection threshold. Two particle groups can be distinguished by their effective densities (assuming spheroid particles). Using samples collected during 44 collection periods (which could each consist of multiple eruptions), the tephra deposit load per particle for each combination of diameter and settling velocity classified by the disdrometer was calculated using multivariate linear regression. Compared to simpler approaches the conversion formulas presented here were found to lead to more accurate estimates. The tephra deposit load values estimated using this method for real-time simultaneous observations were constrained to be within two and six times the sample load. Although the formula is developed based on data from Sakurajima volcano, it can be applied to other volcanoes with similar activity and expected tephra morphology and the methodology presented here can be replicated to produce a tailored formula given enough input data.

Disdrometer

Settling

Sedimentation

10.1016/j.jvolgeores.2021.107442

Cite

Citations (5)

Solidified magma reservoir derived from seismic exploration in the Aira caldera, southern Kyushu, Japan

Research Square (Research Square) (2023)

Hiroki Miyamachi Hiroshi Yakiwara Reiji Kobayashi Shuichiro Hirano Takeshi Kubo

Abstract Seismic exploration was conducted along a profile running through the Aira caldera located in southern Kyushu, Japan. The caldera was formed by an ignimbrite eruption approximately 30 ka BP, namely, the “AT eruption,” which produced the Ito ignimbrite and widespread Aira-Tanzawa ash. This analysis aimed to clarify the detailed P -wave velocity structure beneath the caldera. Accordingly, 829 inland seismic stations and 42 ocean bottom seismographs were deployed along the 195 km-long seismic profile to record seismic waves generated by numerous controlled seismic sources. A detailed velocity structure of the active Aira caldera was successfully obtained to depths of 20 km through travel-time tomography. A substantial structural difference was observed in the thicknesses of the low-velocity zones between the eastern and western sides in the shallowest region of the Aira caldera, suggesting that the Aira caldera is composed of at least two calderas: the AT caldera associated with the AT eruption, and the Wakamiko caldera associated with the post-AT eruption. Perhaps the most interesting feature of the caldera structure is the existence of a substantially high-velocity zone at depths of 6–11 km beneath the center area of the AT caldera, which can be interpreted as the cooled and solidified magma reservoir formed during or after the AT eruption. In addition, a low-velocity region with approximately 15 km depths indicated a deep magma reservoir. Based on these novel and past research results, a new magma supply model in the Aira caldera was proposed. Further, the spatial distribution of the magma reservoir associated with the AT eruption 30 ka BP was estimated, while the future possibility of larger eruptions in this caldera was discussed.

Caldera

Magma chamber

Seismic velocity

10.21203/rs.3.rs-2676096/v1

Cite

Citations (0)

Continuously Operable Simulator and Forecasting the Deposition of Volcanic Ash from Prolonged Eruptions at Sakurajima Volcano, Japan

Journal of Disaster Research (2022)

Masato Iguchi Haruhisa Nakamichi Kosei Takishita Alexandros P. Poulidis

At Sakurajima volcano, frequent Vulcanian eruptions have been seen at the summit crater of Minamidake since 1955. In addition to this eruption style, the eruptive activities of Strombolian type and prolonged ash emission also occur frequently. We studied the design of a simulator of advection-diffusion-fallout of volcanic ash emitted continuously. The time function of volcanic ash eruption rate is given by a linear combination of the volcanic tremor amplitude and the volume change of the pressure source obtained from the ground motion. The simulation is repeated using discretized values of the eruption rate time function at an iteration time interval of the simulation. The integrated value of the volcanic ash deposition on the ground obtained from each individual simulation is used to estimate the value of the ash fallout. The plume height is given by an empirical equation proportional to a quarter of the power of the eruption rate. Since the wind velocity field near the volcano is complicated by the influence of the volcanic topography, the predicted values published by meteorological organizations are made in high resolution by Weather Research and Forecasting (WRF) for the simulation. We confirmed that an individual simulation can be completed within a few minutes of iteration interval time, using the PUFF model as the Lagrangian method and FALL3D-8.0 as the Eulerian method on a general-purpose PC. Except during rainfall, the radar reflectivity, the count of particles per particle size, and fall velocity obtained by the disdrometers can be used for the quasi-real time matching of the plume height calculated from the eruption rate and the ash fall deposition rate obtained from the simulation.

Volcanic ash

Vulcanian eruption

10.20965/jdr.2022.p0805

Cite

Citations (2)

Precise leveling survey in Aso volcano, 2018

Japan Geoscience Union (2019)

Takahiro Ohkura Shin Yoshikawa Hiroyuki Inoue Akihiko Yokoo Mitsuru Utsugi

Source

Cite

Citations (0)

Tephra segregation profiles based on disdrometer observations and tephra dispersal modeling: Vulcanian eruptions of Sakurajima volcano, Japan

Earth Planets and Space (2024)

Kosei Takishita Alexandros-Panagiotis Poulidis Masato Iguchi

Abstract The profile of tephra concentration along a volcanic plume (i.e., the tephra segregation profile) is an important source parameter for the simulation of tephra transport and deposition and thus for the tephra sedimentation load. The most commonly-used approach is to treat an eruption as a single event (i.e., with a time-averaged mass eruption rate; MER). In this case, it is common to use pre-determined profiles that feature most of the tephra segregate at the top of the plume. However, case studies based on observations have revealed that large concentration maxima also appear at the lower part of the plume. To investigate this discrepancy, the impact of plume height on the temporal variations in the MER is examined. To this end, we use the tephra transport and dispersion model Tephra4D with MER estimates obtained from geophysical monitoring and maximum plume height observations to calculate the spatial distribution of the tephra deposit load for 39 eruptive events that consisted of explosions and quasi-steady particle emission from the Sakurajima volcano, Japan. A comparison of the model results with observations from a disdrometer network revealed that for both kinds of activity, maxima in tephra segregation can occur at heights below the reported plume height. The tephra segregation profiles of Vulcanian eruptions at Sakurajima volcano are consistent with most of the modeling studies giving profiles that feature most of the tephra segregating at the top of the plume if the temporal variation of the MER is taken into consideration to properly represent the total series of eruptive events in a sequence. This highlights that even though the activity at Sakurajima volcano is commonly characterized simply as Vulcanian eruptions, in addition to the primary plume developed due to the initial instantaneous release caused by the explosion, the subsequent continuous plume that can accompany the eruption plays an important role in particle emission. Calculations could not reproduce the simultaneous deposition of particles with a wide range of settling velocities in observations, suggesting the importance of volcanic ash fingers caused by gravitational instability in tephra transport simulations. Graphical Abstract

Disdrometer

Vulcanian eruption

10.1186/s40623-023-01952-y

Cite

Citations (1)

Solidified magma reservoir derived from active source seismic experiments in the Aira caldera, southern Kyushu, Japan

Earth Planets and Space (2023)

Hiroki Miyamachi Hiroshi Yakiwara Reiji Kobayashi Shuichiro Hirano Takeshi Kubo

Abstract The Aira caldera, located in southern Kyushu, Japan, originally formed 100 ka, and its current shape reflects the more recent 30 ka caldera-forming eruptions (hereafter, called the AT eruptions). This study aimed to delineate the detailed two-dimensional (2D) seismic velocity structure of the Aira caldera down to approximately 15 km, by means of the travel-time tomography analysis of the seismic profile across the caldera acquired in 2017 and 2018. A substantial structural difference in thickness in the subsurface low-velocity areas in the Aira caldera between the eastern and western sides, suggest that the Aira caldera comprises at least two calderas, identified as the AT and Wakamiko calderas. The most interesting feature of the caldera structure is the existence of a substantial high-velocity zone (HVZ) with a velocity of more than 6.8 km/s at depths of about 6–11 km beneath the central area of the AT caldera. Because no high ratio of P- to S-wave velocity zones in the depth range were detected from the previous three-dimensional velocity model beneath the AT caldera region, we infer that the HVZ is not an active magma reservoir but comprises a solidified and cool remnant. In addition, a poorly resolved low-velocity zone around 15 km in depth suggests the existence of a deep active magma reservoir. By superimposing the distribution of the known pressure sources derived from the observed ground inflation and the volcanic earthquake distribution onto the 2D velocity model, the magma transportation path in the crust was imaged. This image suggested that the HVZ plays an important role in magma transportation in the upper crust. Moreover, we estimated that the AT magma reservoir in the 30 ka Aira caldera-forming eruptions has the total volume of 490 km 3 DRE and is distributed in a depth range of 4–11 km. Graphical Abstract

Caldera

Magma chamber

Seismic velocity

10.1186/s40623-023-01919-z

Cite

Citations (0)

Tephra4D: A Python-Based Model for High-Resolution Tephra Transport and Deposition Simulations—Applications at Sakurajima Volcano, Japan

Atmosphere (2021)

Kosei Takishita Alexandros P. Poulidis Masato Iguchi

Vulcanian eruptions (short-lived explosions consisting of a rising thermal) occur daily in volcanoes around the world. Such small-scale eruptions represent a challenge in numerical modeling due to local-scale effects, such as the volcano’s topography impact on atmospheric circulation and near-vent plume dynamics, that need to be accounted for. In an effort to improve the applicability of Tephra2, a commonly-used advection-diffusion model, in the case of vulcanian eruptions, a number of key modifications were carried out: (i) the ability to solve the equations over bending plume, (ii) temporally-evolving three-dimensional meteorological fields, (iii) the replacement of the particle diameter distribution with observed particle terminal velocity distribution which provides a simple way to account for the settling velocity variation due to particle shape and density. We verified the advantage of our modified model (Tephra4D) in the tephra dispersion from vulcanian eruptions by comparing the calculations and disdrometer observations of tephra sedimentation from four eruptions at Sakurajima volcano, Japan. The simulations of the eruptions show that Tephra4D is useful for eruptions in which small-scale movement contributes significantly to ash transport mainly due to the consideration for orographic winds in advection.

Settling

Orographic lift

Vulcanian eruption

10.3390/atmos12030331

Cite

Citations (2)