Probabilistic study of tephra fallout from Kuju volcano, Japan, using Tephra2

Abstract:

Simulations of tephra dispersal are important to assess tephra fall hazard to electric power constructions. For probabilistic approach to tephra fall hazard, TEPHRA2 is useful that it is a simplified advection-diffusion model to calculate tephra accumulation at localities.

We subjected Kuju volcano (Kuju, located in central-eastern Kyushu Island) that had three big eruptions (VEI=5) after the beginning of the volcanic activity at about 200 ka. If Kuju erupts in the future, the tephra will fall onto eastern Kyushu and the western Shikoku area. Then, we assessed a locality at about 100 km ENE from Kuju and studied probabilistically the biggest plinian eruption in Kuju occurred at 50 ka using TEPHRA2.

The eruption mass is estimated to be 2.03 km³ or 6.2 km³. The input parameters such as wind, eruption mass and plume heights were concerned. We used a long year record of the wind profiles measuring each height from the ground to 30 km during 22 years (from 1989 to 2010) by the Japan Meteorological Agency.

Based on the calculated result using the average wind of each month, the seasonal wind variation exerts a first-order influence on mass distribution. During the winter, winds are strong but blow to east-south-east, so the ash didn’t reach the assessed locality. On the other hand, the winds are so weak during the summer that the area buried by thick ash is limited only close to the eruptive vent. The month that has the thickest ash deposit is September and its thickness is 2.2 cm concerning that the eruption mass is 2.03 km³. As the thickness is 6.9 cm in the case of 6.2 km³, the eruption mass affects the mass distribution.

The hazard curve shows 27% and 1.8% probability of reaching a tephra fall accumulation of 1cm and 5 cm for the assessed locality if Kuju has a VEI5 eruption, which is characterized by low tephra fall accumulation for the eruptive conditions considered in this assessment.