[S-VC27] Mitigation of volcanic disasters - basic and applied researches

[SVC27-P01] Disaster-stricken vehicles dug up for the first time in 30 years and characteristics of pyroclastic surge deposits in 1991 on Unzen Volcano

*Daisuke Nagai¹, Shinichi Sugimoto² (1.Mt.Unzen disaster memorial hall, 2.Unzen volcanic area global geopark)

Keywords:pyroclastic surge, volcanic disaster, disaster prevention education

It has been 30 years since the eruption of Mt. Unzen Fugen killed 43 people. In the local area, the improvement plan of the disaster area is being made and the construction is being carried out. In the construction on February 8, 2021, 4 damaged cars were excavated and visited as they were 30 years ago. The vehicles will be installed on concrete foundations after volcanic ash and other soil is removed. Since the vehicle was salvaged by a heavy machine at the time of this excavation, we had an opportunity to investigate the damage of the entire vehicle which had not been investigated after the disaster, and the pyroclastic surge deposits accumulated in and around the vehicle body in 1991. The investigation was carried out in Kitakamikiba Town, Shimabara City, where many reporters called "fixed point" were present at that time, in a vehicle blown about 70 m downstream from a road by a pyroclastic flow generated on June 3, 1991. Two cars are placed close to each other at the same place, and the cars on the upstream side are placed facing the rear side with the north side facing the front, while the other cars on the downstream side are placed facing the obliquely rear side with the south side facing the front and are placed to lean against the cars on the upstream side. Judging from the testimonies at the time and the pictures and images left by the victims just before the disaster, it is assumed that the two taxis were chartered by the press. Through the excavation, license plates were dug up from cars on the mountain side, and it was discovered that the cars were "Koarashi Taxi".No license plate that could be read from the other car was found, but as a result of identifying the rest of the damaged cars, it was found that the car belonged to the "Maruzen Taxi". The body of the driver of Maruzen Taxi was found near the fixed point. On the other hand, it is probable that the driver of the Koarashi Taxi was in the taxi and suffered from the disaster because he was found somewhere between the fixed point and the excavated vehicle (About 30 m downstream from the fixed point) and communicated with his colleague's vehicle by radio until just before the excavation. In the aerial photograph taken on June 5, there was a damage on the wall surface of the gutter southeast of the location where both cars were supposed to have been, as if something had hit the wall surface, and further downstream, there was a cut mark of about 3 m in width and about 10 m in length, as if the ground had been dragged. Since there is a damaged vehicle on the extension of the cut mark, it is probable that this cut mark was made in the process in which the vehicle was transported by the impact of the pyroclastic surge. The entire engine section and the body of the vehicle were damaged so that they could be broken, and the bumpers, the bonnet, the roof, the bottom surface of the vehicle, and the entire body were found to have large dents that received an impact. It is presumed that the car body was carried rolling because there were relatively uniform dents in the whole car body and no falling marks on the ground. When the car body was dug out, an electric wire was caught around the body. It is known that there was a utility pole nearby, and it is known that the utility pole got entangled with electric wires and wound around the car body as the car moved. A pyroclastic surge deposit on June 3, 1991 around the vehicle was also found. The sediments are about 3 cm thick. The thickness diagram of the pyroclastic surge deposits including the surrounding observation sites suggests that the area was located near the major axis in the distribution basin. The excavated vehicles will be displayed on the site and will be used as a base for disaster learning by the local residents' association. It is hoped that the remains will enhance people's awareness of disaster prevention.