Abstract:
Volumes of lava domes, pyroclastic flow deposits, ash fall deposits and total magma discharged at Unzen Volcano, Nagasaki, western Japan from May 23, 1991 to August 10, 1992 were estimated. Using video records, pictures from helicopter and aerial photos, volumes of domes and pyroclastic flow deposits were directly estimated. In order to estimate continuous volume variations indirectly, tremor energy due to pyroclastic flows were measured using 2397 seismic wave records. We assumed that collapse volume of dome is proportional to the tremor energy associated with pyroclastic flow. Average effusion rate of magma was estimated to be 3.4x10^5m^3/day from May 23 to July 14, 1991. If this effusion rate of magma is nearly constant, the volume of lava domes, pyroclastic flow deposits, ash fall deposits and total magma is calculated to be 4.8x10^7m^3, 7.7x10^7m^3, 2.6x10^7m^3 and 1.5x10^8m^3 (DRE), respectively on August 10, 1992.
The volume estimated from the tremor energy jumped on June 3 and June 8, 1991 as compared with the volume of pyroclastic flow deposits measured from image data. At the same time that these gaps occurred, a horseshoe crater was formed by landslide. Thus, we concluded that landslide triggered the larger pyroclastic flows on June 3 and June 8. On June 8, explosion occurred due to sudden pressure reduction caused by landslide. This explosion and the tremor due to landslide should affect the jump of the tremor energy.
To predict the disaster area due to pyroclastic flow, the relation between the volume calculated from tremor energy and the runout distances of pyroclastic flows were analyzed.