Relationship between Precipitation Just above the Lava Dome and Displacement of the Dome Using X-Band MP Radar at Unzen Fugendake

Abstract

Since its last eruption from 1990-1995, Unzen Volcano (Shimabara Peninsula, Japan) has been quiescent since. At its summit a complex Dacitic dome that expanded towards the East, in the direction of the Mizunashigawa-valley has grown during the eruption onto previously deposited volcanoclastic sediments. As a small portion of the domes have generated rockfalls and as the surrounding gullies have been eroding headwards, the stability of the dome and its evolution is essential for hazards and disaster-risk monitoring and for understanding the decadal-scale volcanic geomorphological change occurring in between eruptions. Therefore, the present contribution aims to (1) quantify the dome movement and (2) separate the different parts of the dome to understand how it deforms; and (3) what is the link between rainfalls and the dome movement. The method relies on the Unzen GbSar system (Ground Based radar interferometry system) and on hourly rainfalls from raingage stations at Unzen Volcano. As a result, the authors have identified that (1) the lower part of the dome rises and falls more rapidly than the upper part of the dome when rainfall is less than 100 mm/48 hours, and (2) the upper and lower parts of the dome move up and down at the same level when rainfall exceeds 100 mm/48 hours. In turn, when rainfall exceeds 250 mm/48 hours, then the upper part of the dome also displays further downward movement, so that the entire dome might be moving down like an accordion.