有珠山2000年噴火活動に対する緊急重力測定ー絶対重力観測網の構築と噴火前後の重力変化ー

概要

論文の詳細を見る
After the eruption of Usu volcano, Japan, on 31 March, 2000, we carried out microgravity measurements around Mt. Usu. Comparing the gravity values acquired by previous measurements in June 1993, we detected significant gravity changes presumably associated with the magma movement. The largest change detected as of May 2000 was - 820μgal at the southern rim of the summit. Both gravity and height changes were successfully reproduced by prescribing a shallow inflation source and a deeper deflation source at depths of 2,000 and 3,000 meters, respectively. Total intruded mass flux is estimated to be 3.6×1010kg. We constructed an absolute gravity network consisting of three sites around Toya lake, and precisely determined absolute gravity values. Moreover, from 16 May to 29 June, we took continuous absolute gravity measurements at Usu volcano observatory, and detected a significant post eruptive gravity change. The absolute gravity continued to decrease by 10μgal until around 25 May when it began to increase After that, the absolute gravity gradually recovered to 10μgal for one month. We can interpret the inferred post-eruptive gravity changes as a continuing crustal uplift followed by a commence of the land subsidence.