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

摘要

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 3.6×10~10kg. 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 lOμgal 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.%2000年3月31日に噴火した有珠山の周辺では顕著な地殼変動が進行したことが，GPS(Global Positioning System)，水準測量，EDM(Electro-Optical Distance Measurement)，セオドライト，航空写真測量，SAR(Synthetic Aperture Radar)によって明らかにされた.航空写真測量によると，西山火口付近は，水平で北方へ20m，上下には60mもの隆起が観測された(国土地理院，2000).これら変形の観測量を，重力変化の観測データと組み合わせることで，地下の質量分布変化に対.