[S-GD01] 重力・ジオイド

[SGD01-02] 富士山科学研究所重力点の設置と「フェーズフリー」重力観測

*本多 亮¹、今西 祐一²、名和 一成³、田中 俊行⁴、兒玉 篤郎⁵、冨山 顕⁵ (1.山梨県富士山科学研究所、2.東京大学地震研究所、3.国立研究開発法人産業技術総合研究所、4.公益財団法人地震予知総合研究振興会東濃地震科学研究所、5.国土地理院)

キーワード：重力、富士山、火山監視、重力計検定、フェーズフリー

Mount Fuji is an active volcano, the difficult situation is that we can’t tell where the vent will open. Moreover, the eruption is thought to occur rapidly after the precursors such as grand deformations or increase of the seismic activities, because of its low viscosity magma. As a part of the volcano monitoring system, we are now constructing the gravity monitoring network. We are planning to install multiple continuous gravity monitoring stations. We installed the First Gravity Station in Mount Fuji Research Institute (MFRI) last year, where the absolute gravity value is measured by GSI. We are going to start the continuous observation in this station. Although low frequency events or tremors are observed sporadically, Mount Fuji is quiet at this point of time. So, the purpose of our observation at the beginning is to comprehend the precipitation and ground water effect, including snow melting effect in the spring time.

There is another interesting point for the gravity observation in Mount Fuji. The gravity difference between MFRI and the fifth station of the Yoshida trail is about 320 mGal, and the travel time between two points is about 30 minutes. It means that this is a good observation line for the verification of the gravimeters. Now we suggest the "phase-free" gravity measurement (https://phasefree.org/). The idea of "phase-free" is to eliminate the social phases (time, state) of normal times and disasters, so that goods or services normally used can be appropriately used at the time of disaster. If we continuously use the gravity observation line for the verification by various gravimeters in the normal time, that will be the important data for the volcano monitoring when the Mount Fuji is starting to prepare the eruption.