[S-VC30] 火山の熱水系

[SVC30-P02] 微動モニタリングによる富士山周辺域の2011年東北地震に伴う時空間変化と回復過程の解明

*垣内 優亮¹ (1.東京大学工学部)

キーワード：Ambient noise monitoring、Postseismic

To monitor the volcanic activity in high spatial and temporal resolution based on ambient noise seismic interferometry, we introduced SVD based Wiener Filter (SVDW) and used multi-component of dense seismometers around the Mt Fuji. We used seismometer data acquired at 63 seismometer stations from January to September 2011. Although most previous studies adopted vertical component cross-correlation and obtained temporal resolution of several 10 days, we can estimate velocity variation in temporal resolution of 1 day by using 18 component seismic velocity changes. In Hakone and Mt Fuji volcanic area, the 2011 Tohoku-oki earthquake triggered seismicity, and 4 days later Mw 5.9 East Shizuoka earthquake occurred at the southern foot of Mt. Fuji. Because of high temporal resolution, we could separate the velocity drop caused by the 2011 Tohoku-oki earthquake and by East Shizuoka earthquake. The velocity change due to East Shizuoka earthquake was local, despite the velocity drop at magma reservoir of Mt. Fuji due to East Shizuoka earthquake is larger than Tohoku earthquake. The larger velocity drop at Mt Fuji during the East Shizuoka earthquake could indicate local pore pressure increasing at magmatic fluid around the Mt Fuji. Furthermore, we characterized the features of postseismic relaxation process around Mt Fuji and Hakone area. After the 2011 Tohoku-oki earthquake, large velocity drop occurred around volcanic regions and likely to be caused by the pressurized volcanic fluids. Furthermore, the velocity dropping seems to occur at the intensive fault area due to the Izu collision. Thus, highly fractured areas (volcanic area and fault area) are sensitive to the remote earthquakes. The recovery of velocity change around volcanic regions and in the east of Fuji-Hakone area is slower than other areas. We interpret that high seismicity or pressurized hydrothermal fluids hinder the closure of cracks in the crust around volcanic regions, and dominant cracks were generated due to large Peak Ground Velocity (PGV) in the east of Fuji-Hakone area. Because our monitoring scheme has high temporal resolution (1 day), it could contribute to the volcano alert system. Furthermore, because we can resolve the temporal variation of seismic velocity change (i.e., pore pressure and stress change) in extensive area, this approach could further contribute to the earthquake triggering process.