2000年秋に富士山の低周波地震が活発化し、同時期にGPSから計算した富士山付近の面積歪が伸びとなっていた。議論では富士山のマグマで脱ガスなどに言及しているが、私は正直なところよくわからない。2000年の夏には伊豆諸島北部イベントがあり、富士山付近にも影響するような地殻変動があったのは事実だろう。

要旨

Areal strain increased noticeably in the region around the northern boundary of the Izu Peninsula in September to December 2000 when a lot of low-frequency earthquakes occurred beneath Mt. Fuji. In the same time the seismic activity in eastern Yamanashi Prefecture became low. Since increase of the areal strain indicates reduction of the pushing force of the colliding Izu block, the decrease of seismicity in eastern Yamanashi Prefecture is easily understood. Further, because diminution of the tectonic stress beneath Mt. Fuji implies reduction of the confining pressure in the magma reservoir, we think it is probable that degassing took place in the magma to build up high pressure in the focal region near the chamber which caused the remarkable activity of the low-frequency earthquakes. We suggest the noticeable increase of the areal strain in late 2000 might be produced by a mechanical separation of the Izu block from the Philippine Sea plate or detachment of the crust of the Izu block as proposed by Seno (2005).

図　GPSから求めた2000年9月から12月の面積歪変化