Internal structure of Mt. Fuji

A talk on the internal structure of Mt. Fuji given at Mt. Fuji Int'l symposium in Fujiyoshida, Yamanashi, Japan.

Transcript

1. 富⼠⼭の内部構造 Internal structure of Mt. Fuji ⻘⽊陽介（東京⼤学地震研究所） Yosuke Aoki (Earthquake Research Institute, The University of Tokyo) Email: yaoki@eri.u-tokyo.ac.jp 8 October 2018 Mt. Fuji Int’l symposium Fujiyoshida, Yamanashi, Japan
2. 富⼠⼭は何が特別なのか What is unique about Mt. Fuji? ü 景観 / Landscape ü ⼤きさ / Size 富⼠⼭のマグマ噴出率は⽇本の⼀般的な⽕⼭の10-100倍 Mt. Fuji has ejected magmas 10-100 times faster than other Japanese volcanoes 疑問 / Questions 1. 富⼠⼭はなぜそこにあるのか︖ / Why does Mt. Fuji exist here? 2. なぜ富⼠⼭は⼤きいのか︖ / Why is Mt. Fuji large? 3. なぜ富⼠⼭のマグマは低粘性なのか︖（なぜ富⼠⼭の⽯は⿊いのか︖） Why is Mt. Fuji dominated by basaltic rocks while arc volcanoes are generally more felsic?
3. ⽇本の⽕⼭のできかた How to make Japanese volcanoes 沈み込み帯の⽕⼭は⾼粘性のマグマを持つものが多 いが、富⼠⼭のマグマは⽞武岩質（低粘性）。 Arc volcanoes tend to be felsic, but Mt. Fuji is dominated by basaltic rocks. Subducting plate Dehydration Magma reservoir Volcano
4. プレートテクトニクスと 富⼠⼭ Tectonic setting around Mt. Fuji ユーラシアプレート Eurasian Plate フィリピン海 プレート Phillippine Sea Plate オホーツク（北⽶）プレート Okhotsk (North American) plate 富⼠⼭ ü 富⼠⼭はプレートの三重会合点付近に 位置する．Mt. Fuji is located near a triple junction of tectonic plates. ü 太平洋プレートが富⼠⼭の下に沈み込 んでいる．Pacific plate subducts beneath Mt. Fuji.
5. 富⼠⼭にかかる⼒ Stress field around Mt. Fuji 北⻄ー南東⽅向に圧縮されている。/ NW-SE compression その結果… / That results in ... ü 上から⾒た富⼠⼭は円形ではなく北⻄ー南東⽅向に伸ば された楕円形をしている。The shape of Mt Fuji is elliptic with a long axis to NW-SE. ü ⽕⼝列は北⻄ー南東⽅向に向いているものが多く、北⻄ 麓・南東麓に多い。/ There more fissures in NW and SE flanks. Many of them strike NW-SE. ü ⼭頂に対して放射状の⽕⼝もある。これは⼭体の荷重に より応⼒場が局所的に乱されているため。/ Fissures near the summit are distributed radially because of the volcano loading. ⾼⽥ (2007) Takada (2007)
6. 富⼠⼭のマグマだまり Magma reservoir of Mt. Fuji 富⼠⼭ 伊⾖諸島 富⼠⼭のマグマだまりは20km付近にあり伊⾖諸島の ⽕⼭のマグマだまりより深い。 Crustal magma reservoir of Mt. Fuji is at ~20 km, deeper than that of surrounding volcanoes. 1707年宝永噴⽕は、深部から上昇したマグ マが浅部の残存マグマと混合して発⽣した。 The 1707 Hoei eruption was triggered by an interaction between deep mafic and shallow felsic magmas. 藤井 (2007) Fujii (2007) Kaneko et al. (2010) between K2 O levels in between compositions of melt-inclusions and the whole rock sample containing their host olivines shown in Fig. 3 is interpreted to result from such a process. Alternatively, the andesitic melt-inclusions could simply have originated from a disequilibrium compositional boundary layer that grew around olivines rapidly due to abrupt decomposition of hydrous basaltic magma. However, the decrease in CaO and Al2 O3 content of the melt-inclusions with increase of SiO2 (Fig. 3) indicates that plagioclase and/or pyroxenes were crystallized alongside olivine, denying this possibility. To evaluate the plotted trends in Fig. 3, petrologic mixing equations (Wright and Doherty, 1970) were applied to the trend of “FG1053” as a test case. Based on the calculation, the most SiO2 -rich melt-inclusion can be successfully derived from the basaltic melt-inclusion assuming fractionation of olivine: 3.6%, clinopyroxene: 3.8%, plagioclase: 15.2%, and magnetite: 3.8% (Appendix D). Scattering observed in each melt- inclusion trend likely results from analytical and corrective errors, as well as from possible entrapment of inclusions generated in the fol- lowing mixing process prior to an eruption. The magmatic process mentioned above is consistent with the zoning pattern of phenocrystic minerals. In our interpretation, pheno- crystic olivine is assumed to be contained in the magma in the shallow magma chamber, which sometimes has andesitic composition. Consis- tent with this, more than two-thirds of the olivine phenocrysts show reverse zoning, which is considered to be a consequence of mixing with the basaltic magma from the deep chamber (Fig. 5, upper right). On the other hand, many of the plagioclase phenocrysts whose compositions lie near the more An-rich peak (∼An86 ) of the bi-modal pattern show normal zoning, while those near the less-calcic peak (∼An70 ) show Fig. 6. Proposed model of the magma plumbing system of Fuji. The magma plumbing system consists of relatively deep (∼20 km) basaltic and shallow SiO2 -rich (often andesitic) magma chambers (∼8–9 km).
7. 富⼠⼭の地下構造 / Mt. Fuji at depth どうやって調べる︖ / Methods ü 地震波速度 / Seismic velocity（温度や流体の有無に敏感 / Sensitive to temperature and the presence of fluids） ü ⽐抵抗 / Resistivity（流体の有無に敏感 / Sensitive to the presence of fluids） Nakamichi et al. (2007) 地震波トモグラフィによる解析 Seismic tomography ⾚︓地震波速度が遅い（低 温・低圧・マグマが少ない） Red: Low semismic velocity (Low temperature, low pressure, less melt fraction) 低周波地震 Low-frequency earthquakes
8. 富⼠⼭の地下構造 / Mt. Fuji at depth ⾚︓強い地震波速度境界（深いほうが速い） Red: Strong velocity contrast (with higher velocity at the bottom of the boundary) 低周波地震 / Low-frequency earthquakes Kinoshita et al. (2015) 地震波レシーバー関数による解析 Receiver function analysis of seismic waves
9. 富⼠⼭のマグマの通り道 Magma pathway beneath Mt. Fuji Kinoshita et al. (2015) 低周波地震 Low frequency earthquakes 富⼠⼭のマグマは東北地⽅や伊⾖諸 島の⽕⼭と同様に太平洋プレートか ら供給されている． Magma in Mt. Fuji is fed from Pacific plate like volcanoes in NE Japan and Izu islands.
10. 富⼠⼭の⽐抵抗構造 Resistivity structure around Mt. Fuji Aizawa et al. (2004, 2016) 富⼠⼭直下には低⽐抵抗領域がある︖-- ⽕⼭性流体︖ Low resistivity beneath Mt. Fuji – Volcanic fluids?
11. まとめ / Summary 富⼠⼭は3つのプレートの会合点付近に位置する． Mt. Fuji is located near a triple junction of tectonic plates. プレート運動により、富⼠⼭は北⻄ー南東⽅向に圧縮を受けている。そのため、⼭体は北⻄ー南東⽅向 に伸びた形になっていて、⽕⼝列も北⻄ー南東⽅向に配列している。Plate motion generates a NW- SE compressional stress field around Mt. Fuji. The stress field makes the volcano in NW-SE direction and fissures alighing NW-SE. 富⼠⼭のマグマだまりは約20kmであり、伊⾖諸島の⽕⼭のマグマだまりより深い。 Magma reservoir of Mt. Fuji is located around 20 km, deeper than that in Izu islands. 富⼠⼭はなぜ巨⼤か、なぜ低粘性のマグマを産出するのか、は重要な問題だが、解決されていない。 Why Mt. Fuji is big and why Mt. Fuji is dominated by basaltic magmas are important research questions, but no definitive answers are given so far. (For scientists) Aoki, Y., Tsunematsu, K., Yoshimoto, M., Recent progress of geophysical and geological studies of Mt. Fuji volcano, Japan, Earth-Science Reviews, in revision.