open access

https://doi.org/10.14943/doctoral.k14645

https://hdl.handle.net/2115/83579

巨大地震直前直後の電離圏異常の三次元トモグラフィー

英語

2021-09-24

Hokkaido University

107p

A dense network of ground global navigation satellite system (GNSS) receivers detected ionospheric total electron content (TEC) changes starting ~40 minutes before the 2011 Tohoku-oki (Mw 9.0) earthquake around the ruptured fault, together with the long-lasting postseismic TEC drop. In this study, I robustly estimate three-dimensional (3D) distribution of both pre- and post-seismic ionospheric anomalies of the 2011 Mw 9.0 Tohoku-oki and the 2010 Mw 8.8 Maule earthquakes by tomographic inversions of electron density anomalies. For the 2011 case, I set up more than 6,000 blocks, as large as 1.0° (east-west) x 0.9° (north-south) x 60 km (vertical), over the Japanese Islands, the Sea of Japan, and the Korean Peninsula, up to 870 km altitude. I used slant-TEC residuals obtained using 8 satellites and 1,493 ground stations as inputs to the 3D tomographic inversion. For the Maule earthquake, TEC data are obtained from 6 GNSS satellites and 89 ground stations mainly in Chile and Argentine. I set up >3,500 blocks, with the size of 1.0° (east-west) x 1.2° (north-south) x 75 km (vertical) for altitudes 75-750 km. I adopted objective standards in determining reference curves of TEC from which the anomalies are defined. I regularized the inversion by introducing two different constraints, the continuity constraint and constraints around zero with altitude-dependent allowances. Performances of the 3D tomography have been confirmed by various resolution tests for artificial patterns. I compare the spatial and temporal distribution of the 3D structure of ionospheric electron density anomalies immediately before these two megathrust earthquakes together with those of another large earthquake (the 2015 Mw 8.3 Illapel) studied by He and Heki (2018). The results of the three cases showed that the preseismic ionospheric anomalies have following common features; (1) they are composed of pairs of low-altitude positive and high-altitude negative electron density anomalies, (2) they occur above the land area close to the submarine faults, and (3) they have clear onsets a few tens of minutes before earthquakes (~40 min before 2011 Tohoku-oki, and Maule, and ~20 minutes before the Illapel earthquakes) and grow with decaying rates. I hypothesize the physical process consistent with such 3D structure as follows. Electric fields made by surface positive charges reach the ionosphere. The field generates electromotive forces and makes electrons move down along geomagnetic fields, and this upward current makes eastward/westward magnetic field in regions to the south/north of the epicenter before earthquakes in northern/southern hemisphere. The current will continue until the induced electric field cancels the external field made by surface charges, making the electric potential uniform along the magnetic field. The current will depend on the along‐field component of the external electric field and the density of free electrons as a function of altitude. The nonuniform electric currents would result in convergence/divergence of electrons and make positive/negative electron density anomalies at the lower/higher ionosphere along the magnetic field, the structure consistent with those found for these three earthquakes by 3D tomography. I will also compare strengths and dimensions of the electron density anomalies before these three earthquakes and discuss future perspective of preseismic ionospheric anomalies.

2021-09-24

甲第14645号

博士(理学)

(主査) 教授 日置 幸介, 教授 古屋 正人, 准教授 高田 陽一郎, 准教授 勝俣 啓, 教授 中谷 正生 （東京大学地震研究所）

理学院（自然史科学専攻）

博士論文

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