JpGU-AGU Joint Meeting 2020

講演情報

[E] ポスター発表

セッション記号 S (固体地球科学) » S-CG 固体地球科学複合領域・一般

[S-CG58] Science of slow earthquakes: Toward unified understandings of whole earthquake process

コンビーナ:井出 哲(東京大学大学院理学系研究科地球惑星科学専攻)、廣瀬 仁(神戸大学都市安全研究センター)、氏家 恒太郎(筑波大学生命環境系)、波多野 恭弘(大阪大学理学研究科)

[SCG58-P18] Long-term Viscoerastic Relaxation and Spatially Inhomogeneous Viscosity Structure

*田中 優作1 (1.東京大学地震研究所)

キーワード:粘弾性緩和、バーガーズレオロジー、地殻変動、GNSS、GRACE、GRACE-FO

Activities of very low-frequency earthquakes after mega-thrust earthquakes can be related to afterslips (e.g., Baba et al., 2020 JGR) but estimation of the spatiotemporal distribution of afterslip is difficult. For instance, GNSS observation can detect the signal of crustal deformation by afterslip + viscoelastic relaxation and their simultaneousness makes it hard to separate those signals.

One method for the separation is to use very long-term data. Time constant of afterslip is relatively short compared to that of viscoelastic relaxation, so the data obtained long after the earthquake do not include the signal of afterslip. By estimating viscoelastic relaxation based on those data and removing its contribution in the whole time series, we can gain the data mainly reflecting afterslip. This concept is very simple and today most of researches attempting to separate afterslip and viscoelastic relaxation are more or less related to it. However, it has one unignorable problem, namely, spatial inhomogeneousness of viscosity structure.

Suito (2017 EPS) suggested that three dimentional rheological heterogeneity is necessary for interpreting viscoelastic relaxation caused by the 2011 Tohoku-Oki earthquake based on GNSS data including ocean bottom stations and also showed that the viscosity of oceanic mantle can be higher than that of mantle wedge. However, it is still unclear if similar phenomena can be observed in different earthquakes and in different places.

In this presentation, I report that spatial inhomogenous viscosity structure is decisively important to interpret GNSS + satellite gravimetry observation results about post-seismic phenomena of the 2004 Sumatra-Andaman earthquake, the 2006-2007 doublet Kuril earthquakes, the 2009 Samoa-Tonga earthquake, the 2010 Maule earthquake, the 2011 Tohoku-Oki earthquake, and the 2012 Indian-Ocean earthquake. I also show the viscosity structure can possibly be even more complex than expected.