[J05-P] Poster

[J05-P-09] Afterslip and viscoelastic components observed in surface gravity change after the 2011
Great Tohoku earthquake

^○Shuhei Okubo¹, Xinlin Zhang², Yoshiyuki
Tanaka¹, Yuichi Imanishi¹, Satoshi Miura³,
Sadato Ueki³, Hiromitsu Oshima¹, Tokumitsu
Maekawa⁴, Kazumi Okada⁴, Miwako Ando¹
(1.The University of Tokyo, Tokyo, Japan, 2.China Earthquake Administration, Wuhan,
China, 3.Tohoku University, Sendai, Japan, 4.Hokkaido University, Sapporo, Japan)

The 2011 earthquake off the Pacific coast of Tohoku imposed significant stress change within the crust and mantle, which results in postseismic crustal deformation and gravity change. In this presentation, we show gravity change on the ground of Tohoku, Hokkaido and Kanto areas during the period between May 2011 and July 2016.
The gravity changes measured with an FG5 absolute gravimeter showed rather monotonous decrease of 30-40 microgals during 2011-2014. Since the elevation changes in the same period are 10 to 20 cm, the gravity decrease can be well explained in terms of afterslip around the fault of the Tohoku earthquake. Surprisingly, however, the gravity trend showed slow but steady increase of 10 microgals since 2014 although the uplifting is still going on in the same period.
Remembering that the gravity change is composed of contributions from elevation change and volumetric strain change (density change), the gravity increase since 2014 is likely to reflect viscoelastic strain change within the mantle. The result will be discussed in more detail based on viscoelastic modeling of a spherical earth.