4:45 PM - 5:00 PM
[SSS09-12] Uplift of southwestern islands of the Ryukyu Arc
Keywords:Okinawa Trough, Vertical movement, Back-arc spreading
As the divergent boundary on land, Iceland and the East African Rift are known, but the GNSS observation in the vicinity has found horizontal movement due to spreading, but the long-term uplift like the southwestern Ryukyu Arc has not been reported. At the plate boundary immediately below Iriomotejima, a slow earthquake (SSE) of about Mw 6.6 occurs once every six months (Heki & Kataoka, 2008 JGR). Tu & Heki (2017 GRL) found that the cumulative amount of slip of these SSEs fluctuates about 10 years from GNSS data of the Ryukyu Arc in the past 20 years. In addition, remarkable earthquake swarms occurred in the Okinawa trough when the slip accumulation accelerated. This suggests that back-arc rifting episodes recur with the period of about ten years in the Okinawa Trough, and subsequent stress diffusion by viscous relaxation of the upper mantle may cause the accelerated southward movement of the block including these islands. In this study, we assume that the uplift in the southwestern Ryukyu Arc was caused by repeating rifting episodes in the Okinawa trough.
The basic processes of the back-arc spreading is the intrusion of the dike, and they bring both horizontal and vertical displacement in Yonaguni about 60 km south of the trough axis as the elastic response to it. However, the velocities far exceed those by simple elastic responses, and we have to consider viscosity relaxation (stress diffusion) after intrusions of the dikes. We stacked viscoelastic responses by numerous past repeating rifting episodes (decadal occurrence of 5 m thick dikes) and investigated whether uplift can also be explained as a part of it. In this study, we used software developed by Dr. Y. Fukahata, Disaster Prevention Research Institute, Kyoto University, to calculate viscoelastic responses at various distances and its time evolution assuming two layers composed of elastic upper layer and viscoelastic lower layer. We compared the calculated and observed velocities and examined the combination of depth and dimension of dikes and viscoelastic structure.