Stress Diffusion from Repeating Back-arc Rifting and Secular Uplift of an Island Arc : Okinawa Trough and Southwestern Ryukyu Arc
Abstract
The Okinawa Trough is the active back-arc spreading axis of the Ryukyu Arc. Its spreading is faster in the western part, and reaches 5 cm/yr in the soutwestern end. The GNSS observations over the past 20 years show fast secular uplift up to 5 mm/yr at the Yonaguni Island, the westernmost island, which gradually decreases as we go eastward (Iriomote, Ishigaki, and Miyako). The uplift rates are roughly proportional to the horizontal speeds coming from the back-arc spreading, suggesting a certain link between them. At the Iriomote segment of the Ryukyu Trench, SSE of Mw 6.6 recur biannually (Heki & Kataoka, 2008 JGR). Tu & Heki (2017 GRL) found that the slip accumulation accelerate once in a decade, and attributed it to the decadal recurrence of rifting episodes at the back arc spreading axis (earthquake swarms also occurr simultaneously). Subsequent stress diffusion by viscous relaxation of the upper mantle may have caused the accelerated southward movement of the arc and the enhanced SSE activity at the trench. In this study, we assume that the uplift in the southwestern Ryukyu Arc was caused by repeating rifting episodes in the Okinawa trough, and model it using a model assuming two layers composed of elastic upper layer and viscoelastic lower layer (Fukahata & Matsu'ura, 2005 GJI; Hashima et al. 2008 GJI). We first calculate viscoelastic responses to a dike intrusion at various distances and its time evolution. Then we stack them assuming occurrence of numerous decadally-repeating rifting episodes. We could reproduce the current fast uplift if the current fast spreading had started in a relatively recent geological time. This suggests that the current fast uplift observed by GNSS might be a temporary phenomenon. This is consistent with the geological observations of marine terraces that uplift of the Yonaguni Island has been non-uniform in space and time.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.G23C0615I
- Keywords:
-
- 1209 Tectonic deformation;
- GEODESY AND GRAVITYDE: 3040 Plate tectonics;
- MARINE GEOLOGY AND GEOPHYSICSDE: 7230 Seismicity and tectonics;
- SEISMOLOGYDE: 8107 Continental neotectonics;
- TECTONOPHYSICS