LAUSR

We studied 38 slow slip events (SSEs) in 1997-2016 beneath the Iriomote Island, southwestern Ryukyu Arc, Japan, using continuous Global Navigation Satellite System (GNSS) data. These SSEs occur biannually on the same fault patch at a depth of ~30 km on the subducting Philippine Sea Plate slab with average moment magnitudes (Mw) of ~6.6. Here we show that the slip accumulation rate (cumulative slip/lapse time) of these SSEs fluctuated over a decadal time scale. The rate increased twice around 2002 and 2013 concurrently with earthquake swarms in the Okinawa Trough. This suggests that episodic activations of the back-arc spreading at the Okinawa Trough caused extra southward movement of the block south of the trough and accelerated convergence at the Ryukyu Trench.