Spatiotemporal Evolution of Recurrent Slow Slip Events Along the Southern Ryukyu Subduction Zone, Japan, From 2010 to 2013

Slow slip events (SSEs) with a moment magnitude of 6.6–6.7 occur at intervals of 5–9 months along the southern Ryukyu subduction zone in southwestern Japan. To obtain detailed image of these SSEs, this study applies a modified Network Inversion Filter to the Global Navigation Satellite System time series from March 2010 to February 2013 and estimates the spatiotemporal evolution of slow slip on the plate interface. Five SSEs are detected during this period. These events have similar cumulative slip distributions and are located near the northwestern coast of Iriomote Island at depths of 30–60 km, indicating that the SSEs recur in the same fault area. The analysis reveals that the spatiotemporal evolution of slip during the five SSEs varies from event to event. The notable feature is that three of the five SSEs nucleate rapidly and reach the maximum slip rate within several days, while the other two SSEs show much slower nucleation, lasting for 25–45 days prior to rapid acceleration of slip. Such variations in spatiotemporal evolution suggest temporal changes of fault properties such as fluid distribution and fault friction. Very low frequency earthquakes and low-frequency earthquakes are activated during one of the five SSEs. The source area of the SSEs is complimentarily located in a deeper part of the very low frequency earthquakes and low-frequency earthquake source areas, and a possible tsunami source region is located near the trench, indicating the depth dependence of the physical properties along the plate interface.