Geodetic Studies of Slow Slip Events in Subduction Zones with Active Back-Arc Opening

  In this study, we review current knowledge regarding the slow slip events (SSE) and their recurrences in the Ryukyu and the Izu-Bonin subduction zones in Japan where active back-arc rifting and spreading occur. We mainly use geodetic data from continuous GNSS observing stations in Japan.

  Behind the southwest Ryukyu Arc, active back-arc spreading goes on at the Okinawa Trough. We updated the past study by Heki and Kataoka (JGR 2008), and found 37 biannually repeating SSEs beneath the Iriomote Island with average moment magnitude (Mw) of 6.6. The curve of the cumulative slip shows significant fluctuations with a decadal period. Seismological studies revealed that the fluctuation might be linked to the activity of back-arc spreading (Tu and Heki, in preparation).

  GNSS observations showed that an early stage back-arc rifting is starting behind the northernmost part of the Izu-Bonin Arc (the Izu Islands) (Nishimura, Tectonophys. 2011). We found transient eastward movements of these islands starting in 2004 July, and this can be explained by a slow slip over a large fault segment extending more than 250 km in north-south. Its Mw is inferred to be 7.5 or more, and would be the largest SSE observed in Japanese subduction zones (Arisa and Heki, under review).

  Distance between the trench and the volcanic arc is about 200 km throughout the Izu-Bonin subduction zone. There is an exception in the southern Izu-Bonin Arc (the Bonin Islands), where we can perform geodetic observations on the Bonin Islands only 100 km apart from the trench axis. There we identified five SSEs repeating every 1.5-2 years since 2007 with Mw of 6.8-6.9. The GNSS data are fairly noisy in these islands, but independent geodetic network of Very Long Baseline Interferometry (VLBI) supported the GNSS observations (Arisa and Heki, in preparation).