Abstract
In this study, short-term slow slip events (SSEs) along the Ryukyu Trench, southwestern Japan were systematically examined using continuous global navigation satellite system (GNSS) data. In total, 130 probable and 93 possible short-term SSEs in the moment magnitude (M w ) range of 5.6 to 6.8 were identified from January 1997 to November 2013 by GNSS time series offset monitoring and elastic dislocation modeling with a rectangular fault located on the subducting Philippine Sea Plate. The detected short-term SSEs were found to have a variety of characteristic recurrence intervals, magnitudes, durations, and coincidental seismic activities. Short-term SSEs without identified low-frequency earthquakes (LFEs) and low-frequency tremors (LFTs) were found to be common along the Ryukyu Trench. The total slip distributions and SSE numbers were heterogeneous and mostly between 10 and 60 km in depth. Although shallow (depth ≤20 km) short-term SSEs have never been detected along the Nankai Trough, it was notable that such SSEs often occur on the shallow plate interface along the Ryukyu Trench. This may be related to the incomplete interplate locking estimated by various geodetic studies. A band of short-term SSEs in the 20 to 40 km depth range extends from west Shikoku through the Bungo Channel to mid-Kyushu and then fades away around the subducted Kyushu-Palau Ridge. SSEs with accompanying LFEs and LFTs were found to be limited to western Shikoku and the Bungo Channel. We found several distinctive clusters of short-term SSEs, in addition to a cluster previously identified in the Yaeyama Islands. The study also identified a cluster northeast of Kikaijima consisting of 20 repeated SSEs at depths in the vicinity of 10 km near the trench where the Amami Plateau subducts, as well as another cluster southeast of Okinawa Island consisting of 29 M w ≤ 6.0 SSEs. The results suggest that the distribution of short-term SSEs, as well as that of large earthquakes, is affected by the topography of the subducting plate.
Highlights
Slow slip events (SSEs) have been observed in subduction zones worldwide by geodetic instruments
The estimated moment magnitude for all identified SSEs ranges from 5.6 to 6.8 (Figure 5). These figures suggest that large short-term SSEs occur frequently along the Ryukyu Trench, which is in contrast with the Nankai Trough along which the largest event is Moment magnitude (Mw) 6.3 (Sekine et al 2010; Nishimura et al 2013)
It is notable that the uncertainties in the parameters for SSEs along the Ryukyu Trench are larger than those along the Nankai Trough because the global navigation satellite system (GNSS) network is less dense along the Ryukyu Trench and the SSEs here occur offshore, away from the GNSS stations
Summary
Slow slip events (SSEs) have been observed in subduction zones worldwide by geodetic instruments (cf., Schwartz and Rokosky 2007; Beroza and Ide 2011; Nishimura et al 2014). Because dense geodetic networks have revealed numerous repeated SSEs in the circum-Pacific seismic belt (e.g., Obara and Hirose 2006; Szeliga et al 2008; Wallace et al 2012; Ozawa et al 2013; Ozawa 2014), they have come to be regarded as playing an important role in releasing strain in subduction zones. Slow earthquakes, including SSEs, low-frequency earthquakes (LFEs), low-frequency tremors (LFTs), and very low frequency earthquakes (VLFEs) along the Nankai Trough, which is located southwest of Japan, have been investigated by numerous researchers (e.g., Hirose et al 1999; Ozawa et al 2001, 2013; Obara 2002, 2010, 2011; Ito et al 2009). Numerous LFEs form a band in western Shikoku and the Bungo Channel (Figure 1), and short-term SSEs
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