Repeating very low frequency earthquakes in the southwestern Ryukyu subduction zone

Very low-frequency earthquakes (VLFEs) are a type of slow earthquakes dominated by the low-frequency energy of 0.1-0.01 Hz, which are important phenomena in discussing how plates converge in subduction zones (Obara and Ito, 2005; Obara and Kato, 2016). Using broadband seismic data of F-net in Japan and BATS (Broadband Array in Taiwan for Seismology) in Taiwan, we detected a total of 29,841 VLFEs in the Ryukyu area for the period of 2005-2012. These VLFEs are located near the Ryukyu Trench axis and have shallow thrust-faulting mechanisms, which indicates that they probably occur in the accretional prism or on the shallow plate interface of the Ryukyu subduction zone.

Among these VLFEs, we note that 97 events, south of the Miyako Island, show notable similarities in waveforms (Cross-correlation coefficient > 0.9), magnitudes (M_w 3.8-4.2), and source mechanisms (thrust faulting). These events recurred in a small area (possibly < 30 km x 10 km). Moreover, they consist of typical mainshock-aftershock sequences decaying within 10 day (54 events) or more or less periodic sequences with an interval of ~50 days (43 events). Such type of VLFEs, named as repeating very low frequency earthquakes (RVLFEs), is similar to small repeating earthquakes in the creeping zone of the plate boundaries. This suggests that the RVLFEs should occur in the same fault patch, and their activity is possibly governed by the plate motion and frictional coefficients of the slip planes. The cumulative seismic slip by the RVLFEs accommodated about 6-26 % (in accretionary prisms) or 3-42 % (on the slab interface) of the time-averaged plate convergence. These two ratios are significantly smaller than that of 72 % released by repeating slow slip events beneath Hateruma Island, southwestern Ryukyu, detected by Tu and Heki (2016).