Distribution of low-frequency earthquakes accompanying the very low frequency earthquakes along the Ryukyu Trench

This study investigated the activity and distribution of low-frequency earthquakes (LFEs) accompanying the very low frequency earthquakes (VLFEs) in the central and southern Ryukyu Trench. This investigation was based on short-period seismometer waveforms obtained by the Japan Meteorological Agency from April 2004 to December 2015. The LFEs were detected using the Envelope Correlation Method, and the hypocenter locations were established using the arrival time difference of the envelope. The arrival times of the P- and S-phases were selected for events in which conspicuous P and S arrivals were observed and their hypocenters were determined. The results showed that LFEs are distributed 30-50 km from the trench axis in the Okinawa and Yaeyama areas. These areas correspond to a slab depth of 12-25 km. The LFEs and VLFEs occurred in association with slow slip events (SSEs) in the Okinawa and Yaeyama areas, indicating that they are induced by SSEs in the Ryukyu Trench. Moreover, the SSEs, LFE-VLFEs, and thrust-type ordinary earthquakes exhibit separate distributions. This suggests change in the frictional condition at the slab depth of 12-25 km along the trench axis in the Ryukyu subduction zone. In the southern Ryukyu Trench, LFEs occur approximately 50 km from the SSE faults, suggesting that SSEs trigger the LFEs near the southern Ryukyu Trench.