3.1 Waveforms of the triggered LFEs
In the Yaeyama area, the LFEs were triggered during the passage of the surface wave of one teleseismic earthquake. Among 45 teleseismic earthquakes, LFEs were detected in five events in the Yaeyama area.
In the Sumatra earthquake (Mw 9.1) of December 26, 2004, the LFEs were observed in the Yaeyama area when the surface waves passed at 1300–1700 s (Fig. 2a). The LFEs first arrived at the HATERS station, and then, they were delayed at stations east and west of the HATERS station. The LFE activity started when the maximum amplitude of the Love wave arrived rather than that of the Rayleigh wave (Fig. 2a). In this earthquake, LFEs were observed even in the vicinity of Okinawa Island when the surface waves passed at 1500–1800 s (Fig. 3a). The LFEs arrived the earliest at ZMM or TAMAG2, and later at KUMEJ2 or KUNIGA stations. At 1500–1800 s, two predominant phases were observed at various stations. At TAMAG2 and ZMM, weak but possible LFEs were detected at 1600–1700 s. These phases were not observed at the other stations. The arrival time of these LFEs corresponded to those of the maximum amplitude of Love waves.
The LFEs were also observed in the waveforms of the Nias earthquake on March 28, 2005 (Mw8.6). However, the amplitudes of the LFE were the smallest among the five events in which LFEs were observed. The LFEs were observed at stations to the northeast of the HATERS station when the maximum amplitude of the Rayleigh wave arrived in the Yaeyama area (1100 s after the earthquake) (Fig. 2b). The LFEs were also observed in the Okinawa area during the passage of the surface waves (at approximately 1700 s) (Fig. 3b). The LFEs rapidly reached TAMAG2 and ZMM in the Okinawa area. The arrival time was closer to the passage time of the maximum amplitude of the Love wave (1600 s) than that of the Rayleigh wave (1200 s).
In the Wenchuan earthquake of 2008 (Mw7.9), LFEs were observed in the Yaeyama area when the surface waves passed at 700–900 s (Fig. 2c). The arrival of the LFEs was the fastest at the HATERS station. The LFEs were observed around the arrival of the maximum amplitude of the Love waves in the Yaeyama area. Similar LFEs were observed in Okinawa at 900–1000 s. The LFEs were first observed at ZMM and TAMAG2 at 950 s, which was the arrival time of the maximum amplitude of the Rayleigh wave in Okinawa.
In the Sumatra earthquake of 2012 (Mw8.6), LFEs were observed in the Yaeyama area at 1500–2000 s (Fig. 2d). The LFEs were first observed at the HATERS station and then at YONAGU. The LFEs intermittently continued after the passage of the maximum amplitude of the surface wave (Rayleigh waves and Love waves). Even in Okinawa, significant arrivals of LFEs were observed at 1300, 1600, and 1800 s (Fig. 3d). The LFEs were observed around the arrival time of the maximum peak of the surface wave.
In the Gorkha earthquake in Nepal on April 25, 2015 (Mw7.8), the LFEs were observed in the Yaeyama area when the surface wave passed at 1200–1400 s (Fig. 2e). In the waveform, the arrival of LFE at 1200 s corresponded to the maximum amplitude of the Love wave, and the arrival of LFE at 1400 s corresponded to the maximum amplitude of the Rayleigh wave. After the passage of the surface wave (1900 s or more), a plurality of LFEs were recorded at KUROSH, IGK, ISHIG2, and ISHIGH (Fig. 2e). The LFEs were observed around the maximum amplitude of the Love wave (1400–1600 s) even in the Okinawa area (Fig. 3e). The LFEs reached the TAMAG3 station the fastest. They were observed in the entire Okinawa area even after the surface wave passed at 1900 s. The LFEs were observed at ZMM, TAMAG2, KUNIGA, and NAGOT at 2150 s.
3.2 Hypocenter distribution
Seven LFEs were identified in the Yaeyama area (Fig. 4a, Table S2). The hypocenters of LFEs generated during the passage of the surface waves were distributed around 24.0°N and 123.6°E. The hypocenters of LFEs, which occurred immediately after the passage of the surface waves, were scattered around 24.0°N and 123.6°E. The error in the determination of the hypocenter of LFEs in the 2004 event is shown by the contour lines in Fig. 4a. Since the depth determination was not accurate, the determination error at the depth of 0 km was plotted. The contour line extended in the N–S direction and had a large error in the direction perpendicular to the trench axis.
Seven LFEs were identified in the Okinawa area (Fig. 4b, Table S2). Most of the LFEs were distributed around 25.8°N and 127.7°E. The error of the hypocenter determination is shown by the contour lines in Fig. 4b. The contours extended obliquely to the trench axis, away from the seismic network near Okinawa Island. There was a large error in the direction away from the island. In the Okinawa area, the LFEs tended to occur separately, except for during the 2015 event. The hypocenters of the LFEs, which occurred immediately after the passage of the surface waves, were also distributed to the south of Okinawa Island. The difference in the distribution of the LFEs during and after the surface wave passage was not significant.