5:15 PM - 6:30 PM
[SCG39-P20] Distribution of Very-Low Frequency Earthquake in the Central Ryukyu Trench
Keywords:Ryukyu Trench, slow earthquake, very-low frequency earthquake, interplate coupling
The distribution of very-low-frequency earthquakes (VLFEs), one type of slow earthquake, illuminates the heterogeneous stress accumulation at the subducting plate interface. VLFEs occurred along the Ryukyu Trench (Ando et al., 2012); however, their detailed distribution has not been clarified because few broadband seismic stations were deployed along the Ryukyu Islands. The detection capability is poor in the Miyako Strait, between Miyako Island and Okinawa Island, where the seismic stations' spacing is over 300 km. In this study, we determined the epicenters of the VLFEs in the Miyako Strait and Okinawa Island using the waveform data from temporary stations in the central Ryukyu arc. We used the F-net stations of the National Research Institute for Earth Science and Disaster Resilience and temporary stations on Okinoerabu Island, Kume Island, Miyako Island, Tarama Island, and Hateruma Island, which were set up by the Association for the Development of Earthquake Prediction. We performed band-pass filtering at a frequency range of 0.05–0.1 Hz for the vertical component waveforms. We then determined the epicenters of the VLFEs using the template matching method (Asano et al., 2015). We adopted the thrust events of regular earthquakes near the Ryukyu Trench in the Miyako Strait for the template. We analyzed the epicenters of VLFEs from January 2015 to December 2019.
The VLFE was distributed in a band along the trench axis near Okinawa Island. On the other hand, in the Miyako Strait, the VLFEs were distributed in spots along the trench axis. In the Miyako Strait, events that could not be detected by the conventional method (Nakamura and Sunagawa, 2015) were also detected. The horizontal location errors in VLFE near Okinawa Island and the Miyako Strait were both 10-12 km in error. The VLFE in the Miyako Strait occurred at a slab depth of 10-15 km. The distribution of VLFEs was complementary to that of reverse fault earthquakes. However, the active area of the VLFEs near Miyako Island was close to the distribution of the reverse fault type earthquakes.
On the other hand, the VLFE activity near Okinawa Island was concentrated at a 12-15 km slab depth. This area was located in a gap between the slow slip events (Nishimura, 2014) and the interplate locked area (Tadokoro et al., 2018). The distribution of activity and reverse fault earthquakes was complementary. This suggests the lateral heterogeneity of the frictional coefficient in the subducted plate interface.
The VLFE was distributed in a band along the trench axis near Okinawa Island. On the other hand, in the Miyako Strait, the VLFEs were distributed in spots along the trench axis. In the Miyako Strait, events that could not be detected by the conventional method (Nakamura and Sunagawa, 2015) were also detected. The horizontal location errors in VLFE near Okinawa Island and the Miyako Strait were both 10-12 km in error. The VLFE in the Miyako Strait occurred at a slab depth of 10-15 km. The distribution of VLFEs was complementary to that of reverse fault earthquakes. However, the active area of the VLFEs near Miyako Island was close to the distribution of the reverse fault type earthquakes.
On the other hand, the VLFE activity near Okinawa Island was concentrated at a 12-15 km slab depth. This area was located in a gap between the slow slip events (Nishimura, 2014) and the interplate locked area (Tadokoro et al., 2018). The distribution of activity and reverse fault earthquakes was complementary. This suggests the lateral heterogeneity of the frictional coefficient in the subducted plate interface.