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[SCG39-P03] Spatio-temporal distribution of very low-frequency earthquakes activity in the north Ryukyu Trench
Keywords:slow earthquake, Ryukyu trench, interplate coupling, very-low frequency earthquake
We investigated the Spatio-temporal changes in the activity of very low-frequency earthquakes (VLFEs) occurring in the northern Ryukyu Trench. We used broadband ocean-bottom seismometers (OBS) installed in the north Ryukyu Trench and the north Okinawa Trough, and broadband seismographs installed on land (F-net and temporary stations). The station intervals of the OBSs were approximately 20 to 60 km. The template matching method (Asano et al., 2015) was used to detect and the epicentral determination of the VLFEs. The vertical component waveforms were band-pass filtered at 0.05-0.1 Hz, and the cross-correlation with the template was calculated. Thrust-type earthquakes that occurred in the Ryukyu Trench were used as the template.
As a result of the analysis, active regions of the VLFEs were distributed off the northeast of Amami Island and from the southeast coast of Amami Island to the offshore of Okinoerabu Island. The errors of the epicentral position were 10-20 km. Off the northeast coast of Amami Island, the swarm of VLFEs started in November 2018. The swarm area corresponds to the slab depth of 10-30 km. The swarm activity concentrated around 28.6N and 130.6E, and the activity of the VLFEs spreaded to the west and south of the active area, avoiding the active area of regular earthquakes. In January 2019, the VLFE swarm activity was observed off the east of Amami Island. The VLFE activity was separated in several clusters along the trench in the east of Okinoerabu Island. The location of the clusters corresponded to a slab depth of 10-20 km.
The activity of these VLFEs did not overlap with the faults of slow slip events (SSEs). However, they tend to occur at the edge of the SSE faults near Kikai Island and Okinoerabu Island. The cluster of the VLFEs was complementary to the distribution of the thrust-type earthquakes.
The magnitude distribution of the VLFEs showed that the large-scale VLFEs occurred in the clusters of VLFEs in the east of Okinoerabu Island and east of Amami Island. The magnitude of VLFEs tended to be larger at shallower slab depths. Variation in the size of maximum earthquake was also observed along the trench axis. This suggests that the frictional condition is changing along the trench axis.
As a result of the analysis, active regions of the VLFEs were distributed off the northeast of Amami Island and from the southeast coast of Amami Island to the offshore of Okinoerabu Island. The errors of the epicentral position were 10-20 km. Off the northeast coast of Amami Island, the swarm of VLFEs started in November 2018. The swarm area corresponds to the slab depth of 10-30 km. The swarm activity concentrated around 28.6N and 130.6E, and the activity of the VLFEs spreaded to the west and south of the active area, avoiding the active area of regular earthquakes. In January 2019, the VLFE swarm activity was observed off the east of Amami Island. The VLFE activity was separated in several clusters along the trench in the east of Okinoerabu Island. The location of the clusters corresponded to a slab depth of 10-20 km.
The activity of these VLFEs did not overlap with the faults of slow slip events (SSEs). However, they tend to occur at the edge of the SSE faults near Kikai Island and Okinoerabu Island. The cluster of the VLFEs was complementary to the distribution of the thrust-type earthquakes.
The magnitude distribution of the VLFEs showed that the large-scale VLFEs occurred in the clusters of VLFEs in the east of Okinoerabu Island and east of Amami Island. The magnitude of VLFEs tended to be larger at shallower slab depths. Variation in the size of maximum earthquake was also observed along the trench axis. This suggests that the frictional condition is changing along the trench axis.