3:30 PM - 3:45 PM
[SCG50-07] Decadal-Scale Changes in Very Low-Frequency Earthquake Activity in the Southwestern Ryukyu Trench
★Invited Papers
Keywords:Ryukyu Trench, Very-low frequency earthquake, Taiwan, Seismicity
In the southwestern Ryukyu Trench, very low-frequency earthquakes (VLFEs) occur in swarms every few months. The spatiotemporal variations in VLFE activity would reflect the spatial distribution and temporal changes in interplate coupling. Therefore, I investigated the spatiotemporal changes in VLFE activity in this region using waveform cross-correlation analysis to reveal temporal variations in interplate coupling.
I used broadband seismic waveform records from the F-net stations in the Ryukyu Islands and the BATS stations in Taiwan for the analysis. I examined VLFE activity over the period from 1998 to 2024. A band-pass filter of 0.02–0.05 Hz was applied to the three-component waveforms at each station, and I computed cross-correlation functions between the continuous waveforms and 120-second template earthquake waveforms. I used both regular thrust-type earthquakes and VLFEs as template events and back-propagated the cross-correlation values to estimate the origin time and epicenter, assuming a surface wave propagation velocity of 3.7 km/s. I then identified the origin times and epicenters where the average cross-correlation values across all stations reached their maximum. I also detected events with average cross-correlation values of 0.5 or higher as VLFEs.
The results showed that VLFEs were concentrated at 10–15 km slab depths along the trench axis, while their activity was low west of 123.0E. Furthermore, the distribution of VLFE occurrences was complementary to that of small- to medium-sized thrust-type earthquakes occurring at the plate interface. This suggests that interplate earthquakes occur less frequently in regions where VLFEs are active. I also found that M6-class intraslab earthquakes occurred near the VLFE region during the study period, but there was no clear evidence that they triggered VLFE activity.
By analyzing the temporal variations in VLFE activity, I found that VLFEs were inactive before 2001. However, their activity significantly increased between 2001 and 2002, coinciding with a period of frequent M6–7-class large earthquakes. Although these large earthquakes were not necessarily interplate events, VLFE swarms occurred immediately after their occurrence. Additionally, interplate earthquakes increased even near the trench axis, where they had not been observed before. This suggests that changes in interplate coupling around the VLFE region may have triggered large earthquakes and activated VLFE activity.
Since around 2022, VLFE activity has intensified southeast of Ishigaki Island in the southwestern Ryukyu Trench. Furthermore, since late 2021, interplate seismic activity has also increased in the southwesternmost part of the Ryukyu Trench (122.2–122.7E). This change may be influenced by aseismic slip along the Longitudinal Valley Fault in Taiwan, which could have altered the stress field and promoted earthquake and VLFE activity.
I used broadband seismic waveform records from the F-net stations in the Ryukyu Islands and the BATS stations in Taiwan for the analysis. I examined VLFE activity over the period from 1998 to 2024. A band-pass filter of 0.02–0.05 Hz was applied to the three-component waveforms at each station, and I computed cross-correlation functions between the continuous waveforms and 120-second template earthquake waveforms. I used both regular thrust-type earthquakes and VLFEs as template events and back-propagated the cross-correlation values to estimate the origin time and epicenter, assuming a surface wave propagation velocity of 3.7 km/s. I then identified the origin times and epicenters where the average cross-correlation values across all stations reached their maximum. I also detected events with average cross-correlation values of 0.5 or higher as VLFEs.
The results showed that VLFEs were concentrated at 10–15 km slab depths along the trench axis, while their activity was low west of 123.0E. Furthermore, the distribution of VLFE occurrences was complementary to that of small- to medium-sized thrust-type earthquakes occurring at the plate interface. This suggests that interplate earthquakes occur less frequently in regions where VLFEs are active. I also found that M6-class intraslab earthquakes occurred near the VLFE region during the study period, but there was no clear evidence that they triggered VLFE activity.
By analyzing the temporal variations in VLFE activity, I found that VLFEs were inactive before 2001. However, their activity significantly increased between 2001 and 2002, coinciding with a period of frequent M6–7-class large earthquakes. Although these large earthquakes were not necessarily interplate events, VLFE swarms occurred immediately after their occurrence. Additionally, interplate earthquakes increased even near the trench axis, where they had not been observed before. This suggests that changes in interplate coupling around the VLFE region may have triggered large earthquakes and activated VLFE activity.
Since around 2022, VLFE activity has intensified southeast of Ishigaki Island in the southwestern Ryukyu Trench. Furthermore, since late 2021, interplate seismic activity has also increased in the southwesternmost part of the Ryukyu Trench (122.2–122.7E). This change may be influenced by aseismic slip along the Longitudinal Valley Fault in Taiwan, which could have altered the stress field and promoted earthquake and VLFE activity.