17:15 〜 18:30
[SCG39-P01] Migration characteristics of shallow very low frequency earthquake episodes southeast off the Kii Peninsula
キーワード：slow earhquake、Nankai Trough、migration、source time function
Recent studies revealed that slow earthquake activity patterns could be related to the tectonic environment, such as frictional conditions and stress accumulation on the plate boundary (e.g., Takemura, Noda et al., 2019; Baba et al., 2020). To reveal spatial heterogeneities on the shallow plate boundary off the Kii Peninsula, where shallow slow earthquakes actively occur, we conducted template matching and cross-correlation analysis of Takemura, Noda et al. (2019). In our analysis, to enhance VLFE signals at NIED F-net stations, we used filtered velocity waveforms with periods of 20-50 s.
Takemura Noda et al. (2019) reported the large VLFE episodes in September 2004 and March 2009. We found another large shallow VLFE episode, which started on 6 December 2020. This episode started in the eastern side of the shallow VLFE active areas, and then VLFEs bilaterally migrated along the strike direction at a speed of approximately 4 km/day (Figure). Several rapid (~50 km/day) VLFE reverse migrations after passing the major migration front were found. Similar characteristics were confirmed in the episodes of 2004 and 2009, but the major fronts of both VLFE episodes were characterized by eastward migration with approximately 8 km/day from the western side of the activity area. These differences among each shallow VLFE episode may be related to local heterogeneities on the shallow plate boundary southeast off the Kii Peninsula.
To investigate this episode more quantitatively, we evaluated moment rate functions of detected VLFEs via the simulated annealing method. Green’s functions from sources to F-net stations in the regional 3D model were evaluated by the OpenSWPC (Maeda et al., 2017). We constructed the 3D model by combining Koketsu et al. (2012) and Tonegawa et al. (2017). Focal mechanisms and source depths were fixed as low-angle thrust-faulting mechanisms on the plate boundary (e.g., Baba et al., 2020). By using estimated moment rate functions, we depicted spatiotemporal characteristics of moment release during this VLFE episode. The total moment of this episode was 3.45×1017 Nm, which is about ten times larger than that of the episode in April 2016 estimated by DONET data (Nakano et al., 2018). The VLFEs with low moment release rates were dominant in the initiation phase of the episode, the VLFEs with high moment rates continuously occurred along the major and rapid reverse migration fronts. This feature is similar to the deep tremor episodes (e.g., Thomas et al., 2013; Yabe and Ide, 2014).
Acknowledgments: We used NIED F-net data https://doi.org/10.17598/NIED.0005. Numerical simulations were conducted on the Fujitsu PRIMERGY CX600M1/CX1640M1 (Oakforest-PAC) in the Information Technology Center, University of Tokyo. This study was also supported by the ERI JURP 2020-S-04
Takemura Noda et al. (2019) reported the large VLFE episodes in September 2004 and March 2009. We found another large shallow VLFE episode, which started on 6 December 2020. This episode started in the eastern side of the shallow VLFE active areas, and then VLFEs bilaterally migrated along the strike direction at a speed of approximately 4 km/day (Figure). Several rapid (~50 km/day) VLFE reverse migrations after passing the major migration front were found. Similar characteristics were confirmed in the episodes of 2004 and 2009, but the major fronts of both VLFE episodes were characterized by eastward migration with approximately 8 km/day from the western side of the activity area. These differences among each shallow VLFE episode may be related to local heterogeneities on the shallow plate boundary southeast off the Kii Peninsula.
To investigate this episode more quantitatively, we evaluated moment rate functions of detected VLFEs via the simulated annealing method. Green’s functions from sources to F-net stations in the regional 3D model were evaluated by the OpenSWPC (Maeda et al., 2017). We constructed the 3D model by combining Koketsu et al. (2012) and Tonegawa et al. (2017). Focal mechanisms and source depths were fixed as low-angle thrust-faulting mechanisms on the plate boundary (e.g., Baba et al., 2020). By using estimated moment rate functions, we depicted spatiotemporal characteristics of moment release during this VLFE episode. The total moment of this episode was 3.45×1017 Nm, which is about ten times larger than that of the episode in April 2016 estimated by DONET data (Nakano et al., 2018). The VLFEs with low moment release rates were dominant in the initiation phase of the episode, the VLFEs with high moment rates continuously occurred along the major and rapid reverse migration fronts. This feature is similar to the deep tremor episodes (e.g., Thomas et al., 2013; Yabe and Ide, 2014).
Acknowledgments: We used NIED F-net data https://doi.org/10.17598/NIED.0005. Numerical simulations were conducted on the Fujitsu PRIMERGY CX600M1/CX1640M1 (Oakforest-PAC) in the Information Technology Center, University of Tokyo. This study was also supported by the ERI JURP 2020-S-04