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[SSS10-P11] Relationship between the Boso slow slip events and the accompanying earthquake swarm: Part 2
Keywords:slow slip, earthquake swarm, ΔCFF, fluid movement
1.Introduction
Slow slip events (SSEs) are phenomena that, unlike normal earthquakes, slowly slide across plate boundaries without radiating seismic waves. SSEs are detected by continuous observation of crustal deformation using geodetic techniques, and their occurrence has been reported in Japan and around the world. Further understanding of SSEs is considered necessary to reveal the mechanisms of stress accumulation and release at plate boundaries, as well as to clarify important issues in seismology, such as the prediction of large earthquakes.
In the off Boso area, short-term SSEs with a duration of several to several tens of days have occurred seven times from 1996 to 2024 with intervals of about 2 to 7 years. The Boso SSEs involve earthquake swarms.
The occurrence of the SSEs and the swarm activities are thought to be related: in the 2007 and 2011 Boso SSEs, the slip of the SSEs and swarm activities were well correlated in time and space, suggesting that the SSEs may have induced the swarm activities (Hirose et al, GRL, 2014). Yasuhara and Sato (JpGU, 2024; Seismological Society of Japan, 2024) obtained ΔCFF for the 2002 and 2007 swarms occurred with the SSEs, suggesting the possibility of fluid movement.
In this study, we examine whether the 2011, 2014, and 2018 Boso SSEs induced the associated swarms and consider the factors that cause the swarms.
2.Analysis method
An ABIC inversion analysis was performed on daily coordinate values (F5 solution) of GNSS data by GSI GEONET to estimate the spatio-temporal distribution of slip of the SSEs. From the obtained slip distribution, we calculated ΔCFFs due to the SSEs on the faults of the swarms. ΔCFFs were calculated for the swarms of Mw 3.5 or greater (5 in 2011, 4 in 2014, and 13 in 2018) that occurred during and one month after the SSEs. We used focal mechanism solutions by the F-net of NIED.
3.Results
Of the ΔCFFs obtained, 40 % (2/5 faults) in 2011, 50 % (2/4 faults) in 2014, and 38 % (5/13 faults) in 2018 were above 0.01 MPa, a condition empirically considered earthquake-prone. These results indicate that the SSEs that occurred in 2011, 2014, and 2018, did not directly trigger the swarms. Therefore, we considered the change in pore pressure that would cause ΔCFF to be above a threshold value of 0.01 MPa for the faults of the swarms.
The change in pore pressure was estimated about 0.05~0.18 MPa increase for the faults of swarms that occurred near SSE’s slip area during the SSEs. Kobayashi and Sato (GRL, 2021) estimated effective normal stress (10~50 MPa) in the area of the Boso SSEs, which is very small compared to lithostatic stress (350~600 MPa), suggesting the presence of high pore pressure. The same analyses as in this study for the 2002 and 2007 SSEs also estimated an increase in pore pressure of about 0.03~0.2 MPa. These results suggest that the slips of the SSEs may have caused fluid migration from its slip area to the fault of the swarms.
Therefore, it is considered that the swarms were triggered by the SSEs through an indirect factor of fluid movement from the slip region of SSEs to the faults of the swarms.
Acknowledgements
Daily coordinates of GSI GEONET (F5 solution) were used. Coulomb 3.4 from the U.S. Geological Survey was used to calculate stresses. Data from F-net of NIED were used for the mechanism solution of earthquake swarms. This research was supported by Grant-in-Aid for Scientific Research (23K03541).
Slow slip events (SSEs) are phenomena that, unlike normal earthquakes, slowly slide across plate boundaries without radiating seismic waves. SSEs are detected by continuous observation of crustal deformation using geodetic techniques, and their occurrence has been reported in Japan and around the world. Further understanding of SSEs is considered necessary to reveal the mechanisms of stress accumulation and release at plate boundaries, as well as to clarify important issues in seismology, such as the prediction of large earthquakes.
In the off Boso area, short-term SSEs with a duration of several to several tens of days have occurred seven times from 1996 to 2024 with intervals of about 2 to 7 years. The Boso SSEs involve earthquake swarms.
The occurrence of the SSEs and the swarm activities are thought to be related: in the 2007 and 2011 Boso SSEs, the slip of the SSEs and swarm activities were well correlated in time and space, suggesting that the SSEs may have induced the swarm activities (Hirose et al, GRL, 2014). Yasuhara and Sato (JpGU, 2024; Seismological Society of Japan, 2024) obtained ΔCFF for the 2002 and 2007 swarms occurred with the SSEs, suggesting the possibility of fluid movement.
In this study, we examine whether the 2011, 2014, and 2018 Boso SSEs induced the associated swarms and consider the factors that cause the swarms.
2.Analysis method
An ABIC inversion analysis was performed on daily coordinate values (F5 solution) of GNSS data by GSI GEONET to estimate the spatio-temporal distribution of slip of the SSEs. From the obtained slip distribution, we calculated ΔCFFs due to the SSEs on the faults of the swarms. ΔCFFs were calculated for the swarms of Mw 3.5 or greater (5 in 2011, 4 in 2014, and 13 in 2018) that occurred during and one month after the SSEs. We used focal mechanism solutions by the F-net of NIED.
3.Results
Of the ΔCFFs obtained, 40 % (2/5 faults) in 2011, 50 % (2/4 faults) in 2014, and 38 % (5/13 faults) in 2018 were above 0.01 MPa, a condition empirically considered earthquake-prone. These results indicate that the SSEs that occurred in 2011, 2014, and 2018, did not directly trigger the swarms. Therefore, we considered the change in pore pressure that would cause ΔCFF to be above a threshold value of 0.01 MPa for the faults of the swarms.
The change in pore pressure was estimated about 0.05~0.18 MPa increase for the faults of swarms that occurred near SSE’s slip area during the SSEs. Kobayashi and Sato (GRL, 2021) estimated effective normal stress (10~50 MPa) in the area of the Boso SSEs, which is very small compared to lithostatic stress (350~600 MPa), suggesting the presence of high pore pressure. The same analyses as in this study for the 2002 and 2007 SSEs also estimated an increase in pore pressure of about 0.03~0.2 MPa. These results suggest that the slips of the SSEs may have caused fluid migration from its slip area to the fault of the swarms.
Therefore, it is considered that the swarms were triggered by the SSEs through an indirect factor of fluid movement from the slip region of SSEs to the faults of the swarms.
Acknowledgements
Daily coordinates of GSI GEONET (F5 solution) were used. Coulomb 3.4 from the U.S. Geological Survey was used to calculate stresses. Data from F-net of NIED were used for the mechanism solution of earthquake swarms. This research was supported by Grant-in-Aid for Scientific Research (23K03541).