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[SCG52-P17] Seismic activity around Suzu city in the northeastern Noto Peninsula
Keywords:swarm earthquake, crustal fluid, fluid migration, fluid pressure
Seismic activities around Suzu City in the northeastern part of the Noto Peninsula have been enhanced since June 2018 and have become more active since December 2020, with earthquakes occurring in four regions. The analysis of GNSS data has revealed that local crustal movement has begun mainly in the northeastern part of the Noto Peninsula, and a spherical pressure source and a slow fault slip model have been proposed as a source of the movement (e.g., Nishimura, 2021). On September 16, 2021, the Mj 5.1 earthquake occurred. We report here on the relationship between seismic activity and a spherical pressure source and migrations of hypocenters.
We focused on the area around Suzu City in the northeastern part of the Noto Peninsula. The JHD method (Kissling et al., 1994) was used to estimate the one-dimensional seismic velocity structure using the JMA data. Based on this velocity structure, we relocated hypocenters by the Double Difference method (Waldhauser and Ellsworth, 2000) using seismic waveforms obtained from JMA, Hi-net, ERI, University of Tokyo, and DPRI, Kyoto University. We also determined the source mechanism of some earthquakes using the initial motion of P-waves (Maeda, 1992).
Seismic activity in the study area is divided into four parts: the southern part (Toyama Bayside), the western part (inland), the northern part (Japan Seaside), and the eastern part (the tip of the Noto Peninsula). The results of the relocation confirm that the hypocenters in the southern part are mainly deeper than 15 km after December 2020, in contrast to the depths of the earthquakes in the other three parts, which are between 10 and 15 km. The distribution of earthquakes in the southern part shows a high dip angle of about 70°, while the other three parts show a dip angle of about 45°. The fault parameters of the F-net CMT solution are generally consistent with these dip angles.
The ΔCFF calculated by the spherical pressure source explains the enhanced seismic activity with reverse faulting mechanisms in the western, northern, and eastern parts. The slow fault slip model is not as good as the spherical pressure source model in explaining the seismic activity. For the southern part, the source mechanism determined by the initial motion provides a lateral-slip or normal-faulting type, and ΔCFF also promotes earthquakes with these types of mechanisms. Lateral-slip and normal-fault mechanisms have also been obtained for earthquakes in the southern part from June 2018 to November 2020 at depths of 15 km or deeper. The ΔCFF from the spherical pressure source inferred from the crustal deformation after December 2020 also fits well with these seismic activities. This fact suggests that a spherical pressure source might exist before December 2020 and has been causing earthquakes in the southern part, although no clear signal was detected by geodetic observations.
We recognize a tendency for the hypocentral depths that become shallower in the southern part from June 2018 to November 2020, and in other areas, there is also a similar tendency for the hypocentral depths after the initiation of seismic activity., This tendency can also be found as an expansion of the hypocentral area. The migration of these hypocenters can be approximated by sqrt(4*pi*D*t) (Shaprio et al., 1997), suggesting that the migration is affected by the diffusion of fluid pressure. Seismic activity has become more active since July 2021, while there is no corresponding change in the crustal movement data. The current seismic activity around Suzu City might be not only influenced by the spherical pressure source but also by fluid movement along faults.
Acknowledgments: We used data provided by JMA, NIED, DPRI, Kyoto University, and ERI, the University of Tokyo. We are grateful to Takuya Nishimura for providing us with the analysis results.
We focused on the area around Suzu City in the northeastern part of the Noto Peninsula. The JHD method (Kissling et al., 1994) was used to estimate the one-dimensional seismic velocity structure using the JMA data. Based on this velocity structure, we relocated hypocenters by the Double Difference method (Waldhauser and Ellsworth, 2000) using seismic waveforms obtained from JMA, Hi-net, ERI, University of Tokyo, and DPRI, Kyoto University. We also determined the source mechanism of some earthquakes using the initial motion of P-waves (Maeda, 1992).
Seismic activity in the study area is divided into four parts: the southern part (Toyama Bayside), the western part (inland), the northern part (Japan Seaside), and the eastern part (the tip of the Noto Peninsula). The results of the relocation confirm that the hypocenters in the southern part are mainly deeper than 15 km after December 2020, in contrast to the depths of the earthquakes in the other three parts, which are between 10 and 15 km. The distribution of earthquakes in the southern part shows a high dip angle of about 70°, while the other three parts show a dip angle of about 45°. The fault parameters of the F-net CMT solution are generally consistent with these dip angles.
The ΔCFF calculated by the spherical pressure source explains the enhanced seismic activity with reverse faulting mechanisms in the western, northern, and eastern parts. The slow fault slip model is not as good as the spherical pressure source model in explaining the seismic activity. For the southern part, the source mechanism determined by the initial motion provides a lateral-slip or normal-faulting type, and ΔCFF also promotes earthquakes with these types of mechanisms. Lateral-slip and normal-fault mechanisms have also been obtained for earthquakes in the southern part from June 2018 to November 2020 at depths of 15 km or deeper. The ΔCFF from the spherical pressure source inferred from the crustal deformation after December 2020 also fits well with these seismic activities. This fact suggests that a spherical pressure source might exist before December 2020 and has been causing earthquakes in the southern part, although no clear signal was detected by geodetic observations.
We recognize a tendency for the hypocentral depths that become shallower in the southern part from June 2018 to November 2020, and in other areas, there is also a similar tendency for the hypocentral depths after the initiation of seismic activity., This tendency can also be found as an expansion of the hypocentral area. The migration of these hypocenters can be approximated by sqrt(4*pi*D*t) (Shaprio et al., 1997), suggesting that the migration is affected by the diffusion of fluid pressure. Seismic activity has become more active since July 2021, while there is no corresponding change in the crustal movement data. The current seismic activity around Suzu City might be not only influenced by the spherical pressure source but also by fluid movement along faults.
Acknowledgments: We used data provided by JMA, NIED, DPRI, Kyoto University, and ERI, the University of Tokyo. We are grateful to Takuya Nishimura for providing us with the analysis results.