5:15 PM - 7:15 PM
[SSS10-P10] Reproduction of interplate slip phenomena along the Nankai Trough: Deep short-term slow slip events
Keywords:Nankai Trough, Short-term slow slip events, Simulation, Cut-off time
Introduction
Various types of slip phenomena occur between plates along the Nankai Trough, including great earthquakes and slow earthquakes, depending on the depth [e.g., Obara & Kato 2016, Science]. In particular, short-term slow slip evens (SSSEs) and deep tremors are distributed in a belt-like zone with several segmentations [e.g., Obara+ 2010, GRL; Okada+ 2022, EPS].
To elucidate the slip phenomena between plates, simulation studies based on the rate- and state-dependent friction (RSF) law have been conducted [e.g., Matsuzawa+ 2013, GRL; Hirose+ 2022, EPS]. Matsuzawa+ [2013] reproduced deep tremors, SSSEs, and LSSEs in the Shikoku region using the RSF law that introduced a cutoff velocity (cutoff time) that represents the characteristics of laboratory experiments using halite [Shimamoto 1986, Science]. In this case, SSSEs were expressed by continuous rupture of tremor patches.
Targeted phenomena
In this study, our targeted phenomena are deep SSSEs along the Nankai Trough. The Mw is about 5.5 to 6, the duration is about 4 days, and the recurrence interval is about 3 to 12 months [e.g., Nishimura+ 2013, JGR]. Slow earthquakes also move along the strike direction at a speed of about 10 km/day [Obara+ 2012, GRL]. The activity and slip amount of SSSEs are large in Ise Bay, eastern Shikoku, and western Shikoku [Okada+ 2022].
The areas where SSSEs and tremors occur largely overlap, but the activity of tremors is low in Ise Bay [Obara+ 2010]. With the same settings as in the previous study [Matsuzawa+ 2013], which expressed SSSEs by continuous rupture of tremor patches, it is not possible to generate SSSEs in areas without tremors. Therefore, we did not place tremor patches and simply gave various parameters as functions of depth.
Model
We modeled the 3-D plate interface from the Tokai region to the Hyuga-Nada Sea, and 53,550 triangular cells with sides of ~2.5 km are arranged. The plate convergence rate is 1.0 cm/y at the eastern end and 5.5 cm/y at the western end [Nishimura+ 2018, Geosphere].
In this study, we adopted the RSF law that introduces a cutoff time with a cutoff time tcx into the aging law [Yoshida+ 2013, JGR]. The friction parameter a is constant at 0.005 for all cells. The friction parameters a-b are set to -0.003 at depths shallower than 35 km and +0.003 at depths deeper than that. The effective normal stress σ , characteristic distance L , and cutoff time tcx are changed in five depth ranges: 1) depths shallower than 10 km, 2) 10–20 km, 3) 20–30 km, 4) 30–35 km, and 5) deeper than 35 km.
Results
In our simulation, Mw 8-class earthquakes occurred at depths shallower than 20 km, LSSEs occurred at depths of 20–30 km, and SSSEs occurred at depths of 30–35 km. The SSSEs were larger in scale in Ise Bay and western Shikoku, which is consistent with the observations. In these regions, the plate boundary is shallower than in the surrounding areas and has width in the dip direction, so it is thought that the SSSEs become larger in scale due to stronger locking after one slip. However, it should be noted that the SSSEs are low in eastern Shikoku and active in the Kii Channel, which is different from the observations. The SSSEs occur at intervals of about 3 months to 1 year, and the slip migration speed is about 10 km/day, which is consistent with the observations. In some cases, SSSEs are excited by the influence of LSSEs on the updip side, shortening the interval between SSSEs.
Various types of slip phenomena occur between plates along the Nankai Trough, including great earthquakes and slow earthquakes, depending on the depth [e.g., Obara & Kato 2016, Science]. In particular, short-term slow slip evens (SSSEs) and deep tremors are distributed in a belt-like zone with several segmentations [e.g., Obara+ 2010, GRL; Okada+ 2022, EPS].
To elucidate the slip phenomena between plates, simulation studies based on the rate- and state-dependent friction (RSF) law have been conducted [e.g., Matsuzawa+ 2013, GRL; Hirose+ 2022, EPS]. Matsuzawa+ [2013] reproduced deep tremors, SSSEs, and LSSEs in the Shikoku region using the RSF law that introduced a cutoff velocity (cutoff time) that represents the characteristics of laboratory experiments using halite [Shimamoto 1986, Science]. In this case, SSSEs were expressed by continuous rupture of tremor patches.
Targeted phenomena
In this study, our targeted phenomena are deep SSSEs along the Nankai Trough. The Mw is about 5.5 to 6, the duration is about 4 days, and the recurrence interval is about 3 to 12 months [e.g., Nishimura+ 2013, JGR]. Slow earthquakes also move along the strike direction at a speed of about 10 km/day [Obara+ 2012, GRL]. The activity and slip amount of SSSEs are large in Ise Bay, eastern Shikoku, and western Shikoku [Okada+ 2022].
The areas where SSSEs and tremors occur largely overlap, but the activity of tremors is low in Ise Bay [Obara+ 2010]. With the same settings as in the previous study [Matsuzawa+ 2013], which expressed SSSEs by continuous rupture of tremor patches, it is not possible to generate SSSEs in areas without tremors. Therefore, we did not place tremor patches and simply gave various parameters as functions of depth.
Model
We modeled the 3-D plate interface from the Tokai region to the Hyuga-Nada Sea, and 53,550 triangular cells with sides of ~2.5 km are arranged. The plate convergence rate is 1.0 cm/y at the eastern end and 5.5 cm/y at the western end [Nishimura+ 2018, Geosphere].
In this study, we adopted the RSF law that introduces a cutoff time with a cutoff time tcx into the aging law [Yoshida+ 2013, JGR]. The friction parameter a is constant at 0.005 for all cells. The friction parameters a-b are set to -0.003 at depths shallower than 35 km and +0.003 at depths deeper than that. The effective normal stress σ , characteristic distance L , and cutoff time tcx are changed in five depth ranges: 1) depths shallower than 10 km, 2) 10–20 km, 3) 20–30 km, 4) 30–35 km, and 5) deeper than 35 km.
Results
In our simulation, Mw 8-class earthquakes occurred at depths shallower than 20 km, LSSEs occurred at depths of 20–30 km, and SSSEs occurred at depths of 30–35 km. The SSSEs were larger in scale in Ise Bay and western Shikoku, which is consistent with the observations. In these regions, the plate boundary is shallower than in the surrounding areas and has width in the dip direction, so it is thought that the SSSEs become larger in scale due to stronger locking after one slip. However, it should be noted that the SSSEs are low in eastern Shikoku and active in the Kii Channel, which is different from the observations. The SSSEs occur at intervals of about 3 months to 1 year, and the slip migration speed is about 10 km/day, which is consistent with the observations. In some cases, SSSEs are excited by the influence of LSSEs on the updip side, shortening the interval between SSSEs.