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[SCG49-08] Cut off depth of seismogenic layer within the upper crust beneath Japanese Islands based on hypocenters determined with three-dimensional seismic velocity structure
Keywords:Cut off depth of seismogenic layer, active fault, coseismic slip region
The subduction of the Pacific plate from the east makes Japanese Islands under compressional stress. The cutoff depth of the seismogenic zone (D90) is important to estimate the maximum size of the earthquake with the active faults. We relocate hypocenters with three-dimensional (3D) seismic velocity structure using airgun (Matsubara et al., 2022) and estimate the cutoff depth of seismogenic zone for the crustal events beneath Japanese Islands.
We relocated 1,914,189 events from October 2000 to August 2024 within 120-150E, 20-50N with 3D seismic velocity structure. We selected events with depths of 0-30 km to be considered as those related to the active faults. The number of events with magnitude larger than 1.4 satisfying Mc is 307,810. We investigated the index D90 as the lower limits of the seismogenic layer defined as the depth above which 90 % of the whole crustal events occurred from the surface. The size of area where we count earthquakes is +/-0.1 degree (approximately 10 km) in case of focusing the active faults and 0.2 degree (approximately 20 km) in case of focusing the area.
We focus 12 large earthquakes, the 2018 Hokkaido Eastern Iburi, the 2008 Iwate-Miyagi, the 2003 North Miyagi, the 2019 Off Yamagata, the 2004 Chuetsu, the 2007 Chuetsu-Oki, the 2007 Noto Peninsula, the 2024 Noto Peninsula, the 2018 Northern Osaka, the 2006 Tottori earthquake, the 2016 Tottori, the 2016 Kumamoto Earthquakes. We calculate D90 for each earthquake using the earthquake catalog before the mainshock. Hypocenters determined with 3D seismic velocity structure are shifted to shallower where the shallow low-velocity zones are resolved such as beneath the hypocenters of 2007 Chuetsu-oki and 2019 Off Yamagata earthquakes. Hypocenters of all large earthquakes are above the D90 estimated from the hypocentral distribution before the mainshocks. Large coseismic slip regions of these large earthquakes are also located above the D90. For example, the hypocenter of the 2007 Chuetsu-oki earthquake is located at depth of 9 km and coseismic slip zone reached 15 km depth. These depths are extremely shallower than D90 as 25 km. The rupture area of the 2019 Off Yamagata prefecture earthquake reached 13 km depth and is above the D90 as 14 km. The hypocenter of the 2024 Noto Peninsula earthquake is at 5 km depth and fault zone was estimated to 12 km. D90 estimated with hypocenters determined by 3D seismic velocity structure around 12 km. Some aftershocks extended below D90 such as the 2016 Kumamoto earthquake.
We obtain D90 estimated using hypocenters determined with 3D seismic velocity structure below the rupture zone of the large earthquakes beneath Japanese Islands. That is important since D90 with hypocenters with 1D velocity structures sometimes within the rupture zone.
References
Matsubara, M., T. Ishiyama, T. No, K. Uehira, M. Mochizuki, T. Kanazawa, N. Takahashi, and S. Kamiya, Seismic velocity structure along the Sea of Japan with large events derived from seismic tomography for whole Japanese Islands including reflection survey data and NIED MOWLAS Hi-net and S-net data, Earth, Planets and Space (2022) 74:171, https://doi.org/10.1186/s40623-022-01724-0
We relocated 1,914,189 events from October 2000 to August 2024 within 120-150E, 20-50N with 3D seismic velocity structure. We selected events with depths of 0-30 km to be considered as those related to the active faults. The number of events with magnitude larger than 1.4 satisfying Mc is 307,810. We investigated the index D90 as the lower limits of the seismogenic layer defined as the depth above which 90 % of the whole crustal events occurred from the surface. The size of area where we count earthquakes is +/-0.1 degree (approximately 10 km) in case of focusing the active faults and 0.2 degree (approximately 20 km) in case of focusing the area.
We focus 12 large earthquakes, the 2018 Hokkaido Eastern Iburi, the 2008 Iwate-Miyagi, the 2003 North Miyagi, the 2019 Off Yamagata, the 2004 Chuetsu, the 2007 Chuetsu-Oki, the 2007 Noto Peninsula, the 2024 Noto Peninsula, the 2018 Northern Osaka, the 2006 Tottori earthquake, the 2016 Tottori, the 2016 Kumamoto Earthquakes. We calculate D90 for each earthquake using the earthquake catalog before the mainshock. Hypocenters determined with 3D seismic velocity structure are shifted to shallower where the shallow low-velocity zones are resolved such as beneath the hypocenters of 2007 Chuetsu-oki and 2019 Off Yamagata earthquakes. Hypocenters of all large earthquakes are above the D90 estimated from the hypocentral distribution before the mainshocks. Large coseismic slip regions of these large earthquakes are also located above the D90. For example, the hypocenter of the 2007 Chuetsu-oki earthquake is located at depth of 9 km and coseismic slip zone reached 15 km depth. These depths are extremely shallower than D90 as 25 km. The rupture area of the 2019 Off Yamagata prefecture earthquake reached 13 km depth and is above the D90 as 14 km. The hypocenter of the 2024 Noto Peninsula earthquake is at 5 km depth and fault zone was estimated to 12 km. D90 estimated with hypocenters determined by 3D seismic velocity structure around 12 km. Some aftershocks extended below D90 such as the 2016 Kumamoto earthquake.
We obtain D90 estimated using hypocenters determined with 3D seismic velocity structure below the rupture zone of the large earthquakes beneath Japanese Islands. That is important since D90 with hypocenters with 1D velocity structures sometimes within the rupture zone.
References
Matsubara, M., T. Ishiyama, T. No, K. Uehira, M. Mochizuki, T. Kanazawa, N. Takahashi, and S. Kamiya, Seismic velocity structure along the Sea of Japan with large events derived from seismic tomography for whole Japanese Islands including reflection survey data and NIED MOWLAS Hi-net and S-net data, Earth, Planets and Space (2022) 74:171, https://doi.org/10.1186/s40623-022-01724-0