5:15 PM - 6:45 PM
[U15-P50] Characterization of crustal structure of Noto peninsula and source fault models constructed before the 2024 Noto peninsula earthquake.
Keywords:the 2024 Noto Peninsula Earthquake, source fault model, crustal structure, Toyama trough, failed rift
Introduction
For the evaluation of seismic hazards in overriding plate, to construct source fault models is a key for calculation of strong ground motions and tsunami. Around the Noto peninsula seismic reflection profiling has been carried out in the Integrated research project on seismic and tsunami hazards around the Sea of Japan (2013-2021) and models of the seismogenic source faults was proposed in 2015 (Sato et al., 2015). The 2024 Noto peninsula earthquake was the first devastative earthquake where the source fault models were already reported. To compare the models with actual source faults is important to improve the models.
Crustal structure around the Noto peninsula
Noto peninsula including shallow water terrene consists of continental crust. Toyama trough, located east and south of the Noto peninsula, shows higher P-wave velocity in the lower crust and interpreted as a failed rift formed in early Miocene (Ishiyama et al., 2017). The active faults along the northern shore of the Noto peninsula are trending parallel to the Toyama trough in the Toyama Bay, has strong relation to the failed rift.
Seismic reflection profiling
Sesimic reflection profiling toeing a long-distance cable was carried out by JAMSTEC in the offshore of the Noto peninsula (No et al., 2019; Kodaira et al., 2015). In the epicentral area on the 2007 Noto Hanto earthquake, the two-ship seismic reflection proofing revealed the image of source fault down to 4 km (Sato et al., 2007). On shore-off shore integrated sesimc survey across the Toyama trough (Ishiyama et al., 2017) and sesimic reflection profiling toeing a 2 km-long cable was carried out by ERI, Univ. Tokyo in the offshore of the Noto peninsula (Sato et al., 2015). North of Noto peninsula, due to shallow water depth (< 200 m), it was very difficult to get the deep image in the seismic sections.
Constructed source fault models before the 2024 Noto peninsula earthquake
By the dense earthquake observation (Sakai et al., 2008) dip angle of the source fault of the 2007 Noto Hanto earthquake was revealed that it dips to the south at 60 degrees (Sato et al., 2007). As the dip angle obtained by seismic sections (< 3 km in depth) is concordant with deep geometry by hypocentral distribution, we estimated the dip angle of source faults based on the result of seismic reflection profiles. We used the information of the active faults reported by Okamura (2002) and Inoue et al. (2010). The locations of the trace of active faults well accord to the newly obtained seismic sections. We extrapolated the obtained dip angle in the seismc section to a source fault dip angle. We determine the depth of the bottom of a source fault using the depth of earthquake distribution (D90). To compare with the hypocentral distribution using 3D velocity structure (Matsubara and Sato, 2024 JpGU), observed dip angle is much shallower in the faults, labeled as NT6 and 5 along the north shore of the Noto peninsula. For the better estimation of the geometry of source faults, we need deep subsurface image by onshore-offshore integrated seismic reflection profiling and integrated examination together with geological cross sections and tectonic geomorphology.
Rift boundary structure may feed liquid to the source faults of 2024 Noto peninsula earthquake
The source faults of 2024 Noto peninsula earthquake run almost parallel to the Toyama trough beneath the Toyama Bay. Earthquake tomography shows that the boundary of the continental crust and crust, showing higher P-wave velocity in the rift, dips outward from the rift axis. It estimated that the boundary of the failed rift is possible pass for deep sited liquid. The earthquake swarm activity in Noto peninsula started from 2020 at the middle crust (Yoshida et al., 2023). As the location of swam sesimc activity is close to the deep boundary of the failed rift and it may suggest that the Rift boundary structure feeds liquid to the source faults of 2024 Noto peninsula earthquake.
For the evaluation of seismic hazards in overriding plate, to construct source fault models is a key for calculation of strong ground motions and tsunami. Around the Noto peninsula seismic reflection profiling has been carried out in the Integrated research project on seismic and tsunami hazards around the Sea of Japan (2013-2021) and models of the seismogenic source faults was proposed in 2015 (Sato et al., 2015). The 2024 Noto peninsula earthquake was the first devastative earthquake where the source fault models were already reported. To compare the models with actual source faults is important to improve the models.
Crustal structure around the Noto peninsula
Noto peninsula including shallow water terrene consists of continental crust. Toyama trough, located east and south of the Noto peninsula, shows higher P-wave velocity in the lower crust and interpreted as a failed rift formed in early Miocene (Ishiyama et al., 2017). The active faults along the northern shore of the Noto peninsula are trending parallel to the Toyama trough in the Toyama Bay, has strong relation to the failed rift.
Seismic reflection profiling
Sesimic reflection profiling toeing a long-distance cable was carried out by JAMSTEC in the offshore of the Noto peninsula (No et al., 2019; Kodaira et al., 2015). In the epicentral area on the 2007 Noto Hanto earthquake, the two-ship seismic reflection proofing revealed the image of source fault down to 4 km (Sato et al., 2007). On shore-off shore integrated sesimc survey across the Toyama trough (Ishiyama et al., 2017) and sesimic reflection profiling toeing a 2 km-long cable was carried out by ERI, Univ. Tokyo in the offshore of the Noto peninsula (Sato et al., 2015). North of Noto peninsula, due to shallow water depth (< 200 m), it was very difficult to get the deep image in the seismic sections.
Constructed source fault models before the 2024 Noto peninsula earthquake
By the dense earthquake observation (Sakai et al., 2008) dip angle of the source fault of the 2007 Noto Hanto earthquake was revealed that it dips to the south at 60 degrees (Sato et al., 2007). As the dip angle obtained by seismic sections (< 3 km in depth) is concordant with deep geometry by hypocentral distribution, we estimated the dip angle of source faults based on the result of seismic reflection profiles. We used the information of the active faults reported by Okamura (2002) and Inoue et al. (2010). The locations of the trace of active faults well accord to the newly obtained seismic sections. We extrapolated the obtained dip angle in the seismc section to a source fault dip angle. We determine the depth of the bottom of a source fault using the depth of earthquake distribution (D90). To compare with the hypocentral distribution using 3D velocity structure (Matsubara and Sato, 2024 JpGU), observed dip angle is much shallower in the faults, labeled as NT6 and 5 along the north shore of the Noto peninsula. For the better estimation of the geometry of source faults, we need deep subsurface image by onshore-offshore integrated seismic reflection profiling and integrated examination together with geological cross sections and tectonic geomorphology.
Rift boundary structure may feed liquid to the source faults of 2024 Noto peninsula earthquake
The source faults of 2024 Noto peninsula earthquake run almost parallel to the Toyama trough beneath the Toyama Bay. Earthquake tomography shows that the boundary of the continental crust and crust, showing higher P-wave velocity in the rift, dips outward from the rift axis. It estimated that the boundary of the failed rift is possible pass for deep sited liquid. The earthquake swarm activity in Noto peninsula started from 2020 at the middle crust (Yoshida et al., 2023). As the location of swam sesimc activity is close to the deep boundary of the failed rift and it may suggest that the Rift boundary structure feeds liquid to the source faults of 2024 Noto peninsula earthquake.