10:45 AM - 11:00 AM
[SSS08-01] Crustal heterogeneity in the 2024 Mutsu Bay earthquake area: effect of fluids from slab dehydration
Keywords:seismic tomography, crustal earthquake, Mutsu Bay, fluid
An M4.6 earthquake occurred at 21:22 on November 16, 2024 at ~8.0 km depth beneath the Mutsu Bay in northern Honshu, and an M5.1 earthquake occurred at 15:40 on November 20 at ~10.0 km depth at almost the same site. These events took place in the upper crust, and their focal mechanisms exhibit a reverse fault type with a compressive axis in the ENE-WSW direction. During November 16 to 29, the seismicity was active near the epicenters of these events, and 9 earthquakes with intensity > 1 occurred there. The M5.1 event is the first earthquake with M > 5.0 in the Mutsu Bay in 90 years since an M5.5 earthquake in 1934. In addition, no earthquake with M > 3.0 had occurred in this area since 2000 till November 2024. Although details on the distribution of active faults in the bay area are unknown, the hypocentral distribution of the events in November 2024 is characterized by a vertical alignment at depths of 5 to 15 km, with almost the same epicenter locations.
It is generally considered that fluids can affect the generation of crustal earthquakes. In the source areas of large crustal earthquakes that have occurred in Japan (e.g., the 1995 M7.3 Kobe earthquake, the 2004 M6.8 Niigata earthquake, and the 2008 M7.2 Iwate-Miyagi earthquake), significant low-velocity (low-V) and high-Poisson’s ratio (PR) anomalies have been detected by seismic tomography (e.g., Zhao et al., 1996; Wang & Zhao, 2006; Cheng et al., 2011). In many cases, the low-V and high-PR anomalies are connected with a low-V zone in the upper mantle wedge above the subducting slab. Therefore, arc magma and fluids derived from slab dehydration may have played an important role in the nucleation of the large crustal earthquakes (Hasegawa & Zhao, 1994; Zhao et al., 2002, 2010, 2015). The Mutsu Bay is currently a non-volcanic area along the volcanic front. However, active volcanoes such as the Osorezan volcano are located in the north, and the Hakkouda and Towada volcanoes are located in the south. An earthquake swarm with a maximum magnitude of 4.9 occurred in 1976 about 20 km northwest of the 2024 Mutsu Bay earthquake area, and low-frequency earthquakes occurred in the lower crust. Although no damage due to the 2024/11 earthquake has been reported, crustal fluids supplied from the mantle wedge are inferred to exist in this area. However, there have been few studies on the crustal structure of the inland area in and around Aomori Prefecture.
In this study, we determine detailed 3-D Vp, Vs, and PR images down to 200 km depth using a great number of arrival time data of local earthquakes that occurred in and around Aomori Prefecture during January 2000 to October 2024. Our results show the subducting Pacific slab clearly as a dipping high-V zone of ~90 km thick. In the central part of the mantle wedge above the Pacific slab, a dipping low-V zone is revealed from ~100 km depth under the back arc, which represents hot and wet upwelling flow associated with the slab dehydration and corner flow in the mantle wedge. Along the volcanic front, significant low-V and high-PR anomalies are detected in the lower crust, which are connected with the low-V zone in the mantle wedge. In contrast, high-V and low-PR anomalies are revealed in the upper crust. These results are in good agreement with those of Zhao, Kitagawa & Toyokuni (2015). The Mutsu Bay earthquakes took place along a boundary between the low-V and high-V zones, which are underlain by significant low-V and PR anomalies in the lower crust. These results indicate that the 2024 Mutsu Bay earthquakes were probably triggered by the intrusion of fluids into an active fault in the upper crust, because the fluids could reduce the fault strength due to high pore pressure.
It is generally considered that fluids can affect the generation of crustal earthquakes. In the source areas of large crustal earthquakes that have occurred in Japan (e.g., the 1995 M7.3 Kobe earthquake, the 2004 M6.8 Niigata earthquake, and the 2008 M7.2 Iwate-Miyagi earthquake), significant low-velocity (low-V) and high-Poisson’s ratio (PR) anomalies have been detected by seismic tomography (e.g., Zhao et al., 1996; Wang & Zhao, 2006; Cheng et al., 2011). In many cases, the low-V and high-PR anomalies are connected with a low-V zone in the upper mantle wedge above the subducting slab. Therefore, arc magma and fluids derived from slab dehydration may have played an important role in the nucleation of the large crustal earthquakes (Hasegawa & Zhao, 1994; Zhao et al., 2002, 2010, 2015). The Mutsu Bay is currently a non-volcanic area along the volcanic front. However, active volcanoes such as the Osorezan volcano are located in the north, and the Hakkouda and Towada volcanoes are located in the south. An earthquake swarm with a maximum magnitude of 4.9 occurred in 1976 about 20 km northwest of the 2024 Mutsu Bay earthquake area, and low-frequency earthquakes occurred in the lower crust. Although no damage due to the 2024/11 earthquake has been reported, crustal fluids supplied from the mantle wedge are inferred to exist in this area. However, there have been few studies on the crustal structure of the inland area in and around Aomori Prefecture.
In this study, we determine detailed 3-D Vp, Vs, and PR images down to 200 km depth using a great number of arrival time data of local earthquakes that occurred in and around Aomori Prefecture during January 2000 to October 2024. Our results show the subducting Pacific slab clearly as a dipping high-V zone of ~90 km thick. In the central part of the mantle wedge above the Pacific slab, a dipping low-V zone is revealed from ~100 km depth under the back arc, which represents hot and wet upwelling flow associated with the slab dehydration and corner flow in the mantle wedge. Along the volcanic front, significant low-V and high-PR anomalies are detected in the lower crust, which are connected with the low-V zone in the mantle wedge. In contrast, high-V and low-PR anomalies are revealed in the upper crust. These results are in good agreement with those of Zhao, Kitagawa & Toyokuni (2015). The Mutsu Bay earthquakes took place along a boundary between the low-V and high-V zones, which are underlain by significant low-V and PR anomalies in the lower crust. These results indicate that the 2024 Mutsu Bay earthquakes were probably triggered by the intrusion of fluids into an active fault in the upper crust, because the fluids could reduce the fault strength due to high pore pressure.