10:00 AM - 10:15 AM
[SSS04-05] 3-D velocity structure beneath Japan derived from local and teleseismic data recorded by S-net and Hi-net
Keywords:S-net, Teleseismic events, seismic tomography, Japan trench
The nucleation and rupture processes of interplate earthquakes in subduction zones are still controversial. Recently, it has been suggested that the presence of hot and wet mantle upwelling in the mantle beneath the slab (subslab hot mantle upwelling, SHMU) may influence the occurrence of slow earthquakes and megathrust earthquakes (e.g., Bodmer et al., 2018; Fan & Zhao, 2021; Zhao & Hua, 2021; Toyokuni et al. 2022). In Japan, a seafloor observation network for earthquakes and tsunami along the Japan Trench (S-net) has been installed in the East Japan forearc and the outer-rise area under the Pacific Ocean after the 2011 Tohoku-Oki Earthquake (Mw 9.0). Recent tomographic studies using the data of local earthquakes recorded by the S-net have improved our understanding of the 3-D forearc structure and provided new insights into seismotectonics and subduction dynamics of the East Japan arc (Yu & Zhao, 2020; Hua et al., 2020; Wang et al., 2022a, 2022b; Zhao et al., 2022; Jia & Zhao, 2023). Teleseismic data recorded at the S-net stations can lead to a much better ray coverage in the forearc region. However, to date only Fan & Zhao (2021) used some teleseismic P-wave data recorded by the S-net to determine Vp tomography beneath East Japan.
In this study, we newly analyze 176 teleseismic events (M > 6.5) recorded at Hi-net (High sensitivity seismograph network in Japan) and 150 S-net stations during January 2017 to June 2024. As a result, 125,725 P-wave relative travel-time residuals from 173 teleseismic events and 85,946 S-wave relative residuals from 167 events are measured. Among them, the number of data recorded at the S-net stations is 14,755 for P-wave and 12,485 for S-wave. We used the tomographic method of Zhao et al. (1994, 2012) to invert the local-earthquake arrival times and the teleseismic relative travel-time residuals simultaneously for 3-D Vp and Vs models down to a depth of 700 km beneath Japan. In our results, the subducting Pacific slab is imaged clearly as a dipping high-velocity zone of ~90 km thick. In the central part of the mantle wedge above the Pacific slab, our results show a dipping low-velocity (low-V) zone from a depth of ~150 km under the back arc, which represents hot and wet upwelling flow associated with the slab dehydration and corner flow in the mantle wedge. These are predominant features of subduction zones and are well consistent with previous results (e.g., Zhao et al., 2012; Liu & Zhao, 2016). Our tomography clearly reveals subslab low-V anomalies (SLVAs) beneath the forearc region at depths of 100 -300 km. The SLVAs have large amplitudes beneath the region off Hokkaido and Boso, but they become disconnected beneath the off-Miyagi area. The SLVA gap appears to correspond to the mainshock rupture zone of the 2011 Tohoku-oki earthquake (Mw 9.0). This result is similar to that of Fan & Zhao (2021). Our tomography also reveals a low-V anomaly subparallel with the slab beneath off Hokkaido from the mantle transition zone (410-660 km depths) to the shallow part. A prominent low-V anomaly is visible beneath southern Tohoku to the Izu Island at depths of 600-700 km. The SLVAs may reflect hot and wet upwelling flows associated with deep subduction of the Pacific plate and its collapsing down to the lower mantle. Previous studies have suggested that the SLVA buoyancy force could cause changes in the overlying slab geometry to increase the shear stress on the megathrust zone, which may limit the large slip area. Our results support the hypothesis of previous studies that SLVA may control the rupture extents of giant megathrust earthquakes.
In this study, we newly analyze 176 teleseismic events (M > 6.5) recorded at Hi-net (High sensitivity seismograph network in Japan) and 150 S-net stations during January 2017 to June 2024. As a result, 125,725 P-wave relative travel-time residuals from 173 teleseismic events and 85,946 S-wave relative residuals from 167 events are measured. Among them, the number of data recorded at the S-net stations is 14,755 for P-wave and 12,485 for S-wave. We used the tomographic method of Zhao et al. (1994, 2012) to invert the local-earthquake arrival times and the teleseismic relative travel-time residuals simultaneously for 3-D Vp and Vs models down to a depth of 700 km beneath Japan. In our results, the subducting Pacific slab is imaged clearly as a dipping high-velocity zone of ~90 km thick. In the central part of the mantle wedge above the Pacific slab, our results show a dipping low-velocity (low-V) zone from a depth of ~150 km under the back arc, which represents hot and wet upwelling flow associated with the slab dehydration and corner flow in the mantle wedge. These are predominant features of subduction zones and are well consistent with previous results (e.g., Zhao et al., 2012; Liu & Zhao, 2016). Our tomography clearly reveals subslab low-V anomalies (SLVAs) beneath the forearc region at depths of 100 -300 km. The SLVAs have large amplitudes beneath the region off Hokkaido and Boso, but they become disconnected beneath the off-Miyagi area. The SLVA gap appears to correspond to the mainshock rupture zone of the 2011 Tohoku-oki earthquake (Mw 9.0). This result is similar to that of Fan & Zhao (2021). Our tomography also reveals a low-V anomaly subparallel with the slab beneath off Hokkaido from the mantle transition zone (410-660 km depths) to the shallow part. A prominent low-V anomaly is visible beneath southern Tohoku to the Izu Island at depths of 600-700 km. The SLVAs may reflect hot and wet upwelling flows associated with deep subduction of the Pacific plate and its collapsing down to the lower mantle. Previous studies have suggested that the SLVA buoyancy force could cause changes in the overlying slab geometry to increase the shear stress on the megathrust zone, which may limit the large slip area. Our results support the hypothesis of previous studies that SLVA may control the rupture extents of giant megathrust earthquakes.