5:15 PM - 7:15 PM
[SSS11-P10] Estimation of the seismic bedrock depth of the Kanto basin using receiver function
of the strong ground motion records
Keywords:Kanto basin, MeSO-net, receiver function, PS converted wave, seismic bedrock depth
In recent years, there have been concerns about earthquakes that occurs beneath the Tokyo metropolitan area and a huge Nankai Trough earthquake, which are expected to cause extensive damage to the Tokyo metropolitan area. Although deep sedimentary layered structure models have been estimated for the Kanto basin, the accuracy of the ground motion prediction could be improved by estimating more accurate seismic bedrock depths. In this study, we estimate the seismic bedrock depth by using a receiver function that can detect PS-converted waves in the sedimentary layer using recently released MeSO-net strong-motion observation records. We then verify the accuracy of the J-SHIS seismic bedrock depth by comparing it with theoretical values obtained using J-SHIS deep sedimentary layered model.
The strong ground motion records from 298 MeSO-net stations were used. The earthquake records used were 10 earthquakes from 2018 to 2020 with 35N to 36.5N,140E to 141.5E, 36 to 97 km, and Mj 6.1 to 5.2. The receiver function for the radial component for vertical motion was calculated. The theoretical Green's functions were calculated using Haskell's method from J-SHIS model (NIED, 2019).
The PS-P time becomes shorter from the Shimousa region toward Kasumigaura, suggesting that the seismic bedrock depth becomes shallower. In the Boso Peninsula, the PS-P time tends to become longer toward the center of the western side, with the longest station having a PS-P time of 2.20 s, suggesting that the seismic bedrock depth is deepening. In Tokyo, Saitama, and Kanagawa Prefectures, PS-P time becomes shorter in the Kanto Mountains, and the time difference of PS-P time between adjacent stations changes by more than 0.25 s around 139.5E, suggesting that the seismic bedrock depth is rapidly becoming shallower in the western part.
To verify the accuracy of the J-SHIS seismic bedrock depth, the difference between the PS-P time was investigated. The regions where less difference are Chiba City, Kashiwa City, southern Ibaraki Prefecture, Ota Ward, and the Kanto Mountains. About half of the stations in Chiba, Ibaraki, and Tokyo prefectures had time differences between 0 and ±0.10 s, while about 70% of the stations in Saitama and Kanagawa prefectures had time differences between 0 and ±0.10 s. However, it was found that the J-SHIS seismic bedrock depths tend to be estimated deeper in Chiba and southeastern Ibaraki prefectures, while those in southwestern Ibaraki, Saitama, Tokyo, and Kanagawa prefectures tend to be estimated shallower. Furthermore, in central Chiba Prefecture, Tako Town and the eastern Tokyo and the southern and eastern parts of Saitama Prefecture, the time differences are large, ranging from about ±0.25 to ±0.45 s.
Since the appearance of PS-converted waves is due to the impedance contrast of the seismic bedrock, we focused on the Miura Group. Comparing the stations where PS-converted waves are significant and those where they are not, the thickness of the Miura Group tends to exceed about 1100 m. The thicker Miura Formation making it difficult for PS-converted waves to appear clearly. The obscuration of the PS converted wave may also be due to the effect of artificial noise in the seismograph installation environment or other factors.
From this study, it is clear that the seismic bedrock depth should be further verified in areas where the difference of PS-P time is large, but the reliability of the results of this study is high in the Shimousa region and southern Ibaraki Prefecture. This study indicates that the J-SHIS seismic bedrock depths are generally reasonable. However, the areas where the observed and theoretical PS-P times were large were the same areas where the PS converted wave were unclear.
The strong ground motion records from 298 MeSO-net stations were used. The earthquake records used were 10 earthquakes from 2018 to 2020 with 35N to 36.5N,140E to 141.5E, 36 to 97 km, and Mj 6.1 to 5.2. The receiver function for the radial component for vertical motion was calculated. The theoretical Green's functions were calculated using Haskell's method from J-SHIS model (NIED, 2019).
The PS-P time becomes shorter from the Shimousa region toward Kasumigaura, suggesting that the seismic bedrock depth becomes shallower. In the Boso Peninsula, the PS-P time tends to become longer toward the center of the western side, with the longest station having a PS-P time of 2.20 s, suggesting that the seismic bedrock depth is deepening. In Tokyo, Saitama, and Kanagawa Prefectures, PS-P time becomes shorter in the Kanto Mountains, and the time difference of PS-P time between adjacent stations changes by more than 0.25 s around 139.5E, suggesting that the seismic bedrock depth is rapidly becoming shallower in the western part.
To verify the accuracy of the J-SHIS seismic bedrock depth, the difference between the PS-P time was investigated. The regions where less difference are Chiba City, Kashiwa City, southern Ibaraki Prefecture, Ota Ward, and the Kanto Mountains. About half of the stations in Chiba, Ibaraki, and Tokyo prefectures had time differences between 0 and ±0.10 s, while about 70% of the stations in Saitama and Kanagawa prefectures had time differences between 0 and ±0.10 s. However, it was found that the J-SHIS seismic bedrock depths tend to be estimated deeper in Chiba and southeastern Ibaraki prefectures, while those in southwestern Ibaraki, Saitama, Tokyo, and Kanagawa prefectures tend to be estimated shallower. Furthermore, in central Chiba Prefecture, Tako Town and the eastern Tokyo and the southern and eastern parts of Saitama Prefecture, the time differences are large, ranging from about ±0.25 to ±0.45 s.
Since the appearance of PS-converted waves is due to the impedance contrast of the seismic bedrock, we focused on the Miura Group. Comparing the stations where PS-converted waves are significant and those where they are not, the thickness of the Miura Group tends to exceed about 1100 m. The thicker Miura Formation making it difficult for PS-converted waves to appear clearly. The obscuration of the PS converted wave may also be due to the effect of artificial noise in the seismograph installation environment or other factors.
From this study, it is clear that the seismic bedrock depth should be further verified in areas where the difference of PS-P time is large, but the reliability of the results of this study is high in the Shimousa region and southern Ibaraki Prefecture. This study indicates that the J-SHIS seismic bedrock depths are generally reasonable. However, the areas where the observed and theoretical PS-P times were large were the same areas where the PS converted wave were unclear.