首都圏に来るべき地震とその強震動

The Tokyo Metropolitan Area (TMA) is located close to the triple junction of the Pacific, Philippine Sea, and continental plates. Various earthquakes occur in this complex plate system. Those generated by active faults might be candidates for future earthquakes beneath TMA, but their occurrence probabilities are not high. A deeper part of the Philippine Sea slab is also located beneath TMA. This part can generate a large earthquake with a high probability, so a prediction of strong shaking has been carried out following the recipe for strong ground motions. The occurrence probability of a subduction-zone earthquake along the Sagami trough is low because the 1923 Kanto earthquake occurred only 80 years ago. The probabilities of earthquakes along the Nankai trough are high. In particular, the Tokai earthquake has a probability as high as 86%. The Dai-DaiToku project has made models of the Philippine Sea slab shape and the Kanto basin structure, and a 250 m-mesh geomorphological site classification map, which will enhance the accuracy of strong ground motion prediction

新しいフィリピン海プレート形状と統合三次元速度構造モデルを用いた1923年関東地震の強震動評価

The Tokyo metropolitan area is under constant threat of strong ground motions from future plate-boundary earthquakes along the subducting Philippine sea slab. Here, we upgrade a ground motion simulation of the 1923 Kanto earthquake using a source model along the new geometry of the Philippine sea slab, geophysical-based velocity model, and effcient computational tool. The source process was inferred from strong-motion, teleseismic, and geodetic data with the new geometry of the slab. The 3D velocity-structure model beneath the Tokyo metropolitan area has been constructed using integrating refraction, reflection, borehole, microtremor, and gravity data, as well as ground motion spectra. We introduce a low-frequency ground motion simulation using these models and the finite element method with a voxel mesh. The western basin edge complicated wave propagation, and excited long-period motions were found within the basin. We confirmed that the simulated ground motions are sensitive to the distribution of asperities of the source model along the shallower plate geometry where the eastern major asperity is located closer to downtown Tokyo than in previous models. Because high-frequency components are essential for seismic intensity measurements, source modeling using the pseudo-dynamic approach and ground-motion simulation using the hybrid method combining deterministic and stochastic approaches are strong candidates to complete a broadband ground motion simulation

首都圏・近畿圏における大都市圏地殻構造調査

Deep seismic profiling was performed in the Kanto and Kinki areas to obtain a better estimation of strong ground motions. In the Kanto area, we identify the seismogenic source fault on the upper surface of the Philippine Sea plate. The depth to the top of this plate, 4 to 26 km, is much shallower than previously estimated from the distribution of seismicity. This shallower plate geometry changes the location of the maximum finite slip of the 1923 Kanto earthquake, and its location corresponds to a zone of poor reflection on the mega-thrust, namely, a strong reflectivity zone along the mega-thrust coincidences with aseismic slip zone. In the Kinki area, 120-km-long seismic reflection profiling was carried out from Osaka to Suzuka across the Osaka and Ise basins and several active faults. Deep sub-horizontal reflectors are found at 26 and 16km in depth. The shallower re flectors correspond to the base of the seismogenic zone. Dipping reflectors, probably deeper extensions of active faults, merge into the mid-crustal reflectors

Rupture processes of the 2021 and 2022 Fukushima-oki earthquakes: adjacent events on the complex fault system in the subducting slab

Abstract The 2021 M w 7.1 and 2022 M w 7.4 Fukushima-oki earthquakes ruptured adjacent regions in the subducting slab, which gave us a good opportunity to better understand the rupture process of an intraslab earthquake and the fault system in a subducting slab hosting such large earthquakes. We developed source models of the two earthquakes by constructing fault models based on the distributions of relocated aftershocks and performing joint source inversion using strong motion, teleseismic and geodetic data. The results showed that the 2021 earthquake was initiated by the west-northwest dipping fault and that it then ruptured the east-southeast dipping fault. The rupture propagated to the southwest and up-dip directions. For the 2022 earthquake, the rupture primarily propagated to the north-northeast and up-dip directions on another east-southeast dipping fault, but a delayed rupture occurred around the hypocenter approximately 12 s after the rupture initiation. This was probably due to the complex fault system around the hypocenter. Our source models accurately reproduced observed data for both earthquakes, indicating that the fault geometry was appropriate. We found that the source faults of these earthquakes had similarities to faults in the outer-rise region, which suggests that the 2021 and 2022 earthquakes occurred on faults that originally formed in the outer-rise region and reactivated in the subducting slab. Such a fault system in the subducting slab was probably one of the factors that controlled the rupture processes of the two earthquakes. Graphical Abstrac